Table Of Contents

Configuring IP Unicast Routing

Understanding IP Routing

Steps for Configuring Routing

Configuring IP Addressing

Default Addressing Configuration

Assigning IP Addresses to Network Interfaces

Use of Subnet Zero

Classless Routing

Configuring Address Resolution Methods

Define a Static ARP Cache

Set ARP Encapsulation

Enable Proxy ARP

Routing Assistance When IP Routing is Disabled

Proxy ARP

Default Gateway

ICMP Router Discovery Protocol (IRDP)

Configuring Broadcast Packet Handling

Enabling Directed Broadcast-to-Physical Broadcast Translation

Forwarding UDP Broadcast Packets and Protocols

Establishing an IP Broadcast Address

Flooding IP Broadcasts

Monitoring and Maintaining IP Addressing

Enabling IP Unicast Routing

Configuring RIP

Default RIP Configuration

Configuring Basic RIP Parameters

Configuring RIP Authentication

Configuring Summary Addresses and Split Horizon

Configuring Split Horizon

Configuring OSPF

Default OSPF Configuration

Nonstop Forwarding Awareness

Configuring Basic OSPF Parameters

Configuring OSPF Interfaces

Configuring OSPF Network Types

Configuring OSPF for Nonbroadcast Networks

Configuring Network Types for OSPF Interfaces

Configuring OSPF Area Parameters

Configuring Other OSPF Parameters

Changing LSA Group Pacing

Configuring a Loopback Interface

Monitoring OSPF

Configuring EIGRP

Default EIGRP Configuration

Nonstop Forwarding Awareness

Configuring Basic EIGRP Parameters

Configuring EIGRP Interfaces

Configuring EIGRP Route Authentication

Monitoring and Maintaining EIGRP

Configuring BGP

Default BGP Configuration

Nonstop Forwarding Awareness

Enabling BGP Routing

Managing Routing Policy Changes

Configuring BGP Decision Attributes

Configuring BGP Filtering with Route Maps

Configuring BGP Filtering by Neighbor

Configuring Prefix Lists for BGP Filtering

Configuring BGP Community Filtering 

Configuring BGP Neighbors and Peer Groups

Configuring Aggregate Addresses

Configuring Routing Domain Confederations

Configuring BGP Route Reflectors

Configuring Route Dampening

Monitoring and Maintaining BGP

Configuring ISO CLNS Routing

Configuring IS-IS Dynamic Routing

Default IS-IS Configuration

Nonstop Forwarding Awareness

Enabling IS-IS Routing

Configuring IS-IS Global Parameters

Configuring IS-IS Interface Parameters

Monitoring and Maintaining ISO IGRP and IS-IS

Configuring Multi-VRF CE

Understanding Multi-VRF CE

Default Multi-VRF CE Configuration

Multi-VRF CE Configuration Guidelines

Configuring VRFs

Configuring a VPN Routing Session

Configuring BGP PE to CE Routing Sessions

Multi-VRF CE Configuration Example

Displaying Multi-VRF CE Status

Configuring Protocol-Independent Features

Configuring Cisco Express Forwarding

Configuring the Number of Equal-Cost Routing Paths

Configuring Static Unicast Routes

Specifying Default Routes and Networks

Using Route Maps to Redistribute Routing Information

Configuring Policy-Based Routing

PBR Configuration Guidelines

Enabling PBR

Filtering Routing Information

Setting Passive Interfaces

Controlling Advertising and Processing in Routing Updates

Filtering Sources of Routing Information

Managing Authentication Keys

Monitoring and Maintaining the IP Network

Configuring IP Unicast Routing

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This chapter describes how to configure IP unicast routing on the Catalyst 3750 Metro switch.

For more detailed IP unicast configuration information, see the Cisco IOS IP Configuration Guide, Release 12.2. For complete syntax and usage information for the commands used in this chapter, see these command references:

•Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2

•Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2

•Cisco IOS IP Command Reference, Volume 3 of 3: Multicast, Release 12.2

This chapter consists of these sections:

•Understanding IP Routing

•Steps for Configuring Routing

•Configuring IP Addressing

•Enabling IP Unicast Routing

•Configuring RIP

•Configuring OSPF

•Configuring EIGRP

•Configuring BGP

•Configuring ISO CLNS Routing

•Configuring Multi-VRF CE

•Configuring Protocol-Independent Features

•Monitoring and Maintaining the IP Network

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Note When configuring routing parameters on the switch and to allocate system resources to maximize the number of unicast routes allowed, you can use the sdm prefer routing global configuration command to set the Switch Database Management (SDM) feature to the routing template. For more information on the SDM templates, see Chapter 7, "Configuring SDM Templates," or see the sdm prefer command in the command reference for this release.

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Understanding IP Routing

In some network environments, VLANs are associated with individual networks or subnetworks. In an IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the size of the broadcast domain and keeps local traffic local. However, network devices in different VLANs cannot communicate with one another without a Layer 3 device (router) to route traffic between the VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN.

Figure 34-1 shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The router has an interface in each VLAN.

Figure 34-1 Routing Topology Example

When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet addressed to that host. Switch A forwards the packet directly to Host B, without sending it to the router.

When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which receives the traffic on the VLAN 10 interface. The router checks the routing table, determines the correct outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives the packet and forwards it to Host C.

Routers and Layer 3 switches can route packets in three different ways:

•By using default routing

•By using preprogrammed static routes for the traffic

•By dynamically calculating routes by using a routing protocol

Default routing refers to sending traffic with a destination unknown to the router to a default outlet or destination.

Static unicast routing forwards packets from predetermined ports through a single path into and out of a network. Static routing is secure and uses little bandwidth, but does not automatically respond to changes in the network, such as link failures, and therefore, might result in unreachable destinations. As networks grow, static routing becomes a labor-intensive liability.

Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding traffic. There are two types of dynamic routing protocols:

•Routers using distance-vector protocols maintain routing tables with distance values of networked resources, and periodically pass these tables to their neighbors. Distance-vector protocols use one or a series of metrics for calculating the best routes. These protocols are easy to configure and use.

•Routers using link-state protocols maintain a complex database of network topology, based on the exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in the network, which speeds up the convergence time or time required to respond to these changes. Link-state protocols respond quickly to topology changes, but require greater bandwidth and more resources than distance-vector protocols.

Distance-vector protocols supported by the Catalyst 3750 switch are Routing Information Protocol (RIP), which uses a single distance metric (cost) to determine the best path and Border Gateway Protocol (BGP), which adds a path vector mechanism. The switch also supports the Open Shortest Path First (OSPF) link-state protocol and Enhanced IGRP (EIGRP), which adds some link-state routing features to traditional IGRP to improve efficiency. The switch supports ISO connectionless dynamic routing protocols, such as Interior Gateway Routing Protocol (IGRP), which is a distance vector protocol, and Intermediate System-to-Intermediate System (IS-IS) routing.

Steps for Configuring Routing

By default, IP routing is disabled on the switch, and you must enable it before routing can take place. For detailed IP routing configuration information, see the Cisco IOS IP Configuration Guide, Release 12.2.

In the following procedures, the specified interface must be one of these Layer 3 interfaces:

•A routed port: a physical port configured as a Layer 3 port by using the no switchport interface configuration command.

•A switch virtual interface (SVI): a VLAN interface created by using the interface vlan vlan_id global configuration command and by default a Layer 3 interface.

•An EtherChannel port channel in Layer 3 mode: a port-channel logical interface created by using the interface port-channel port-channel-number global configuration command and binding the Ethernet interface into the channel group. For more information, see the "Configuring Layer 3 EtherChannels" section.

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Note The switch does not support tunnel interfaces for unicast routed traffic.

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All Layer 3 interfaces on which routing will occur must have IP addresses assigned to them. See the "Assigning IP Addresses to Network Interfaces" section.

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Note A Layer 3 switch can have an IP address assigned to each routed port and SVI. The number of routed ports and SVIs that you can configure is not limited by software. However, the interrelationship between this number and the number and volume of features being implemented might have an impact on CPU utilization because of hardware limitations. To optimize system memory for routing, use the sdm prefer routing global configuration command.

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Configuring routing consists of several main procedures:

•To support VLAN interfaces, create and configure VLANs on the switch, and assign VLAN membership to Layer 2 interfaces. For more information, see Chapter 12, "Configuring VLANs."

•Configure Layer 3 interfaces.

•Enable IP routing on the switch.

•Assign IP addresses to the Layer 3 interfaces.

•Enable selected routing protocols on the switch.

•Configure routing protocol parameters (optional).

Configuring IP Addressing

A required task for configuring IP routing is to assign IP addresses to Layer 3 network interfaces to enable the interfaces and allow communication with the hosts on those interfaces that use IP. These sections describe how to configure various IP addressing features. Assigning IP addresses to the interface is required; the other procedures are optional.

•Default Addressing Configuration

•Assigning IP Addresses to Network Interfaces

•Configuring Address Resolution Methods

•Routing Assistance When IP Routing is Disabled

•Configuring Broadcast Packet Handling

•Monitoring and Maintaining IP Addressing

Default Addressing Configuration

Table 34-1 shows the default addressing configuration.

Table 34-1 Default Addressing Configuration 

Feature	Default Setting
IP address	None defined.
ARP	No permanent entries in the Address Resolution Protocol (ARP) cache. Encapsulation: Standard Ethernet-style ARP. Timeout: 14400 seconds (4 hours).
IP broadcast address	255.255.255.255 (all ones).
IP classless routing	Enabled.
IP default gateway	Disabled.
IP directed broadcast	Disabled (all IP directed broadcasts are dropped).
IP domain	Domain list: No domain names defined. Domain lookup: Enabled. Domain name: Enabled.
IP forward-protocol	If a helper address is defined or User Datagram Protocol (UDP) flooding is configured, UDP forwarding is enabled on default ports. Any-local-broadcast: Disabled. Spanning Tree Protocol (STP): Disabled. Turbo-flood: Disabled.
IP helper address	Disabled.
IP host	Disabled.
IRDP	Disabled. Defaults when enabled: •Broadcast IRDP advertisements. •Maximum interval between advertisements: 600 seconds. •Minimum interval between advertisements: 0.75 times max interval •Preference: 0.
IP proxy ARP	Enabled.
IP routing	Disabled.
IP subnet-zero	Disabled.

Assigning IP Addresses to Network Interfaces

An IP address identifies a location to which IP packets can be sent. Some IP addresses are reserved for special uses and cannot be used for host, subnet, or network addresses. RFC 1166, "Internet Numbers," contains the official description of IP addresses.

An interface can have one primary IP address. A mask identifies the bits that denote the network number in an IP address. When you use the mask to subnet a network, the mask is referred to as a subnet mask. To receive an assigned network number, contact your Internet service provider.

Beginning in privileged EXEC mode, follow these steps to assign an IP address and a network mask to a Layer 3 interface:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	no switchport	Remove the interface from Layer 2 configuration mode (if it is a physical interface).
Step 4	ip address ip-address subnet-mask	Configure the IP address and IP subnet mask.
Step 5	no shutdown	Enable the interface.
Step 6	end	Return to privileged EXEC mode.
Step 7	show interfaces [interface-id] show ip interface [interface-id] show running-config interface [interface-id]	Verify your entries.
Step 8	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use of Subnet Zero

Subnetting with a subnet address of zero is strongly discouraged because of the problems that can arise if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as 255.255.255.0, subnet zero would be written as 131.108.0.0, which is the same as the network address.

You can use the all ones subnet (131.108.255.0) and even though it is discouraged, you can enable the use of subnet zero if you need the entire subnet space for your IP address.

Beginning in privileged EXEC mode, follow these steps to enable subnet zero:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip subnet-zero	Enable the use of subnet zero for interface addresses and routing updates.
Step 3	end	Return to privileged EXEC mode.
Step 4	show running-config	Verify your entry.
Step 5	copy running-config startup-config	(Optional) Save your entry in the configuration file.

Use the no ip subnet-zero global configuration command to restore the default and disable the use of subnet zero.

Classless Routing

By default, classless routing behavior is enabled on the switch when it is configured to route. With classless routing, if a router receives packets for a subnet of a network with no default route, the router forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C address spaces used to simulate a single, larger address space and is designed to relieve the pressure on the rapidly depleting Class B address space.

In Figure 34-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of discarding the packet, the router forwards it to the best supernet route. If you disable classless routing and a router receives packets destined for a subnet of a network with no network default route, the router discards the packet.

Figure 34-2 IP Classless Routing

In Figure 34-3, the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and 128.20.3.0. If the host sends a packet to 120.20.4.1, because there is no network default route, the router discards the packet.

Figure 34-3 No IP Classless Routing

To prevent the switch from forwarding packets destined for unrecognized subnets to the best supernet route possible, you can disable classless routing behavior.

Beginning in privileged EXEC mode, follow these steps to disable classless routing:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	no ip classless	Disable classless routing behavior.
Step 3	end	Return to privileged EXEC mode.
Step 4	show running-config	Verify your entry.
Step 5	copy running-config startup-config	(Optional) Save your entry in the configuration file.

To restore the default and have the switch forward packets destined for a subnet of a network with no network default route to the best supernet route possible, use the ip classless global configuration command.

Configuring Address Resolution Methods

You can control interface-specific handling of IP by using address resolution. A device using IP can have both a local address or MAC address, which uniquely defines the device on its local segment or LAN, and a network address, which identifies the network to which the device belongs.

The local address or MAC address is known as a data link address because it is contained in the data link layer (Layer 2) section of the packet header and is read by data link (Layer 2) devices. To communicate with a device on Ethernet, the software must determine the MAC address of the device. The process of determining the MAC address from an IP address is called address resolution. The process of determining the IP address from the MAC address is called reverse address resolution.

The switch can use these forms of address resolution:

•Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses. Taking an IP address as input, ARP determines the associated MAC address and then stores the IP address/MAC address association in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests or replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP).

•Proxy ARP helps hosts with no routing tables determine the MAC addresses of hosts on other networks or subnets. If the switch (router) receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to the host through other interfaces, it generates a proxy ARP packet giving its own local data link address. The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host.

Catalyst 3750 Metro switches also use the Reverse Address Resolution Protocol (RARP), which functions the same as ARP does, except that the RARP packets request an IP address instead of a local MAC address. Using RARP requires a RARP server on the same network segment as the router interface. Use the ip rarp-server address interface configuration command to identify the server.

For more information on RARP, see the Cisco IOS Configuration Fundamentals Configuration Guide, Release 12.2.

You can perform these tasks to configure address resolution:

•Define a Static ARP Cache

•Set ARP Encapsulation

•Enable Proxy ARP

Define a Static ARP Cache

ARP and other address resolution protocols provide dynamic mapping between IP addresses and MAC addresses. Because most hosts support dynamic address resolution, you usually do not need to specify static ARP cache entries. If you must define a static ARP cache entry, you can do so globally, which installs a permanent entry in the ARP cache that the switch uses to translate IP addresses into MAC addresses. Optionally, you can also specify that the switch respond to ARP requests as if it were the owner of the specified IP address. If you do not want the ARP entry to be permanent, you can specify a timeout period for the ARP entry.

Beginning in privileged EXEC mode, follow these steps to provide static mapping between IP addresses and MAC addresses:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	arp ip-address hardware-address type	Globally associate an IP address with a MAC (hardware) address in the ARP cache, and specify encapsulation type as one of these: •arpa—ARP encapsulation for Ethernet interfaces •snap—Subnetwork Address Protocol encapsulation for Token Ring and FDDI interfaces •sap—HP's ARP type
Step 3	arp ip-address hardware-address type [alias]	(Optional) Specify that the switch respond to ARP requests as if it were the owner of the specified IP address.
Step 4	interface interface-id	Enter interface configuration mode, and specify the interface to configure.
Step 5	arp timeout seconds	(Optional) Set the length of time an ARP cache entry will stay in the cache. The default is 14400 seconds (4 hours). The range is 0 to 2147483 seconds.
Step 6	end	Return to privileged EXEC mode.
Step 7	show interfaces [interface-id]	Verify the type of ARP and the timeout value used on all interfaces or a specific interface.
Step 8	show arp or show ip arp	View the contents of the ARP cache.
Step 9	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To remove an entry from the ARP cache, use the no arp ip-address hardware-address type global configuration command. To remove all nonstatic entries from the ARP cache, use the clear arp-cache privileged EXEC command.

Set ARP Encapsulation

By default, Ethernet ARP encapsulation (represented by the arpa keyword) is enabled on an IP interface. You can change the encapsulation methods to SNAP if required by your network.

Beginning in privileged EXEC mode, follow these steps to specify the ARP encapsulation type:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	arp {arpa | snap}	Specify the ARP encapsulation method: •arpa—Address Resolution Protocol •snap—Subnetwork Address Protocol
Step 4	end	Return to privileged EXEC mode.
Step 5	show interfaces [interface-id]	Verify ARP encapsulation configuration on all interfaces or the specified interface.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable an encapsulation type, use the no arp arpa or no arp snap interface configuration command.

Enable Proxy ARP

By default, the switch uses proxy ARP to help hosts determine MAC addresses of hosts on other networks or subnets.

Beginning in privileged EXEC mode, follow these steps to enable proxy ARP if it has been disabled:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	ip proxy-arp	Enable proxy ARP on the interface.
Step 4	end	Return to privileged EXEC mode.
Step 5	show ip interface [interface-id]	Verify the configuration on the interface or all interfaces.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable proxy ARP on the interface, use the no ip proxy-arp interface configuration command.

Routing Assistance When IP Routing is Disabled

These mechanisms allow the switch to learn about routes to other networks when it does not have IP routing enabled:

•Proxy ARP

•Default Gateway

•ICMP Router Discovery Protocol (IRDP)

Proxy ARP

Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no routing information to communicate with hosts on other networks or subnets. The host assumes that all hosts are on the same local Ethernet and that they can use ARP to determine their MAC addresses. If a switch receives an ARP request for a host that is not on the same network as the sender, the switch evaluates whether it has the best route to that host. If it does, it sends an ARP reply packet with its own Ethernet MAC address, and the host that sent the request sends the packet to the switch, which forwards it to the intended host. Proxy ARP treats all networks as if they are local and performs ARP requests for every IP address.

Proxy ARP is enabled by default. To enable it after it has been disabled, see the "Enable Proxy ARP" section. Proxy ARP works as long as other routers support it.

Default Gateway

Another method for locating routes is to define a default router or default gateway. All nonlocal packets are sent to this router, which either routes them appropriately or sends an IP Control Message Protocol (ICMP) redirect message back, defining which local router the host should use. The switch caches the redirect messages and forwards each packet as efficiently as possible. A limitation of this method is that there is no means of detecting when the default router has gone down or is unavailable.

Beginning in privileged EXEC mode, follow these steps to define a default gateway (router) when IP routing is disabled:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip default-gateway ip-address	Set up a default gateway (router).
Step 3	end	Return to privileged EXEC mode.
Step 4	show ip redirects	Display the address of the default gateway router to verify the setting.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no ip default-gateway global configuration command to disable this function.

ICMP Router Discovery Protocol (IRDP)

Router discovery allows the switch to dynamically learn about routes to other networks using IRDP. IRDP allows hosts to locate routers. When operating as a client, the switch generates router discovery packets. When operating as a host, the switch receives router discovery packets. The switch can also listen to Routing Information Protocol (RIP) updates and use this information to infer locations of routers. The switch does not actually store the routing tables sent by routing devices; it merely keeps track of which systems are sending the data. The advantage of using IRDP is that it allows each router to specify both a priority and the time after which a device is assumed to be down if no further packets are received.

Each device discovered becomes a candidate for the default router, and a new highest-priority router is selected when a higher priority router is discovered, when the current default router is declared down, or when a TCP connection is about to time out because of excessive retransmissions.

The only required task for IRDP routing on an interface is to enable IRDP processing on that interface. When enabled, the default parameters apply. You can optionally change any of these parameters.

Beginning in privileged EXEC mode, follow these steps to enable and configure IRDP on an interface:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	ip irdp	Enable IRDP processing on the interface.
Step 4	ip irdp multicast	(Optional) Send IRDP advertisements to the multicast address (224.0.0.1) instead of IP broadcasts. Note This command allows for compatibility with Sun Microsystems Solaris, which requires IRDP packets to be sent out as multicasts. Many implementations cannot receive these multicasts; ensure end-host ability before using this command.
Step 5	ip irdp holdtime seconds	(Optional) Set the IRDP period for which advertisements are valid. The default is three times the maxadvertinterval value. It must be greater than maxadvertinterval and cannot be greater than 9000 seconds. If you change the maxadvertinterval value, this value also changes.
Step 6	ip irdp maxadvertinterval seconds	(Optional) Set the IRDP maximum interval between advertisements. The default is 600 seconds.
Step 7	ip irdp minadvertinterval seconds	(Optional) Set the IRDP minimum interval between advertisements. The default is 0.75 times the maxadvertinterval. If you change the maxadvertinterval, this value changes to the new default (0.75 of maxadvertinterval).
Step 8	ip irdp preference number	(Optional) Set a device IRDP preference level. The allowed range is -231 to 231. The default is 0. A higher value increases the router preference level.
Step 9	ip irdp address address [number]	(Optional) Specify an IRDP address and preference to proxy-advertise.
Step 10	end	Return to privileged EXEC mode.
Step 11	show ip irdp	Verify settings by displaying IRDP values.
Step 12	copy running-config startup-config	(Optional) Save your entries in the configuration file.

If you change the maxadvertinterval value, the holdtime and minadvertinterval values also change, so it is important to first change the maxadvertinterval value, before manually changing either the holdtime or minadvertinterval values.

Use the no ip irdp interface configuration command to disable IRDP routing.

Configuring Broadcast Packet Handling

After configuring an IP interface address, you can enable routing and configure one or more routing protocols, or you can configure the way the switch responds to network broadcasts. A broadcast is a data packet destined for all hosts on a physical network. The switch supports two kinds of broadcasting:

•A directed broadcast packet is sent to a specific network or series of networks. A directed broadcast address includes the network or subnet fields.

•A flooded broadcast packet is sent to every network.

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Note You can also limit broadcast, unicast, and multicast traffic on Layer 2 interfaces by using the storm-control interface configuration command to set traffic suppression levels. For more information, see Chapter 24, "Configuring Port-Based Traffic Control."

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Routers provide some protection from broadcast storms by limiting their extent to the local cable. Bridges (including intelligent bridges), because they are Layer 2 devices, forward broadcasts to all network segments, thus propagating broadcast storms. The best solution to the broadcast storm problem is to use a single broadcast address scheme on a network. In most modern IP implementations, you can set the address to be used as the broadcast address. Many implementations, including the one in the Catalyst 3750 switch, support several addressing schemes for forwarding broadcast messages.

Perform the tasks in these sections to enable these schemes:

•Enabling Directed Broadcast-to-Physical Broadcast Translation

•Forwarding UDP Broadcast Packets and Protocols

•Establishing an IP Broadcast Address

•Flooding IP Broadcasts

Enabling Directed Broadcast-to-Physical Broadcast Translation

By default, IP directed broadcasts are dropped; they are not forwarded. Dropping IP-directed broadcasts makes routers less susceptible to denial-of-service attacks.

You can enable forwarding of IP-directed broadcasts on an interface where the broadcast becomes a physical (MAC-layer) broadcast. Only those protocols configured by using the ip forward-protocol global configuration command are forwarded.

You can specify an access list to control which broadcasts are forwarded. When an access list is specified, only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts. For more information on access lists, see "Configuring Network Security with ACLs."

Beginning in privileged EXEC mode, follow these steps to enable forwarding of IP-directed broadcasts on an interface:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the interface to configure.
Step 3	ip directed-broadcast [access-list-number]	Enable directed broadcast-to-physical broadcast translation on the interface. You can include an access list to control which broadcasts are forwarded. When you specify an access list, only IP packets permitted by the access list can be translated. Note The ip directed-broadcast interface configuration command can be configured on a VPN routing/forwarding (VRF) interface and is VRF aware. Directed broadcast traffic is routed only within the VRF.
Step 4	exit	Return to global configuration mode.
Step 5	ip forward-protocol {udp [port] | nd | sdns}	Specify which protocols and ports the router forwards when forwarding broadcast packets. •udp—Forward UPD datagrams. port: (Optional) Destination port that controls which UDP services are forwarded. •nd—Forward ND datagrams. •sdns—Forward SDNS datagrams
Step 6	end	Return to privileged EXEC mode.
Step 7	show ip interface [interface-id] or show running-config	Verify the configuration on the interface or all interfaces.
Step 8	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no ip directed-broadcast interface configuration command to disable translation of directed broadcast to physical broadcasts. Use the no ip forward-protocol global configuration command to remove a protocol or port.

Forwarding UDP Broadcast Packets and Protocols

User Datagram Protocol (UDP) is an IP host-to-host layer protocol, as is TCP. UDP provides a low-overhead, connectionless session between two end systems and does not provide for acknowledgment of received datagrams. Network hosts occasionally use UDP broadcasts to determine address, configuration, and name information. If such a host is on a network segment that does not include a server, UDP broadcasts are normally not forwarded. You can remedy this situation by configuring an interface on a router to forward certain classes of broadcasts to a helper address. You can use more than one helper address per interface.

You can specify a UDP destination port to control which UDP services are forwarded. You can specify multiple UDP protocols. You can also specify the Network Disk (ND) protocol, which is used by older diskless Sun workstations and the network security protocol SDNS.

By default, both UDP and ND forwarding are enabled if a helper address has been defined for an interface. The description for the ip forward-protocol interface configuration command in the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2 lists the ports that are forwarded by default if you do not specify any UDP ports.

If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry DHCP information.

Beginning in privileged EXEC mode, follow these steps to enable forwarding UDP broadcast packets on an interface and specify the destination address:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	ip helper-address address	Enable forwarding and specify the destination address for forwarding UDP broadcast packets, including BOOTP.
Step 4	exit	Return to global configuration mode.
Step 5	ip forward-protocol {udp [port] | nd | sdns}	Specify which protocols the router forwards when forwarding broadcast packets.
Step 6	end	Return to privileged EXEC mode.
Step 7	show ip interface [interface-id] or show running-config	Verify the configuration on the interface or all interfaces.
Step 8	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no ip helper-address interface configuration command to disable the forwarding of broadcast packets to specific addresses. Use the no ip forward-protocol global configuration command to remove a protocol or port.

Establishing an IP Broadcast Address

The most popular IP broadcast address (and the default) is an address consisting of all ones (255.255.255.255). However, the switch can be configured to generate any form of IP broadcast address.

Beginning in privileged EXEC mode, follow these steps to set the IP broadcast address on an interface:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the interface to configure.
Step 3	ip broadcast-address ip-address	Enter a broadcast address different from the default, for example 128.1.255.255.
Step 4	end	Return to privileged EXEC mode.
Step 5	show ip interface [interface-id]	Verify the broadcast address on the interface or all interfaces.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To restore the default IP broadcast address, use the no ip broadcast-address interface configuration command.

Flooding IP Broadcasts

You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion by using the database created by the bridging STP. Using this feature also prevents loops. To support this capability, bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured on an interface, it still can receive broadcasts. However, the interface never forwards broadcasts it receives, and the router never uses that interface to send broadcasts received on a different interface.

Packets that are forwarded to a single network address using the IP helper-address mechanism can be flooded. Only one copy of the packet is sent on each network segment.

To be considered for flooding, packets must meet these criteria. (Note that these are the same conditions used to consider packet forwarding using IP helper addresses.)

•The packet must be a MAC-level broadcast.

•The packet must be an IP-level broadcast.

•The packet must be a TFTP, DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP specified by the ip forward-protocol udp global configuration command.

•The time-to-live (TTL) value of the packet must be at least two.

A flooded UDP datagram is given the destination address specified with the ip broadcast-address interface configuration command on the output interface. The destination address can be set to any address. Thus, the destination address might change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.

When a flooded UDP datagram is sent out an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they are present on the output interface.

Beginning in privileged EXEC mode, follow these steps to use the bridging spanning-tree database to flood UDP datagrams:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip forward-protocol spanning-tree	Use the bridging spanning-tree database to flood UDP datagrams.
Step 3	end	Return to privileged EXEC mode.
Step 4	show running-config	Verify your entry.
Step 5	copy running-config startup-config	(Optional) Save your entry in the configuration file.

Use the no ip forward-protocol spanning-tree global configuration command to disable the flooding of IP broadcasts.

In a Catalyst 3750 Metro switch, the majority of packets are forwarded in hardware; most packets do not go through the switch CPU. For those packets that do go to the CPU, you can speed up spanning tree-based UDP flooding by a factor of about four to five times by using turbo-flooding. This feature is supported over Ethernet interfaces configured for ARP encapsulation.

Beginning in privileged EXEC mode, follow these steps to increase spanning-tree-based flooding:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode
Step 2	ip forward-protocol turbo-flood	Use the spanning-tree database to speed up flooding of UDP datagrams.
Step 3	end	Return to privileged EXEC mode.
Step 4	show running-config	Verify your entry.
Step 5	copy running-config startup-config	(Optional) Save your entry in the configuration file.

To disable this feature, use the no ip forward-protocol turbo-flood global configuration command.

Monitoring and Maintaining IP Addressing

When the contents of a particular cache, table, or database have become or are suspected to be invalid, you can remove all its contents by using the clear privileged EXEC commands. Table 34-2 lists the commands for clearing contents.

Table 34-2 Commands to Clear Caches, Tables, and Databases

Command	Purpose
clear arp-cache	Clear the IP ARP cache and the fast-switching cache.
clear host {name | *}	Remove one or all entries from the hostname and the address cache.
clear ip route {network [mask] |*}	Remove one or more routes from the IP routing table.

You can display specific statistics, such as the contents of IP routing tables, caches, and databases; the reachability of nodes; and the routing path that packets are taking through the network. Table 34-3 lists the privileged EXEC commands for displaying IP statistics.

Table 34-3 Commands to Display Caches, Tables, and Databases  

Command	Purpose
show arp	Display the entries in the ARP table.
show hosts	Display the default domain name, style of lookup service, name server hosts, and the cached list of hostnames and addresses.
show ip aliases	Display IP addresses mapped to TCP ports (aliases).
show ip arp	Display the IP ARP cache.
show ip interface [interface-id]	Display the IP status of interfaces.
show ip irdp	Display IRDP values.
show ip masks address	Display the masks used for network addresses and the number of subnets using each mask.
show ip redirects	Display the address of a default gateway.
show ip route [address [mask]] | [protocol]	Display the current state of the routing table.
show ip route summary	Display the current state of the routing table in summary form.

Enabling IP Unicast Routing

By default, the switch is in Layer 2 switching mode and IP routing is disabled. To use the Layer 3 capabilities of the switch, you must enable IP routing.

Beginning in privileged EXEC mode, follow these steps to enable IP routing:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip routing	Enable IP routing.
Step 3	router ip_routing_protocol	Specify an IP routing protocol. This step might include other commands, such as specifying the networks to route with the network (RIP) router configuration command. For information on specific protocols, see sections later in this chapter and to the Cisco IOS IP Configuration Guide, Release 12.2.
Step 4	end	Return to privileged EXEC mode.
Step 5	show running-config	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no ip routing global configuration command to disable routing.

This example shows how to enable IP routing using RIP as the routing protocol:

Switch# configure terminal 

Enter configuration commands, one per line.  End with CNTL/Z.

Switch(config)# ip routing 

Switch(config)# router rip 

Switch(config-router)# network 10.0.0.0 

Switch(config-router)# end 

You can now set up parameters for the selected routing protocols as described in these sections:

•Configuring RIP

•Configuring OSPF

•Configuring EIGRP

•Configuring BGP

•Configuring ISO CLNS Routing

•Configuring Multi-VRF CE

You can also configure nonprotocol-specific features:

•Configuring Protocol-Independent Features

Configuring RIP

The Routing Information Protocol (RIP) is an interior gateway protocol (IGP) created for use in small, homogeneous networks. It is a distance-vector routing protocol that uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. The protocol is documented in RFC 1058. You can find detailed information about RIP in IP Routing Fundamentals, published by Cisco Press.

Using RIP, the switch sends routing information updates (advertisements) every 30 seconds. If a router does not receive an update from another router for 180 seconds or more, it marks the routes served by that router as unusable. If there is still no update after 240 seconds, the router removes all routing table entries for the non-updating router.

RIP uses hop counts to rate the value of different routes. The hop count is the number of routers that can be traversed in a route. A directly connected network has a hop count of zero; a network with a hop count of 16 is unreachable. This small range (0 to 15) makes RIP unsuitable for large networks.

If the router has a default network path, RIP advertises a route that links the router to the pseudonetwork 0.0.0.0. The 0.0.0.0 network does not exist; it is treated by RIP as a network to implement the default routing feature. The switch advertises the default network if a default was learned by RIP or if the router has a gateway of last resort and RIP is configured with a default metric. RIP sends updates to the interfaces in specified networks. If an interface's network is not specified, it is not advertised in any RIP update.

This section briefly describes how to configure RIP. It includes this information:

•Default RIP Configuration

•Configuring Basic RIP Parameters

•Configuring RIP Authentication

•Configuring Summary Addresses and Split Horizon

Default RIP Configuration

Table 34-4 shows the default RIP configuration.

Table 34-4 Default RIP Configuration 

Feature	Default Setting
Auto summary	Enabled.
Default-information originate	Disabled.
Default metric	Built-in; automatic metric translations.
IP RIP authentication key-chain	No authentication. Authentication mode: clear text.
IP RIP receive version	According to the version router configuration command.
IP RIP send version	According to the version router configuration command.
IP RIP triggered	According to the version router configuration command.
IP split horizon	Varies with media.
Neighbor	None defined.
Network	None specified.
Offset list	Disabled.
Output delay	0 milliseconds.
Timers basic	Update: 30 seconds. Invalid: 180 seconds. Hold-down: 180 seconds. Flush: 240 seconds.
Validate-update-source	Enabled.
Version	Receives RIP Version 1 and 2 packets; sends Version 1 packets.

Configuring Basic RIP Parameters

To configure RIP, you enable RIP routing for a network and optionally configure other parameters.

Beginning in privileged EXEC mode, follow these steps to enable and configure RIP:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip routing	Enable IP routing. (Required only if IP routing is disabled.)
Step 3	router rip	Enable a RIP routing process, and enter router configuration mode.
Step 4	network network number	Associate a network with a RIP routing process. You can specify multiple network commands. RIP routing updates are sent and received through interfaces only on these networks.
Step 5	neighbor ip-address	(Optional) Define a neighboring router with which to exchange routing information. This step allows routing updates from RIP (normally a broadcast protocol) to reach nonbroadcast networks.
Step 6	offset list [access-list number | name] {in | out} offset [type number]	(Optional) Apply an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through RIP. You can limit the offset list with an access list or an interface.
Step 7	timers basic update invalid holddown flush	(Optional) Adjust routing protocol timers. Valid ranges for all timers are 0 to 4294967295 seconds. •update—The time between sending routing updates. The default is 30 seconds. •invalid—The timer after which a route is declared invalid. The default is 180 seconds. •holddown—The time before a route is removed from the routing table. The default is 180 seconds. •flush—The amount of time for which routing updates are postponed. The default is 240 seconds.
Step 8	version {1 | 2}	(Optional) Configure the switch to receive and send only RIP Version 1 or RIP Version 2 packets. By default, the switch receives Version 1 and 2 but sends only Version 1. You can also use the interface commands ip rip {send | receive} version 1 | 2 | 1 2} to control what versions are used for sending and receiving on interfaces.
Step 9	no auto summary	(Optional) Disable automatic summarization. By default, the switch summarizes subprefixes when crossing classful network boundaries. Disable summarization (RIP Version 2 only) to advertise subnet and host routing information to classful network boundaries.
Step 10	no validate-update-source	(Optional) Disable validation of the source IP address of incoming RIP routing updates. By default, the switch validates the source IP address of incoming RIP routing updates and discards the update if the source address is not valid. Under normal circumstances, disabling this feature is not recommended. However, if you have a router that is off-network and you want to receive its updates, you can use this command.
Step 11	output-delay delay	(Optional) Add interpacket delay for RIP updates sent. By default, packets in a multiple-packet RIP update have no delay added between packets. If you are sending packets to a lower-speed device, you can add an interpacket delay in the range of 8 to 50 milliseconds.
Step 12	end	Return to privileged EXEC mode.
Step 13	show ip protocols	Verify your entries.
Step 14	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To turn off the RIP routing process, use the no router rip global configuration command.

To display the parameters and current state of the active routing protocol process, use the show ip protocols privileged EXEC command. Use the show ip rip database privileged EXEC command to display summary address entries in the RIP database.

Configuring RIP Authentication

RIP Version 1 does not support authentication. If you are sending and receiving RIP Version 2 packets, you can enable RIP authentication on an interface. The key chain determines the set of keys that can be used on the interface. If a key chain is not configured, no authentication is performed, not even the default. Therefore, you must also perform the tasks in the "Managing Authentication Keys" section.

The switch supports two modes of authentication on interfaces for which RIP authentication is enabled: plain text and MD5. The default is plain text.

Beginning in privileged EXEC mode, follow these steps to configure RIP authentication on an interface:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the interface to configure.
Step 3	ip rip authentication key-chain name-of-chain	Enable RIP authentication.
Step 4	ip rip authentication mode [text | md5}	Configure the interface to use plain text authentication (the default) or MD5 digest authentication.
Step 5	end	Return to privileged EXEC mode.
Step 6	show running-config interface [interface-id]	Verify your entries.
Step 7	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To restore clear text authentication, use the no ip rip authentication mode interface configuration command. To prevent authentication, use the no ip rip authentication key-chain interface configuration command.

Configuring Summary Addresses and Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router on any interface from which that information originated. This feature usually optimizes communication among multiple routers, especially when links are broken.

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Note In general, disabling split horizon is not recommended unless you are certain that your application requires it to properly advertise routes.

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If you want to configure an interface running RIP to advertise a summarized local IP address pool on a network access server for dial-up clients, use the ip summary-address rip interface configuration command.

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Note If split horizon is enabled, neither autosummary nor interface IP summary addresses are advertised.

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Beginning in privileged EXEC mode, follow these steps to set an interface to advertise a summarized local IP address and to disable split horizon on the interface:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	ip address ip-address subnet-mask	Configure the IP address and IP subnet.
Step 4	ip summary-address rip ip address ip-network mask	Configure the IP address to be summarized and the IP network mask.
Step 5	no ip split horizon	Disable split horizon on the interface.
Step 6	end	Return to privileged EXEC mode.
Step 7	show ip interface interface-id	Verify your entries.
Step 8	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable IP summarization, use the no ip summary-address rip router configuration command.

In this example, the major net is 10.0.0.0. The summary address 10.2.0.0 overrides the autosummary address of 10.0.0.0 so that 10.2.0.0 is advertised out Gigabit Ethernet port 2, and 10.0.0.0 is not advertised. In the example, if the interface is still in Layer 2 mode (the default), you must enter a no switchport interface configuration command before entering the ip address interface configuration command.

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Note If split horizon is enabled, neither autosummary nor interface summary addresses (those configured with the ip summary-address rip router configuration command) are advertised.

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Switch(config)# router rip

Switch(config-router)# interface gigabitethernet1/0/2

Switch(config-if)# ip address 10.1.5.1 255.255.255.0

Switch(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0

Switch(config-if)# no ip split-horizon

Switch(config-if)# exit

Switch(config)# router rip

Switch(config-router)# network 10.0.0.0

Switch(config-router)# neighbor 2.2.2.2 peer-group mygroup

Switch(config-router)# end

Configuring Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router on any interface from which that information originated. This feature can optimize communication among multiple routers, especially when links are broken.

---

Note In general, we do not recommend disabling split horizon unless you are certain that your application requires it to properly advertise routes.

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Beginning in privileged EXEC mode, follow these steps to disable split horizon on the interface:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the interface to configure.
Step 3	ip address ip-address subnet-mask	Configure the IP address and IP subnet.
Step 4	no ip split-horizon	Disable split horizon on the interface.
Step 5	end	Return to privileged EXEC mode.
Step 6	show ip interface interface-id	Verify your entries.
Step 7	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To enable the split horizon mechanism, use the ip split-horizon interface configuration command.

Configuring OSPF

This section briefly describes how to configure Open Shortest Path First (OSPF). For a complete description of the OSPF commands, see the "OSPF Commands" chapter of the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

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Note OSPF classifies different media into broadcast, nonbroadcast multiaccess (NBMA), or point-to-point networks. Broadcast and nonbroadcast networks can also be configured as point-to-multipoint networks. Starting with Cisco IOS release 12.2(25) SEE, the switch supports all these network types.

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OSPF is an Interior Gateway Protocol (IGP) designed expressly for IP networks, supporting IP subnetting and tagging of externally derived routing information. OSPF also allows packet authentication and uses IP multicast when sending and receiving packets. The Cisco Systems implementation supports RFC 1253, OSPF MIB.

The Cisco implementation conforms to the OSPF Version 2 specifications with these key features:

•Definition of stub areas is supported.

•Routes learned through any IP routing protocol can be redistributed into another IP routing protocol. At the intradomain level, this means that OSPF can import routes learned through EIGRP and RIP. OSPF routes can also be exported into RIP.

•Plain text and MD5 authentication among neighboring routers within an area is supported.

•Configurable routing interface parameters include interface output cost, retransmission interval, interface transmit delay, router priority, router dead and hello intervals, and authentication key.

•Virtual links are supported.

•Not-so-stubby-areas (NSSAs) per RFC 1587are supported.

OSPF typically requires coordination among many internal routers, area border routers (ABRs) connected to multiple areas, and autonomous system boundary routers (ASBRs). The minimum configuration would use all default parameter values, no authentication, and interfaces assigned to areas. If you customize your environment, you must ensure coordinated configuration of all routers.

This section briefly describes how to configure OSPF. It includes this information:

•Default OSPF Configuration

•Configuring Basic OSPF Parameters

•Configuring OSPF Interfaces

•Configuring OSPF Area Parameters

•Configuring Other OSPF Parameters

•Changing LSA Group Pacing

•Configuring a Loopback Interface

•Monitoring OSPF

Default OSPF Configuration

Table 34-5 shows the default OSPF configuration.

Table 34-5 Default OSPF Configuration 

Feature	Default Setting
Interface parameters	Cost: No default cost predefined. Retransmit interval: 5 seconds. Transmit delay: 1 second. Priority: 1. Hello interval: 10 seconds. Dead interval: 4 times the hello interval. No authentication. No password specified. MD5 authentication disabled.
Area	Authentication type: 0 (no authentication). Default cost: 1. Range: Disabled. Stub: No stub area defined. NSSA: No NSSA area defined.
Auto cost	100 Mbps.
Default-information originate	Disabled. When enabled, the default metric setting is 10, and the external route type default is Type 2.
Default metric	Built-in, automatic metric translation, as appropriate for each routing protocol.
Distance OSPF	dist1 (all routes within an area): 110. dist2 (all routes from one area to another): 110. and dist3 (routes from other routing domains): 110.
OSPF database filter	Disabled. All outgoing link-state advertisements (LSAs) are flooded to the interface.
IP OSPF name lookup	Disabled.
Log adjacency changes	Enabled.
Neighbor	None specified.
Neighbor database filter	Disabled. All outgoing LSAs are flooded to the neighbor.
Network area	Disabled.
NSF1 awareness	Enabled2 . Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes.
Router ID	No OSPF routing process defined.
Summary address	Disabled.
Timers LSA group pacing	240 seconds.
Timers shortest path first (spf)	spf delay: 5 seconds. spf-holdtime: 10 seconds.
Virtual link	No area ID or router ID defined. Hello interval: 10 seconds. Retransmit interval: 5 seconds. Transmit delay: 1 second. Dead interval: 40 seconds. Authentication key: no key predefined. Message-digest key (MD5): no key predefined.

1 NSF = Nonstop forwarding 2 OSPF NSF awareness is enabled for IPv4 on Catalyst 3750 Metro switches, Cisco IOS Release 12.2(25)SEG or later.

Nonstop Forwarding Awareness

The OSPF NSF Awareness feature is supported for IPv4, beginning with Cisco IOS Release 12.2(25)SEG. When the neighboring router is NSF-capable, the Layer 3 switch continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in a router crashing and the backup RP taking over, or while the primary RP is manually reloaded for a non-disruptive software upgrade.

This feature cannot be disabled. For more information on this feature see the OSPF Nonstop Forwarding (NSF) Awareness Feature Guide at this URL:

http://cisco.com/en/US/products/sw/iosswrel/ps1839/products_white_paper09186a0080153edd.shtml

Configuring Basic OSPF Parameters

Enabling OSPF requires that you create an OSPF routing process, specify the range of IP addresses to be associated with the routing process, and assign area IDs to be associated with that range.

Beginning in privileged EXEC mode, follow these steps to enable OSPF:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router ospf process-id	Enable OSPF routing, and enter router configuration mode. The process ID is an internally used identification parameter that is locally assigned and can be any positive integer. Each OSPF routing process has a unique value.
Step 3	network address wildcard-mask area area-id	Define an interface on which OSPF runs and the area ID for that interface. You can use the wildcard-mask to use a single command to define one or more multiple interfaces to be associated with a specific OSPF area. The area ID can be a decimal value or an IP address.
Step 4	end	Return to privileged EXEC mode.
Step 5	show ip protocols	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To terminate an OSPF routing process, use the no router ospf process-id global configuration command.

This example shows how to configure an OSPF routing process and assign it a process number of 109:

Switch(config)# router ospf 109

Switch(config-router)# network 131.108.0.0 255.255.255.0 area 24

Configuring OSPF Interfaces

You can use the ip ospf interface configuration commands to modify interface-specific OSPF parameters. You are not required to modify any of these parameters, but some interface parameters (hello interval, dead interval, and authentication key) must be consistent across all routers in an attached network. If you modify these parameters, be sure all routers in the network have compatible values.

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Note The ip ospf interface configuration commands are all optional.

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Beginning in privileged EXEC mode, follow these steps to modify OSPF interface parameters:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	ip ospf cost	(Optional) Explicitly specify the cost of sending a packet on the interface.
Step 4	ip ospf retransmit-interval seconds	(Optional) Specify the number of seconds between link state advertisement transmissions. The range is 1 to 65535 seconds. The default is 5 seconds.
Step 5	ip ospf transmit-delay seconds	(Optional) Set the estimated number of seconds to wait before sending a link state update packet. The range is 1 to 65535 seconds. The default is 1 second.
Step 6	ip ospf priority number	(Optional) Set priority to help determine the OSPF designated router for a network. The range is from 0 to 255. The default is 1.
Step 7	ip ospf hello-interval seconds	(Optional) Set the number of seconds between hello packets sent on an OSPF interface. The value must be the same for all nodes on a network. The range is 1 to 65535 seconds. The default is 10 seconds.
Step 8	ip ospf dead-interval seconds	(Optional) Set the number of seconds after the last device hello packet was seen before its neighbors declare the OSPF router to be down. The value must be the same for all nodes on a network. The range is 1 to 65535 seconds. The default is 4 times the hello interval.
Step 9	ip ospf authentication-key key	(Optional) Assign a password to be used by neighboring OSPF routers. The password can be any string of keyboard-entered characters up to 8 bytes in length. All neighboring routers on the same network must have the same password to exchange OSPF information.
Step 10	ip ospf message digest-key keyid md5 key	(Optional) Enable MDS authentication. •keyid—An identifier from 1 to 255. •key—An alphanumeric password of up to 16 bytes.
Step 11	ip ospf database-filter all out	(Optional) Block flooding of OSPF LSA packets to the interface. By default, OSPF floods new LSAs over all interfaces in the same area, except the interface on which the LSA arrives.
Step 12	end	Return to privileged EXEC mode.
Step 13	show ip ospf interface [interface-name]	Display OSPF-related interface information.
Step 14	show ip ospf neighbor detail	Display NSF awareness status of neighbor switch. The output matches one of these examples: •Options is 0x52 LLS Options is 0x1 (LR) When both of these lines appear, the neighbor switch is NSF aware. •Options is 0x42—This means the neighbor switch is not NSF aware.
Step 15	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no form of these commands to remove the configured parameter value or return to the default value.

Configuring OSPF Network Types

OSPF classifies different media into the three types of networks by default:

•Broadcast networks (Ethernet, Token Ring, and FDDI)

•Nonbroadcast multiaccess (NBMA) networks (Switched Multimegabit Data Service [SMDS], Frame Relay, and X.25)

•Point-to-point networks (High-Level Data Link Control [HDLC], PPP)

You can also configure network interfaces as either a broadcast or an NBMA network and as point-to point or point-to-multipoint, regardless of the default media type.

Configuring OSPF for Nonbroadcast Networks

Because many routers might be attached to an OSPF network, a designated router is selected for the network. If broadcast capability is not configured in the network, the designated router selection requires special configuration parameters. You need to configure these parameters only for devices that are eligible to become the designated router or backup designated router (in other words, routers with a nonzero router priority value).

Beginning in privileged EXEC mode, follow these steps to configure routers that interconnect to nonbroadcast networks:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router ospf process-id	Configure an OSPF routing process and enter router configuration mode.
Step 3	neighbor ip-address [priority number] [poll-interval seconds]	Specify an OSPF neighbor with neighbor parameters as required. •ip-address—Enter the interface IP address of the OSPF neighbor. •(Optional) priority number—Specify the router priority value of the nonbroadcast neighbor associated with the IP address. The range is 0 to 255; the default is 0. •(Optional) poll-interval seconds—Specify a number that represents the poll interval time (in seconds). This value should be much larger than the hello interval. The range is 0-4294967295; the default is 120 seconds (2 minutes).
Step 4	end	Return to privileged EXEC mode.
Step 5	show ip ospf [process-id]	Display OSPF-related information.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

On point-to-multipoint, nonbroadcast networks, you then use the neighbor router configuration command to identify neighbors. Assigning a cost to a neighbor is optional.

Configuring Network Types for OSPF Interfaces

You can configure network interfaces as either broadcast or NBMA and as point-to point or point-to-multipoint, regardless of the default media type.

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface with one or more neighbors. On point-to-multipoint broadcast networks, specifying neighbors is optional. When you configure an interface as point-to-multipoint when the media does not support broadcast, you should use the neighbor command to identify neighbors.

Beginning in privileged EXEC mode, follow these steps to configure OSPF network type for an interface:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	ip ospf network {broadcast | non-broadcast | {point-to-multipoint [non-broadcast] | point-to-point}}	Configure the OSFP network type for the specified interface. Select one of these network types: •broadcast—Specify an OSPF broadcast multi-access network. •non-broadcast—Specify an OSPF NBMA network. •point-to-multipoint—Specify an OSPF point-to-multipoint network. If you do not enter another keyword, the interface is point-to-multipoint for broadcast media. •point-to-multipoint non-broadcast—Specify an OSPF nonbroadcast point-to-multipoint network. •point-to-point—Specify an OSPF point-to-point network.
Step 4	exit	Return to global configuration mode.
Step 5	router ospf process-id	(Optional for point-to-multipoint; required for point-to-multipoint nonbroadcast) Configure an OSPF routing process and enter router configuration mode.
Step 6	neighbor ip-address cost number	(Optional for point-to-multipoint; required for point-to-multipoint nonbroadcast). Specify a configured OSPF neighbor and assign a cost to the neighbor. •ip-address—Enter the interface IP address of the OSPF neighbor. •cost number—Specify a cost for the neighbor as an integer from 1 to 65535. Note On point-to-multipoint broadcast networks, specifying a neighbor is optional, but if you do specify a neighbor, you must specify a cost for that neighbor. On point-to-multipoint nonbroadcast neighbors, you must specify a neighbor, but assigning a cost to the neighbor is optional. If not specified, neighbors assume the cost of the interface, based on the ip ospf cost interface configuration command.
Step 7	end	Return to privileged EXEC mode.
Step 8	show ip ospf interface [interface-id]	Display OSPF-related interface information.
Step 9	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no form of the ip ospf network command to return to the default network type for the media.

Configuring OSPF Area Parameters

You can optionally configure several OSPF area parameters. These parameters include authentication for password-based protection against unauthorized access to an area, stub areas, and not-so-stubby-areas (NSSAs). Stub areas are areas into which information on external routes is not sent. Instead, the area border router (ABR) generates a default external route into the stub area for destinations outside the autonomous system (AS). An NSSA does not flood all LSAs from the core into the area, but can import AS external routes within the area by redistribution.

Route summarization is the consolidation of advertised addresses into a single summary route to be advertised by other areas. If network numbers are contiguous, you can use the area range router configuration command to configure the ABR to advertise a summary route that covers all networks in the range.

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Note The OSPF area router configuration commands are all optional.

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Beginning in privileged EXEC mode, follow these steps to configure area parameters:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router ospf process-id	Enable OSPF routing, and enter router configuration mode.
Step 3	area area-id authentication	(Optional) Allow password-based protection against unauthorized access to the identified area. The identifier can be either a decimal value or an IP address.
Step 4	area area-id authentication message-digest	(Optional) Enable MD5 authentication on the area.
Step 5	area area-id stub [no-summary]	(Optional) Define an area as a stub area. The no-summary keyword prevents an ABR from sending summary link advertisements into the stub area.
Step 6	area area-id nssa [no-redistribution] [default-information-originate] [no-summary]	(Optional) Defines an area as a not-so-stubby-area. Every router within the same area must agree that the area is NSSA. Select one of these keywords: •no-redistribution—Select when the router is an NSSA ABR and you want the redistribute command to import routes into normal areas, but not into the NSSA. •default-information-originate—Select on an ABR to allow importing type 7 LSAs into the NSSA. •no-redistribution—Select to not send summary LSAs into the NSSA.
Step 7	area area-id range address mask	(Optional) Specify an address range for which a single route is advertised. Use this command only with area border routers.
Step 8	end	Return to privileged EXEC mode.
Step 9	show ip ospf [process-id] show ip ospf [process-id [area-id]] database	Display information about the OSPF routing process in general or for a specific process ID to verify configuration. Display lists of information related to the OSPF database for a specific router.
Step 10	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no form of these commands to remove the configured parameter value or to return to the default value.

Configuring Other OSPF Parameters

You can optionally configure other OSPF parameters in router configuration mode.

•Route summarization: When redistributing routes from other protocols as described in the "Using Route Maps to Redistribute Routing Information" section, each route is advertised individually in an external LSA. To help decrease the size of the OSPF link state database, you can use the summary-address router configuration command to advertise a single router for all the redistributed routes included in a specified network address and mask.

•Virtual links: In OSPF, all areas must be connected to a backbone area. You can establish a virtual link in case of a backbone-continuity break by configuring two Area Border Routers as endpoints of a virtual link. Configuration information includes the identity of the other virtual endpoint (the other ABR) and the nonbackbone link that the two routers have in common (the transit area). Virtual links cannot be configured through a stub area.

•Default route: When you specifically configure redistribution of routes into an OSPF routing domain, the route automatically becomes an autonomous system boundary router (ASBR). You can force the ASBR to generate a default route into the OSPF routing domain.

•Domain Name Server (DNS) names for use in all OSPF show privileged EXEC command displays makes it easier to identify a router than displaying it by router ID or neighbor ID.

•Default Metrics: OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. The metric is calculated as ref-bw divided by bandwidth, where ref is 10 by default, and bandwidth (bw) is determined by the bandwidth interface configuration command. For multiple links with high bandwidth, you can specify a larger number to differentiate the cost on those links.

•Administrative distance is a rating of the trustworthiness of a routing information source, an integer between 0 and 255, with a higher value meaning a lower trust rating. An administrative distance of 255 means the routing information source cannot be trusted at all and should be ignored. OSPF uses three different administrative distances: routes within an area (interarea), routes to another area (interarea), and routes from another routing domain learned through redistribution (external). You can change any of the distance values.

•Passive interfaces: Because interfaces between two devices on an Ethernet represent only one network segment, to prevent OSPF from sending hello packets for the sending interface, you must configure the sending device to be a passive interface. Both devices can identify each other through the hello packet for the receiving interface.

•Route calculation timers: You can configure the delay time between when OSPF receives a topology change and when it starts the shortest path first (SPF) calculation and the hold time between two SPF calculations.

•Log neighbor changes: You can configure the router to send a syslog message when an OSPF neighbor state changes, providing a high-level view of changes in the router.

Beginning in privileged EXEC mode, follow these steps to configure these OSPF parameters:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router ospf process-id	Enable OSPF routing, and enter router configuration mode.
Step 3	summary-address address mask	(Optional) Specify an address and IP subnet mask for redistributed routes so that only one summary route is advertised.
Step 4	area area-id virtual-link router-id [hello-interval seconds] [retransmit-interval seconds] [trans] [[authentication-key key] | message-digest-key keyid md5 key]]	(Optional) Establish a virtual link and set its parameters. See the "Configuring OSPF Interfaces" section for parameter definitions and Table 34-5 for virtual link defaults.
Step 5	default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name]	(Optional) Force the ASBR to generate a default route into the OSPF routing domain. Parameters are all optional.
Step 6	ip ospf name-lookup	(Optional) Configure DNS name lookup. The default is disabled.
Step 7	ip auto-cost reference-bandwidth ref-bw	(Optional) Specify an address range for which a single route will be advertised. Use this command only with area border routers.
Step 8	distance ospf {[inter-area dist1] [inter-area dist2] [external dist3]}	(Optional) Change the OSPF distance values. The default distance for each type of route is 110. The range is 1 to 255.
Step 9	passive-interface type number	(Optional) Suppress the sending of hello packets through the specified interface.
Step 10	timers spf spf-delay spf-holdtime	(Optional) Configure route calculation timers. •spf-delay—Enter an integer from 0 to 65535. The default is 5 seconds; 0 means no delay. •spf-holdtime—Enter an integer from 0 to 65535. The default is 10 seconds; 0 means no delay.
Step 11	ospf log-adj-changes	(Optional) Send syslog message when a neighbor state changes.
Step 12	end	Return to privileged EXEC mode.
Step 13	show ip ospf [process-id [area-id]] database	Display lists of information related to the OSPF database for a specific router. For some of the keyword options, see to the "Monitoring OSPF" section.
Step 14	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Changing LSA Group Pacing

The OSPF LSA group pacing feature allows the router to group OSPF LSAs and pace the refreshing, check-summing, and aging functions for more efficient router use. This feature is enabled by default with a 4-minute default pacing interval, and you will not usually need to modify this parameter. The optimum group pacing interval is inversely proportional to the number of LSAs the router is refreshing, check-summing, and aging. For example, if you have approximately 10,000 LSAs in the database, decreasing the pacing interval would benefit you. If you have a very small database (40 to 100 LSAs), increasing the pacing interval to 10 to 20 minutes might benefit you slightly.

Beginning in privileged EXEC mode, follow these steps to configure OSPF LSA pacing:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router ospf process-id	Enable OSPF routing, and enter router configuration mode.
Step 3	timers lsa-group-pacing seconds	Change the group pacing of LSAs.
Step 4	end	Return to privileged EXEC mode.
Step 5	show running-config	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return to the default value, use the no timers lsa-group-pacing router configuration command.

Configuring a Loopback Interface

OSPF uses the highest IP address configured on the interfaces as its router ID. If this interface is down or removed, the OSPF process must recalculate a new router ID and resend all its routing information out its interfaces. If a loopback interface is configured with an IP address, OSPF uses this IP address as its router ID, even if other interfaces have higher IP addresses. Because loopback interfaces never fail, this provides greater stability. OSPF automatically prefers a loopback interface over other interfaces, and it chooses the highest IP address among all loopback interfaces.

Beginning in privileged EXEC mode, follow these steps to configure a loopback interface:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface loopback 0	Create a loopback interface, and enter interface configuration mode.
Step 3	ip address address mask	Assign an IP address to this interface.
Step 4	end	Return to privileged EXEC mode.
Step 5	show ip interface	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no interface loopback 0 global configuration command to disable the loopback interface.

Monitoring OSPF

You can display specific statistics such as the contents of IP routing tables, caches, and databases.

Table 34-6 lists some of the privileged EXEC commands for displaying statistics. For more show ip ospf database privileged EXEC command options and for explanations of fields in the resulting display, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

Table 34-6 Show IP OSPF Statistics Commands

Command	Purpose
show ip ospf [process-id]	Display general information about OSPF routing processes.
show ip ospf [process-id] database [router] [link-state-id] show ip ospf [process-id] database [router] [self-originate] show ip ospf [process-id] database [router] [adv-router [ip-address]] show ip ospf [process-id] database [network] [link-state-id] show ip ospf [process-id] database [summary] [link-state-id] show ip ospf [process-id] database [asbr-summary] [link-state-id] show ip ospf [process-id] database [external] [link-state-id] show ip ospf [process-id area-id] database [database-summary]	Display lists of information related to the OSPF database.
show ip ospf border-routes	Display the internal OSPF routing ABR and ASBR table entries.
show ip ospf interface [interface-name]	Display OSPF-related interface information.
show ip ospf neighbor [interface-name] [neighbor-id] detail	Display OSPF interface neighbor information.
show ip ospf virtual-links	Display OSPF-related virtual links information.

Configuring EIGRP

Enhanced IGRP (EIGRP) is a Cisco proprietary enhanced version of the IGRP. EIGRP uses the same distance vector algorithm and distance information as IGRP; however, the convergence properties and the operating efficiency of EIGRP are significantly improved.

The convergence technology employs an algorithm referred to as the Diffusing Update Algorithm (DUAL), which guarantees loop-free operation at every instant throughout a route computation and allows all devices involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations.

IP EIGRP provides increased network width. With RIP, the largest possible width of your network is 15 hops. Because the EIGRP metric is large enough to support thousands of hops, the only barrier to expanding the network is the transport-layer hop counter. EIGRP increments the transport control field only when an IP packet has traversed 15 routers and the next hop to the destination was learned through EIGRP. When a RIP route is used as the next hop to the destination, the transport control field is incremented as usual.

EIGRP offers these features:

•Fast convergence.

•Incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table, minimizing the bandwidth required for EIGRP packets.

•Less CPU usage because full update packets need not be processed each time they are received.

•Protocol-independent neighbor discovery mechanism to learn about neighboring routers.

•Variable-length subnet masks (VLSMs).

•Arbitrary route summarization.

•EIGRP scales to large networks.

Enhanced IGRP has these four basic components:

•Neighbor discovery and recovery is the process that routers use to dynamically learn of other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. Neighbor discovery and recovery is achieved with low overhead by periodically sending small hello packets. As long as hello packets are received, the Cisco IOS software can determine that a neighbor is alive and functioning. When this status is determined, the neighboring routers can exchange routing information.

•The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP packets must be sent reliably, and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multiaccess network that has multicast capabilities (such as Ethernet), it is not necessary to send hellos reliably to all neighbors individually. Therefore, EIGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets (such as updates) require acknowledgment, which is shown in the packet. The reliable transport has a provision to send multicast packets quickly when there are unacknowledged packets pending. Doing so helps ensure that convergence time remains low in the presence of varying speed links.

•The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses the distance information (known as a metric) to select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors, but there are neighbors advertising the destination, a recomputation must occur. This is the process whereby a new successor is determined. The amount of time it takes to recompute the route affects the convergence time. Recomputation is processor-intensive; it is advantageous to avoid recomputation if it is not necessary. When a topology change occurs, DUAL tests for feasible successors. If there are feasible successors, it uses any it finds to avoid unnecessary recomputation.

•The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the IP EIGRP module, which is responsible for sending and receiving EIGRP packets that are encapsulated in IP. It is also responsible for parsing EIGRP packets and informing DUAL of the new information received. EIGRP asks DUAL to make routing decisions, but the results are stored in the IP routing table. EIGRP is also responsible for redistributing routes learned by other IP routing protocols.

This section briefly describes how to configure EIGRP. It includes this information:

•Default EIGRP Configuration

•Configuring Basic EIGRP Parameters

•Configuring EIGRP Interfaces

•Configuring EIGRP Route Authentication

•Monitoring and Maintaining EIGRP

Default EIGRP Configuration

Table 34-7 shows the default EIGRP configuration.

Table 34-7 Default EIGRP Configuration 

Feature	Default Setting
Auto summary	Enabled. Subprefixes are summarized to the classful network boundary when crossing classful network boundaries.
Default-information	Exterior routes are accepted and default information is passed between EIGRP processes when doing redistribution.
Default metric	Only connected routes and interface static routes can be redistributed without a default metric. The metric includes: •Bandwidth: 0 or greater kbps. •Delay (tens of microseconds): 0 or any positive number that is a multiple of 39.1 nanoseconds. •Reliability: any number between 0 and 255 (255 means 100 percent reliability). •Loading: effective bandwidth as a number between 0 and 255 (255 is 100 percent loading). •MTU: maximum transmission unit size of the route in bytes. 0 or any positive integer.
Distance	Internal distance: 90. External distance: 170.
EIGRP log-neighbor changes	Disabled. No adjacency changes logged.
IP authentication key-chain	No authentication provided.
IP authentication mode	No authentication provided.
IP bandwidth-percent	50 percent.
IP hello interval	For low-speed nonbroadcast multiaccess (NBMA) networks: 60 seconds; all other networks: 5 seconds.
IP hold-time	For low-speed NBMA networks: 180 seconds; all other networks: 15 seconds.
IP split-horizon	Enabled.
IP summary address	No summary aggregate addresses are predefined.
Metric weights	tos: 0; k1 and k3: 1; k2, k4, and k5: 0
Network	None specified.
NSF1 Awareness	Enabled2 . Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes.
Offset-list	Disabled.
Router EIGRP	Disabled.
Set metric	No metric set in the route map.
Traffic-share	Distributed proportionately to the ratios of the metrics.
Variance	1 (equal-cost load balancing).

1 NSF = Nonstop Forwarding 2 EIGRP NSF awareness is enabled for IPv4 on Catalyst 3750 Metro switches, Cisco IOS Release 12.2(25)SEG or later.

To create an EIGRP routing process, you must enable EIGRP and associate networks. EIGRP sends updates to the interfaces in the specified networks. If you do not specify an interface network, it is not advertised in any EIGRP update.

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Note If you have routers on your network that are configured for IGRP, and you want to change to EIGRP, you must designate transition routers that have both IGRP and EIGRP configured. In these cases, perform Steps 1 through 3 in the next section and also see the "Configuring Split Horizon" section. You must use the same AS number for routes to be automatically redistributed.

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Nonstop Forwarding Awareness

The EIGRP NSF Awareness feature is supported for IPv4, beginning with Cisco IOS Release 12.2(25)SEG. When the neighboring router is NSF-capable, the Layer 3 switch continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in a router failing and the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software upgrade.

This feature cannot be disabled. For more information on this feature, see the EIGRP Nonstop Forwarding (NSF) Awareness Feature Guide at this URL: http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a0080160010.html

Configuring Basic EIGRP Parameters

Beginning in privileged EXEC mode, follow these steps to configure EIGRP. Configuring the routing process is required; other steps are optional:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router eigrp autonomous-system	Enable an EIGRP routing process, and enter router configuration mode. The AS number identifies the routes to other EIGRP routers and is used to tag routing information.
Step 3	network network-number	Associate networks with an EIGRP routing process. EIGRP sends updates to the interfaces in the specified networks.
Step 4	eigrp log-neighbor-changes	(Optional) Enable logging of EIGRP neighbor changes to monitor routing system stability.
Step 5	metric weights tos k1 k2 k3 k4 k5	(Optional) Adjust the EIGRP metric. Although the defaults have been carefully determined to provide excellent operation in most networks, you can adjust them. Caution Determining metrics is complex and is not recommended without guidance from an experienced network designer.
Step 6	offset list [access-list number | name] {in | out} offset [type number]	(Optional) Apply an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through EIGRP. You can limit the offset list with an access list or an interface.
Step 7	no auto-summary	(Optional) Disable automatic summarization of subnet routes into network-level routes.
Step 8	ip summary-address eigrp autonomous-system-number address mask	(Optional) Configure a summary aggregate.
Step 9	end	Return to privileged EXEC mode.
Step 10	show ip protocols	Verify your entries.
Step 11	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no forms of these commands to disable the feature or return the setting to the default value.

Configuring EIGRP Interfaces

Other optional EIGRP parameters can be configured on an interface basis.

Beginning in privileged EXEC mode, follow these steps to configure EIGRP interfaces:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	ip bandwidth-percent eigrp percent	(Optional) Configure the percentage of bandwidth that can be used by EIGRP on an interface. The default is 50 percent.
Step 4	ip summary-address eigrp autonomous-system-number address mask	(Optional) Configure a summary aggregate address for a specified interface (not usually necessary if auto-summary is enabled).
Step 5	ip hello-interval eigrp autonomous-system-number seconds	(Optional) Change the hello time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 60 seconds for low-speed NBMA networks and 5 seconds for all other networks.
Step 6	ip hold-time eigrp autonomous-system-number seconds	(Optional) Change the hold time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 180 seconds for low-speed NBMA networks and 15 seconds for all other networks. Caution Do not adjust the hold time without consulting Cisco technical support.
Step 7	no ip split-horizon eigrp autonomous-system-number	(Optional) Disable split horizon to allow route information to be advertised by a router out any interface from which that information originated.
Step 8	end	Return to privileged EXEC mode.
Step 9	show ip eigrp interface	Display which interfaces EIGRP is active on and information about EIGRP relating to those interfaces.
Step 10	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no forms of these commands to disable the feature or return the setting to the default value.

Configuring EIGRP Route Authentication

EIGRP route authentication provides MD5 authentication of routing updates from the EIGRP routing protocol to prevent the introduction of unauthorized or false routing messages from unapproved sources.

Beginning in privileged EXEC mode, follow these steps to enable authentication:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Enter interface configuration mode, and specify the Layer 3 interface to configure.
Step 3	ip authentication mode eigrp autonomous-system md5	Enable MD5 authentication in IP EIGRP packets.
Step 4	ip authentication key-chain eigrp autonomous-system key-chain	Enable authentication of IP EIGRP packets.
Step 5	exit	Return to global configuration mode.
Step 6	key chain name-of-chain	Identify a key chain and enter key-chain configuration mode. Match the name configured in Step 4.
Step 7	key number	In key-chain configuration mode, identify the key number.
Step 8	key-string text	In key-chain key configuration mode, identify the key string.
Step 9	accept-lifetime start-time {infinite | end-time | duration seconds}	(Optional) Specify the time period during which the key can be received. The start-time and end-time syntax can be either hh:mm:ss Month date year or hh:mm:ss date Month year. The default is forever with the default start-time and the earliest acceptable date as January 1, 1993. The default end-time and duration is infinite.
Step 10	send-lifetime start-time {infinite | end-time | duration seconds}	(Optional) Specify the time period during which the key can be sent. The start-time and end-time syntax can be either hh:mm:ss Month date year or hh:mm:ss date Month year. The default is forever with the default start-time and the earliest acceptable date as January 1, 1993. The default end-time and duration is infinite.
Step 11	end	Return to privileged EXEC mode.
Step 12	show key chain	Display authentication key information.
Step 13	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no forms of these commands to disable the feature or to return the setting to the default value.

Monitoring and Maintaining EIGRP

You can delete neighbors from the neighbor table. You can also display various EIGRP routing statistics. Table 34-8 lists the privileged EXEC commands for deleting neighbors and displaying statistics. For explanations of fields in the resulting display, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

Table 34-8  IP EIGRP Clear and Show Commands  

Command	Purpose
clear ip eigrp neighbors [if-address | interface]	Delete neighbors from the neighbor table.
show ip eigrp interface [interface] [as number]	Display information about interfaces configured for EIGRP.
show ip eigrp neighbors [type-number]	Display EIGRP discovered neighbors.
show ip eigrp topology [autonomous-system-number] | [[ip-address] mask]]	Display the EIGRP topology table for a given process.
show ip eigrp traffic [autonomous-system-number]	Display the number of packets sent and received for all or a specified EIGRP process.

Configuring BGP

The Border Gateway Protocol (BGP) is an exterior gateway protocol used to set up an interdomain routing system that guarantees the loop-free exchange of routing information between autonomous systems. Autonomous systems are made up of routers that operate under the same administration and that run Interior Gateway Protocols (IGPs), such as RIP or OSPF, within their boundaries and that interconnect by using an Exterior Gateway Protocol (EGP). BGP Version 4 is the standard EGP for interdomain routing in the Internet. The protocol is defined in RFCs 1163, 1267, and 1771. You can find detailed information about BGP in Internet Routing Architectures, published by Cisco Press, and in the "Configuring BGP" chapter in the Cisco IOS IP and IP Routing Configuration Guide.

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Note For details about BGP commands and keywords, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2. For a list of BGP commands that are visible but not supported by the switch, see Appendix C, "Unsupported Commands in Cisco IOS Release12.2(25)SEG."

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Routers that belong to the same autonomous system (AS) and that exchange BGP updates run internal BGP (IBGP), and routers that belong to different autonomous systems and that exchange BGP updates run external BGP (EBGP). Most configuration commands are the same for configuring EBGP and IBGP. The difference is that the routing updates are exchanged either between autonomous systems (EBGP) or within an AS (IBGP). Figure 34-4 shows a network that is running both EBGP and IBGP.

Figure 34-4 EBGP, IBGP, and Multiple Autonomous Systems

Before exchanging information with an external AS, BGP ensures that networks within the AS can be reached by defining internal BGP peering among routers within the AS and by redistributing BGP routing information to IGPs that run within the AS, such as IGRP and OSPF.

Routers that run a BGP routing process are often referred to as BGP speakers. BGP uses the Transmission Control Protocol (TCP) as its transport protocol (specifically port 179). Two BGP speakers that have a TCP connection to each other for exchanging routing information are known as peers or neighbors. In Figure 34-4, Routers A and B are BGP peers, as are Routers B and C and Routers C and D. The routing information is a series of AS numbers that describe the full path to the destination network. BGP uses this information to construct a loop-free map of autonomous systems.

The network has these characteristics:

•Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP peers are directly connected and that the IBGP peers are not. As long as there is an IGP running that allows the two neighbors to reach one another, IBGP peers do not have to be directly connected.

•All BGP speakers within an AS must establish a peer relationship with each other. That is, the BGP speakers within an AS must be fully meshed logically. BGP4 provides two techniques that reduce the requirement for a logical full mesh: confederations and route reflectors.

•AS 200 is a transit AS for AS 100 and AS 300—that is, AS 200 is used to transfer packets between AS 100 and AS 300.

BGP peers initially exchange their full BGP routing tables and then send only incremental updates. BGP peers also exchange keepalive messages (to ensure that the connection is up) and notification messages (in response to errors or special conditions).

In BGP, each route consists of a network number, a list of autonomous systems that information has passed through (the autonomous system path), and a list of other path attributes. The primary function of a BGP system is to exchange network reachability information, including information about the list of AS paths, with other BGP systems. This information can be used to determine AS connectivity, to prune routing loops, and to enforce AS-level policy decisions.

A router or switch running Cisco IOS does not select or use an IBGP route unless it has a route available to the next-hop router and it has received synchronization from an IGP (unless IGP synchronization is disabled). When multiple routes are available, BGP bases its path selection on attribute values. See the "Configuring BGP Decision Attributes" section for information about BGP attributes.

BGP Version 4 supports classless interdomain routing (CIDR) so you can reduce the size of your routing tables by creating aggregate routes, resulting in supernets. CIDR eliminates the concept of network classes within BGP and supports the advertising of IP prefixes.

These sections briefly describe how to configure BGP and supported BGP features:

•Default BGP Configuration

•Enabling BGP Routing

•Managing Routing Policy Changes

•Configuring BGP Decision Attributes

•Configuring BGP Filtering with Route Maps

•Configuring BGP Filtering by Neighbor

•Configuring Prefix Lists for BGP Filtering

•Configuring BGP Community Filtering

•Configuring BGP Neighbors and Peer Groups

•Configuring Aggregate Addresses

•Configuring Routing Domain Confederations

•Configuring BGP Route Reflectors

•Configuring Route Dampening

•Monitoring and Maintaining BGP

For detailed descriptions of BGP configuration, see the "Configuring BGP" chapter in the Cisco IOS IP Configuration Guide, Release 12.2. For details about specific commands, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

For a list of BGP commands that are visible but not supported by the switch, see Appendix C, "Unsupported Commands in Cisco IOS Release12.2(25)SEG."

Default BGP Configuration

Table 34-9 shows the basic default BGP configuration. For the defaults for all characteristics, see the specific commands in the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

Table 34-9 Default BGP Configuration 

Feature	Default Setting
Aggregate address	Disabled: None defined.
AS path access list	None defined.
Auto summary	Enabled.
Best path	The router considers as-path in choosing a route and does not compare similar routes from external BGP peers. Compare router ID: Disabled.
BGP community list	Number: None defined. When you permit a value for the community number, the list defaults to an implicit deny for everything else that has not been permitted. Format: Cisco default format (32-bit number).
BGP confederation identifier/peers	Identifier: None configured. Peers: None identified.
BGP Fast external fallover	Enabled.
BGP local preference	100. The range is 0 to 4294967295 with the higher value preferred.
BGP network	None specified; no backdoor route advertised.
BGP route dampening	Disabled by default. When enabled: •Half-life is 15 minutes. •Re-use is 750 (10-second increments). •Suppress is 2000 (10-second increments). •Max-suppress-time is 4 times half-life; 60 minutes.
BGP router ID	The IP address of a loopback interface if one is configured or the highest IP address configured for a physical interface on the router.
Default information originate (protocol or network redistribution)	Disabled.
Default metric	Built-in, automatic metric translations.
Distance	External route administrative distance: 20 (acceptable values are from 1 to 255). Internal route administrative distance: 200 (acceptable values are from 1 to 255). Local route administrative distance: 200 (acceptable values are from 1 to 255).
Distribute list	In (filter networks received in updates): Disabled. Out (suppress networks from being advertised in updates): Disabled.
Internal route redistribution	Disabled.
IP prefix list	None defined.
Multi exit discriminator (MED)	Always compare: Disabled. Does not compare MEDs for paths from neighbors in different autonomous systems. Best path compare: Disabled. MED missing as worst path: Disabled. Deterministic MED comparison is disabled.
Neighbor	These are default neighbor settings: •Advertisement interval: 30 seconds for external peers; 5 seconds for internal peers. •Change logging: Enabled. •Conditional advertisement: Disabled. •Default originate: No default route is sent to the neighbor. •Description: None. •Distribute list: None defined. •External BGP multihop: Only directly connected neighbors are allowed. •Filter list: None used. •Maximum number of prefixes received: No limit. •Next hop (router as next hop for BGP neighbor): Disabled. •Password: Disabled. •Peer group: None defined; no members assigned. •Prefix list: None specified. •Remote AS (add entry to neighbor BGP table): No peers defined. •Private AS number removal: Disabled. •Route maps: None applied to a peer. •Send community attributes: None sent to neighbors. •Shutdown or soft reconfiguration: Not enabled. •Timers: keepalive: 60 seconds; holdtime: 180 seconds. •Update source: Best local address. •Version: BGP Version 4. •Weight: Routes learned through BGP peer: 0; routes sourced by the local router: 32768.
NSF1 Awareness	Disabled2 . Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes.
Route reflector	None configured.
Synchronization (BGP and IGP)	Enabled.
Table map update	Disabled.
Timers	Keepalive: 60 seconds Holdtime: 180 seconds.

1 NSF = Nonstop Forwarding 2 NSF Awareness can be enabled for IPv4 on Catalyst 3750 Metro switches with the Cisco IOS Release 12.2(25)SEG image by enabling Graceful Restart.

Nonstop Forwarding Awareness

The BGP NSF Awareness feature is supported for IPv4, beginning with Cisco IOS Release 12.2(25)SEG. To enable this feature with BGP routing, you need to enable Graceful Restart. When the neighboring router is NSF-capable, and this feature is enabled, the Layer 3 switch continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in a router failing and the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software upgrade.

Enabling BGP Routing

To enable BGP routing, you establish a BGP routing process and define the local network. Because BGP must completely recognize the relationships with its neighbors, you must also specify a BGP neighbor.

BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same AS; external neighbors are in different autonomous systems. External neighbors are usually adjacent to each other and share a subnet, but internal neighbors can be anywhere in the same AS.

The switch supports the use of private AS numbers, usually assigned by service providers and given to systems whose routes are not advertised to external neighbors. The private AS numbers are from 64512 to 65535. You can configure external neighbors to remove private AS numbers from the AS path by using the neighbor remove-private-as router configuration command. Then when an update is passed to an external neighbor, if the AS path includes private AS numbers, these numbers are dropped.

If your AS will be passing traffic through it from another AS to a third AS, it is important to be consistent about the routes it advertises. If BGP advertised a route before all routers in the network had learned about the route through the IGP, the AS might receive traffic that some routers could not yet route. To prevent this from happening, BGP must wait until the IGP has propagated information across the AS so that BGP is synchronized with the IGP. Synchronization is enabled by default. If your AS does not pass traffic from one AS to another AS, or if all routers in your autonomous systems are running BGP, you can disable synchronization, which allows your network to carry fewer routes in the IGP and allows BGP to converge more quickly.

Beginning in privileged EXEC mode, follow these steps to enable BGP routing, establish a BGP routing process, and specify a neighbor:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip routing	Enable IP routing (required only if IP routing is disabled).
Step 3	router bgp autonomous-system	Enable a BGP routing process, assign it an AS number, and enter router configuration mode. The AS number can be from 1 to 65535, with 64512 to 65535 designated as private autonomous numbers.
Step 4	network network-number [mask network-mask] [route-map route-map-name]	Configure a network as local to this AS, and enter it in the BGP table.
Step 5	neighbor {ip-address | peer-group-name} remote-as number	Add an entry to the BGP neighbor table specifying that the neighbor identified by the IP address belongs to the specified AS. For EBGP, neighbors are usually directly connected, and the IP address is the address of the interface at the other end of the connection. For IBGP, the IP address can be the address of any of the router interfaces.
Step 6	neighbor {ip-address | peer-group-name} remove-private-as	(Optional) Remove private AS numbers from the AS-path in outbound routing updates.
Step 7	no synchronization	(Optional) Disable synchronization between BGP and an IGP.
Step 8	no auto-summary	(Optional) Disable automatic network summarization. By default, when a subnet is redistributed from an IGP into BGP, only the network route is inserted into the BGP table.
Step 9	bgp fast-external-fallover	(Optional) Automatically reset a BGP session when a link between external neighbors goes down. By default, the session is not immediately reset.
Step 10	bgp graceful-restart	(Optional) Enable NSF awareness on switch. By default, NSF awareness is disabled.
Step 11	end	Return to privileged EXEC mode.
Step 12	show ip bgp network network-number or show ip bgp neighbor	Verify the configuration. Verify that NSF awareness (Graceful Restart) is enabled on the neighbor. If NSF awareness is enabled on the switch and the neighbor, this message appears: Graceful Restart Capability: advertised and received If NSF awareness is enabled on the switch, but not on the neighbor, this message appears: Graceful Restart Capability: advertised
Step 13	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no router bgp autonomous-system global configuration command to remove a BGP AS. Use the no network network-number router configuration command to remove the network from the BGP table. Use the no neighbor {ip-address | peer-group-name} remote-as number router configuration command to remove a neighbor. Use the no neighbor {ip-address | peer-group-name} remove-private-as router configuration command to include private AS numbers in updates to a neighbor. Use the synchronization router configuration command to re-enable synchronization.

These examples show how to configure BGP on the routers in Figure 34-4.

Router A:

Switch(config)# router bgp 100

Switch(config-router)# neighbor 129.213.1.1 remote-as 200

Router B:

Switch(config)# router bgp 200

Switch(config-router)# neighbor 129.213.1.2 remote-as 100

Switch(config-router)# neighbor 175.220.1.2 remote-as 200

Router C:

Switch(config)# router bgp 200

Switch(config-router)# neighbor 175.220.212.1 remote-as 200

Switch(config-router)# neighbor 192.208.10.1 remote-as 300

Router D:

Switch(config)# router bgp 300

Switch(config-router)# neighbor 192.208.10.2 remote-as 200

To verify that BGP peers are running, use the show ip bgp neighbors privileged EXEC command. This is the output of this command on Router A:

Switch# show ip bgp neighbors

BGP neighbor is 129.213.1.1, remote AS 200, external link

 BGP version 4, remote router ID 175.220.212.1

 BGP state = established, table version = 3, up for 0:10:59

 Last read 0:00:29, hold time is 180, keepalive interval is 60 seconds

 Minimum time between advertisement runs is 30 seconds

 Received 2828 messages, 0 notifications, 0 in queue

 Sent 2826 messages, 0 notifications, 0 in queue

 Connections established 11; dropped 10

Anything other than state = established means that the peers are not running. The remote router ID is the highest IP address on that router (or the highest loopback interface). Each time the table is updated with new information, the table version number increments. A table version number that continually increments means that a route is flapping, causing continual routing updates.

For exterior protocols, a reference to an IP network from the network router configuration command controls only which networks are advertised. This is in contrast to Interior Gateway Protocols (IGPs), such as EIGRP, which also use the network command to determine where to send updates.

For detailed descriptions of BGP configuration, see the "IP Routing Protocols" chapter in the Cisco IOS IP Configuration Guide, Release 12.2. For details about specific commands, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2. See Appendix C, "Unsupported Commands in Cisco IOS Release12.2(25)SEG," for a list of BGP commands that are visible but not supported by the switch.

Managing Routing Policy Changes

Routing policies for a peer include all the configurations that might affect inbound or outbound routing table updates. When you have defined two routers as BGP neighbors, they form a BGP connection and exchange routing information. If you later change a BGP filter, weight, distance, version, or timer, or make a similar configuration change, you must reset the BGP sessions so that the configuration changes take effect.

There are two types of reset, hard reset and soft reset. Cisco IOS software releases 12.1 and later support a soft reset without any prior configuration. To use a soft reset without preconfiguration, both BGP peers must support the soft route refresh capability, which is advertised in the OPEN message sent when the peers establish a TCP session. A soft reset allows the dynamic exchange of route refresh requests and routing information between BGP routers and the subsequent re-advertisement of the respective outbound routing table.

•When soft reset generates inbound updates from a neighbor, it is called dynamic inbound soft reset.

•When soft reset sends a set of updates to a neighbor, it is called outbound soft reset.

A soft inbound reset causes the new inbound policy to take effect. A soft outbound reset causes the new local outbound policy to take effect without resetting the BGP session. As a new set of updates is sent during outbound policy reset, a new inbound policy can also take effect.

Table 34-10 lists the advantages and disadvantages hard reset and soft reset.

Table 34-10 Advantages and Disadvantages of Hard and Soft Resets

Type of Reset	Advantages	Disadvantages
Hard reset	No memory overhead	The prefixes in the BGP, IP, and FIB tables provided by the neighbor are lost. Not recommended.
Outbound soft reset	No configuration, no storing of routing table updates	Does not reset inbound routing table updates.
Dynamic inbound soft reset	Does not clear the BGP session and cache Does not require storing of routing table updates and has no memory overhead	Both BGP routers must support the route refresh capability (in Cisco IOS Release 12.1 and later).

Beginning in privileged EXEC mode, follow these steps to determine if a BGP peer supports the route refresh capability and to reset the BGP session:

	Command	Purpose
Step 1	show ip bgp neighbors	Display whether a neighbor supports the route refresh capability. When supported, this message appears for the router: Received route refresh capability from peer.
Step 2	clear ip bgp {* | address | peer-group-name}	Reset the routing table on the specified connection. •Enter an asterisk (*) to specify that all connections be reset. •Enter an IP address to specify the connection to be reset. •Enter a peer group name to reset the peer group.
Step 3	clear ip bgp {* | address | peer-group-name} soft out	(Optional) Perform an outbound soft reset to reset the inbound routing table on the specified connection. Use this command if route refresh is supported. •Enter an asterisk (*) to specify that all connections be reset. •Enter an IP address to specify the connection to be reset. •Enter a peer group name to reset the peer group.
Step 4	show ip bgp show ip bgp neighbors	Verify the reset by checking information about the routing table and about BGP neighbors.

Configuring BGP Decision Attributes

When a BGP speaker receives updates from multiple autonomous systems that describe different paths to the same destination, it must choose the single best path for reaching that destination. When chosen, the selected path is entered into the BGP routing table and propagated to its neighbors. The decision is based on the value of attributes that the update contains and other BGP-configurable factors.

When a BGP peer learns two EBGP paths for a prefix from a neighboring AS, it chooses the best path and inserts that path in the IP routing table. If BGP multipath support is enabled and the EBGP paths are learned from the same neighboring autonomous systems, instead of a single best path, multiple paths are installed in the IP routing table. Then, during packet switching, per-packet or per-destination load balancing is performed among the multiple paths. The maximum-paths router configuration command controls the number of paths allowed.

These factors summarize the order in which BGP evaluates the attributes for choosing the best path:

1. If the path specifies a next hop that is inaccessible, drop the update. The BGP next-hop attribute, automatically determined by the software, is the IP address of the next hop that is going to be used to reach a destination. For EBGP, this is usually the IP address of the neighbor specified by the neighbor remote-as router configuration command. You can disable next-hop processing by using route maps or the neighbor next-hop-self router configuration command.

2. Prefer the path with the largest weight (a Cisco proprietary parameter). The weight attribute is local to the router and not propagated in routing updates. By default, the weight attribute is 32768 for paths that the router originates and zero for other paths. Routes with the largest weight are preferred. You can use access lists, route maps, or the neighbor weight router configuration command to set weights.

3. Prefer the route with the highest local preference. Local preference is part of the routing update and exchanged among routers in the same AS. The default value of the local preference attribute is 100. You can set local preference by using the bgp default local-preference router configuration command or by using a route map.

4. Prefer the route that was originated by BGP running on the local router.

5. Prefer the route with the shortest AS path.

6. Prefer the route with the lowest origin type. An interior route or IGP is lower than a route learned by EGP, and an EGP-learned route is lower than one of unknown origin or learned in another way.

7. Prefer the route with the lowest multi -exit discriminator (MED) metric attribute if the neighboring AS is the same for all routes considered. You can configure the MED by using route maps or by using the default-metric router configuration command. When an update is sent to an IBGP peer, the MED is included.

8. Prefer the external (EBGP) path over the internal (IBGP) path.

9. Prefer the route that can be reached through the closest IGP neighbor (the lowest IGP metric). This means that the router will prefer the shortest internal path within the AS to reach the destination (the shortest path to the BGP next-hop).

10. If the following conditions are all true, insert the route for this path into the IP routing table:

•Both the best route and this route are external.

•Both the best route and this route are from the same neighboring autonomous system.

•maximum-paths is enabled.

11. If multipath is not enabled, prefer the route with the lowest IP address value for the BGP router ID. The router ID is usually the highest IP address on the router or the loopback (virtual) address, but might be implementation-specific.

Beginning in privileged EXEC mode, follow these steps to configure some decision attributes:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router bgp autonomous-system	Enable a BGP routing process, assign it an AS number, and enter router configuration mode.
Step 3	bgp best-path as-path ignore	(Optional) Configure the router to ignore AS path length in selecting a route.
Step 4	neighbor {ip-address | peer-group-name} next-hop-self	(Optional) Disable next-hop processing on BGP updates to a neighbor by entering a specific IP address to be used instead of the next-hop address.
Step 5	neighbor {ip-address | peer-group-name} weight weight	(Optional) Assign a weight to a neighbor connection. Acceptable values are from 0 to 65535; the largest weight is the preferred route. Routes learned through another BGP peer have a default weight of 0; routes sourced by the local router have a default weight of 32768.
Step 6	default-metric number	(Optional) Set a MED metric to set preferred paths to external neighbors. All routes without a MED will also be set to this value. The range is 1 to 4294967295. The lowest value is the most desirable.
Step 7	bgp bestpath med missing-as-worst	(Optional) Configure the switch to consider a missing MED as having a value of infinity, making the path without a MED value the least desirable path.
Step 8	bgp always-compare med	(Optional) Configure the switch to compare MEDs for paths from neighbors in different autonomous systems. By default, MED comparison is only done among paths in the same AS.
Step 9	bgp bestpath med confed	(Optional) Configure the switch to consider the MED in choosing a path from among those advertised by different subautonomous systems within a confederation.
Step 10	bgp deterministic med	(Optional) Configure the switch to consider the MED variable when choosing among routes advertised by different peers in the same AS.
Step 11	bgp default local-preference value	(Optional) Change the default local preference value. The range is 0 to 4294967295; the default value is 100. The highest local preference value is preferred.
Step 12	maximum-paths number	(Optional) Configure the number of paths to be added to the IP routing table. The default is to only enter the best path in the routing table. The range is from 1 to 8. Having multiple paths allows load balancing among the paths.
Step 13	end	Return to privileged EXEC mode.
Step 14	show ip bgp show ip bgp neighbors	Verify the reset by checking information about the routing table and about BGP neighbors.
Step 15	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no form of each command to return to the default state.

Configuring BGP Filtering with Route Maps

Within BGP, route maps can be used to control and to modify routing information and to define the conditions by which routes are redistributed between routing domains. See the "Using Route Maps to Redistribute Routing Information" section for more information about route maps. Each route map has a name that identifies the route map (map tag) and an optional sequence number.

Beginning in privileged EXEC mode, follow these steps to use a route map to disable next-hop processing:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	route-map map-tag [[permit | deny] | sequence-number]]	Create a route map, and enter route-map configuration mode.
Step 3	set ip next-hop ip-address [...ip-address] [peer-address]	(Optional) Set a route map to disable next-hop processing •In an inbound route map, set the next hop of matching routes to be the neighbor peering address, overriding third-party next hops. •In an outbound route map of a BGP peer, set the next hop to the peering address of the local router, disabling the next-hop calculation.
Step 4	end	Return to privileged EXEC mode.
Step 5	show route-map [map-name]	Display all route maps configured or only the one specified to verify configuration.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no route-map map-tag command to delete the route map. Use the no set ip next-hop ip-address command to re-enable next-hop processing.

Configuring BGP Filtering by Neighbor

You can filter BGP advertisements by using AS-path filters, such as the as-path access-list global configuration command and the neighbor filter-list router configuration command. You can also use access lists with the neighbor distribute-list router configuration command. Distribute-list filters are applied to network numbers. See the "Controlling Advertising and Processing in Routing Updates" section for information about the distribute-list command.

You can use route maps on a per-neighbor basis to filter updates and to modify various attributes. A route map can be applied to either inbound or outbound updates. Only the routes that pass the route map are sent or accepted in updates. On both inbound and outbound updates, matching is supported based on AS path, community, and network numbers. Autonomous system path matching requires the match as-path access-list route-map command, community based matching requires the match community-list route-map command, and network-based matching requires the ip access-list global configuration command.

Beginning in privileged EXEC mode, follow these steps to apply a per-neighbor route map:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router bgp autonomous-system	Enable a BGP routing process, assign it an AS number, and enter router configuration mode.
Step 3	neighbor {ip-address | peer-group name} distribute-list {access-list-number | name} {in | out}	(Optional) Filter BGP routing updates to or from neighbors as specified in an access list. Note You can also use the neighbor prefix-list router configuration command to filter updates, but you cannot use both commands to configure the same BGP peer.
Step 4	neighbor {ip-address | peer-group name} route-map map-tag {in | out}	(Optional) Apply a route map to filter an incoming or outgoing route.
Step 5	end	Return to privileged EXEC mode.
Step 6	show ip bgp neighbors	Verify the configuration.
Step 7	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Use the no neighbor distribute-list command to remove the access list from the neighbor. Use the no neighbor route-map map-tag router configuration command to remove the route map from the neighbor.

Another method of filtering is to specify an access list filter on both incoming and outbound updates, based on the BGP autonomous system paths. Each filter is an access list based on regular expressions. (For more information on forming regular expressions, see the "Regular Expressions" appendix in the Cisco IOS Dial Services Command Reference.) To use this method, define an autonomous system path access list, and apply it to updates to and from particular neighbors.

Beginning in privileged EXEC mode, follow these steps to configure BGP path filtering:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip as-path access-list access-list-number {permit | deny} as-regular-expressions	Define a BGP-related access list.
Step 3	router bgp autonomous-system	Enter BGP router configuration mode.
Step 4	neighbor {ip-address | peer-group name} filter-list {access-list-number | name} {in | out | weight weight}	Establish a BGP filter based on an access list.
Step 5	end	Return to privileged EXEC mode.
Step 6	show ip bgp neighbors [paths regular-expression]	Verify the configuration.
Step 7	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Configuring Prefix Lists for BGP Filtering

You can use prefix lists as an alternative to access lists in many BGP route filtering commands, including the neighbor distribute-list router configuration command. The advantages of using prefix lists include performance improvements in loading and lookup of large lists, incremental update support, easier CLI configuration, and greater flexibility.

Filtering by a prefix list involves matching the prefixes of routes with those listed in the prefix list, as when matching access lists. When there is a match, the route is used. Whether a prefix is permitted or denied is based upon these rules:

•An empty prefix list permits all prefixes.

•An implicit deny is assumed if a given prefix does not match any entries in a prefix list.

•When multiple entries of a prefix list match a given prefix, the sequence number of a prefix list entry identifies the entry with the lowest sequence number.

By default, sequence numbers are generated automatically and incremented in units of five. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry. You can specify sequence values in any increment. If you specify increments of one, you cannot insert additional entries into the list; if you choose very large increments, you might run out of values.

You do not need to specify a sequence number when removing a configuration entry. Show commands include the sequence numbers in their output.

Before using a prefix list in a command, you must set up the prefix list. Beginning in privileged EXEC mode, follow these steps to create a prefix list or to add an entry to a prefix list:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip prefix-list list-name [seq seq-value] deny | permit network/len [ge ge-value] [le le-value]	Create a prefix list with an optional sequence number to deny or permit access for matching conditions. You must enter at least one permit or deny clause. •network/len is the network number and length (in bits) of the network mask. •(Optional) ge and le values specify the range of the prefix length to be matched.The specified ge-value and le-value must satisfy this condition: len < ge-value < le-value < 32
Step 3	ip prefix-list list-name seq seq-value deny | permit network/len [ge ge-value] [le le-value]	(Optional) Add an entry to a prefix list, and assign a sequence number to the entry.
Step 4	end	Return to privileged EXEC mode.
Step 5	show ip prefix list [detail | summary] name [network/len] [seq seq-num] [longer] [first-match]	Verify the configuration by displaying information about a prefix list or prefix list entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To delete a prefix list and all of its entries, use the no ip prefix-list list-name global configuration command. To delete an entry from a prefix list, use the no ip prefix-list seq seq-value global configuration command. To disable automatic generation of sequence numbers, use the no ip prefix-list sequence number command; to reenable automatic generation, use the ip prefix-list sequence number command. To clear the hit-count table of prefix list entries, use the clear ip prefix-list privileged EXEC command.

Configuring BGP Community Filtering 

One way that BGP controls the distribution of routing information based on the value of the COMMUNITIES attribute. The attribute is a way to groups destinations into communities and to apply routing decisions based on the communities. This method simplifies configuration of a BGP speaker to control distribution of routing information.

A community is a group of destinations that share some common attribute. Each destination can belong to multiple communities. AS administrators can define to which communities a destination belongs. By default, all destinations belong to the general Internet community. The community is identified by the COMMUNITIES attribute, an optional, transitive, global attribute in the numerical range from 1 to 4294967200. These are some predefined, well-known communities:

•internet—Advertise this route to the Internet community. All routers belong to it.

•no-export—Do not advertise this route to EBGP peers.

•no-advertise—Do not advertise this route to any peer (internal or external).

•local-as—Do not advertise this route to peers outside the local autonomous system.

Based on the community, you can control which routing information to accept, prefer, or distribute to other neighbors. A BGP speaker can set, append, or modify the community of a route when learning, advertising, or redistributing routes. When routes are aggregated, the resulting aggregate has a COMMUNITIES attribute that contains all communities from all the initial routes.

You can use community lists to create groups of communities to use in a match clause of a route map. As with an access list, a series of community lists can be created. Statements are checked until a match is found. As soon as one statement is satisfied, the test is concluded.

To set the COMMUNITIES attribute and match clauses based on communities, see the match community-list and set community route-map configuration commands in the "Using Route Maps to Redistribute Routing Information" section.

By default, no COMMUNITIES attribute is sent to a neighbor. You can specify that the COMMUNITIES attribute be sent to the neighbor at an IP address by using the neighbor send-community router configuration command.

Beginning in privileged EXEC mode, follow these steps to create and to apply a community list:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	ip community-list community-list-number {permit | deny} community-number	Create a community list, and assign it a number. •The community-list-number is an integer from 1 to 99 that identifies one or more permit or deny groups of communities. •The community-number is the number configured by a set community route-map configuration command.
Step 3	router bgp autonomous-system	Enter BGP router configuration mode.
Step 4	neighbor {ip-address | peer-group name} send-community	Specify that the COMMUNITIES attribute be sent to the neighbor at this IP address.
Step 5	set comm-list list-num delete	(Optional) Remove communities from the community attribute of an inbound or outbound update that match a standard or extended community list specified by a route map.
Step 6	exit	Return to global configuration mode.
Step 7	ip bgp-community new-format	(Optional) Display and parse BGP communities in the format AA:NN. A BGP community is displayed in a two-part format 2 bytes long. The Cisco default community format is in the format NNAA. In the most recent RFC for BGP, a community takes the form AA:NN, where the first part is the AS number and the second part is a 2-byte number.
Step 8	end	Return to privileged EXEC mode.
Step 9	show ip bgp community	Verify the configuration.
Step 10	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Configuring BGP Neighbors and Peer Groups

Often many BGP neighbors are configured with the same update policies (that is, the same outbound route maps, distribute lists, filter lists, update source, and so on). Neighbors with the same update policies can be grouped into peer groups to simplify configuration and to make updating more efficient. When you have configured many peers, we recommend this approach.

To configure a BGP peer group, you create the peer group, assign options to the peer group, and add neighbors as peer group members. You configure the peer group by using the neighbor router configuration commands. By default, peer group members inherit all the configuration options of the peer group, including the remote-as (if configured), version, update-source, out-route-map, out-filter-list, out-dist-list, minimum-advertisement-interval, and next-hop-self. All peer group members also inherit changes made to the peer group. Members can also be configured to override the options that do not affect outbound updates.

To assign configuration options to an individual neighbor, specify any of these router configuration commands by using the neighbor IP address. To assign the options to a peer group, specify any of the commands by using the peer group name. You can disable a BGP peer or peer group without removing all the configuration information by using the neighbor shutdown router configuration command.

Beginning in privileged EXEC mode, use these commands to configure BGP peers:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router bgp autonomous-system	Enter BGP router configuration mode.
Step 3	neighbor peer-group-name peer-group	Create a BGP peer group.
Step 4	neighbor ip-address peer-group peer-group-name	Make a BGP neighbor a member of the peer group.
Step 5	neighbor {ip-address | peer-group-name} remote-as number	Specify a BGP neighbor. If a peer group is not configured with a remote-as number, use this command to create peer groups containing EBGP neighbors. The range is 1 to 65535.
Step 6	neighbor {ip-address | peer-group-name} description text	(Optional) Associate a description with a neighbor.
Step 7	neighbor {ip-address | peer-group-name} default-originate [route-map map-name]	(Optional) Allow a BGP speaker (the local router) to send the default route 0.0.0.0 to a neighbor for use as a default route.
Step 8	neighbor {ip-address | peer-group-name} send-community	(Optional) Specify that the COMMUNITIES attribute be sent to the neighbor at this IP address.
Step 9	neighbor {ip-address | peer-group-name} update-source interface	(Optional) Allow internal BGP sessions to use any operational interface for TCP connections.
Step 10	neighbor {ip-address | peer-group-name} ebgp-multihop	(Optional) Allow BGP sessions, even when the neighbor is not on a directly connected segment. The multihop session is not established if the only route to the multihop peer's address is the default route (0.0.0.0).
Step 11	neighbor {ip-address | peer-group-name} local-as number	(Optional) Specify an AS number to use as the local AS. The range is 1 to 65535.
Step 12	neighbor {ip-address | peer-group-name} advertisement-interval seconds	(Optional) Set the minimum interval between sending BGP routing updates.
Step 13	neighbor {ip-address | peer-group-name} maximum-prefix maximum [threshold]	(Optional) Control how many prefixes can be received from a neighbor. The range is 1 to 4294967295. The threshold (optional) is the percentage of maximum at which a warning message is generated. The default is 75 percent.
Step 14	neighbor {ip-address | peer-group-name} next-hop-self	(Optional) Disable next-hop processing on the BGP updates to a neighbor.
Step 15	neighbor {ip-address | peer-group-name} password string	(Optional) Set MD5 authentication on a TCP connection to a BGP peer. The same password must be configured on both BGP peers, or the connection between them is not made.
Step 16	neighbor {ip-address | peer-group-name} route-map map-name {in | out}	(Optional) Apply a route map to incoming or outgoing routes.
Step 17	neighbor {ip-address | peer-group-name} send-community	(Optional) Specify that the COMMUNITIES attribute be sent to the neighbor at this IP address.
Step 18	neighbor {ip-address | peer-group-name} timers keepalive holdtime	(Optional) Set timers for the neighbor or peer group. •The keepalive interval is the time within which keepalive messages are sent to peers. The range is 1 to 4294967295 seconds; the default is 60. •The holdtime is the interval after which a peer is declared inactive after not receiving a keepalive message from it. The range is 1 to 4294967295 seconds; the default is 180.
Step 19	neighbor {ip-address | peer-group-name} weight weight	(Optional) Specify a weight for all routes from a neighbor.
Step 20	neighbor {ip-address | peer-group-name} distribute-list {access-list-number | name} {in | out}	(Optional) Filter BGP routing updates to or from neighbors, as specified in an access list.
Step 21	neighbor {ip-address | peer-group-name} filter-list access-list-number {in | out | weight weight}	(Optional) Establish a BGP filter.
Step 22	neighbor {ip-address | peer-group-name} version value	(Optional) Specify the BGP version to use when communicating with a neighbor.
Step 23	neighbor {ip-address | peer-group-name} soft-reconfiguration inbound	(Optional) Configure the software to start storing received updates.
Step 24	end	Return to privileged EXEC mode.
Step 25	show ip bgp neighbors	Verify the configuration.
Step 26	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable an existing BGP neighbor or neighbor peer group, use the neighbor shutdown router configuration command. To enable a previously existing neighbor or neighbor peer group that had been disabled, use the no neighbor shutdown router configuration command.

Configuring Aggregate Addresses

Classless interdomain routing (CIDR) enables you to create aggregate routes (or supernets) to minimize the size of routing tables. You can configure aggregate routes in BGP either by redistributing an aggregate route into BGP or by creating an aggregate entry in the BGP routing table. An aggregate address is added to the BGP table when there is at least one more specific entry in the BGP table.

Beginning in privileged EXEC mode, use these commands to create an aggregate address in the routing table:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router bgp autonomous-system	Enter BGP router configuration mode.
Step 3	aggregate-address address mask	Create an aggregate entry in the BGP routing table. The aggregate route is advertised as coming from the AS, and the atomic aggregate attribute is set to indicate that information might be missing.
Step 4	aggregate-address address mask as-set	(Optional) Generate AS set path information. This command creates an aggregate entry following the same rules as the previous command, but the advertised path will be an AS_SET consisting of all elements contained in all paths. Do not use this keyword when aggregating many paths because this route must be continually withdrawn and updated.
Step 5	aggregate-address address-mask summary-only	(Optional) Advertise summary addresses only.
Step 6	aggregate-address address mask suppress-map map-name	(Optional) Suppress selected, more specific routes.
Step 7	aggregate-address address mask advertise-map map-name	(Optional) Generate an aggregate based on conditions specified by the route map.
Step 8	aggregate-address address mask attribute-map map-name	(Optional) Generate an aggregate with attributes specified in the route map.
Step 9	end	Return to privileged EXEC mode.
Step 10	show ip bgp neighbors [advertised-routes]	Verify the configuration.
Step 11	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To delete an aggregate entry, use the no aggregate-address address mask router configuration command. To return options to the default values, use the command with keywords.

Configuring Routing Domain Confederations

One way to reduce the IBGP mesh is to divide an autonomous system into multiple subautonomous systems and to group them into a single confederation that appears as a single autonomous system. Each autonomous system is fully meshed within itself and has a few connections to other autonomous systems in the same confederation. Even though the peers in different autonomous systems have EBGP sessions, they exchange routing information as if they were IBGP peers. Specifically, the next hop, MED, and local preference information is preserved. You can then use a single IGP for all of the autonomous systems.

To configure a BGP confederation, you must specify a confederation identifier that acts as the autonomous system number for the group of autonomous systems.

Beginning in privileged EXEC mode, use these commands to configure a BGP confederation:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router bgp autonomous-system	Enter BGP router configuration mode.
Step 3	bgp confederation identifier autonomous-system	Configure a BGP confederation identifier.
Step 4	bgp confederation peers autonomous-system [autonomous-system ...]	Specify the autonomous systems that belong to the confederation and that will be treated as special EBGP peers.
Step 5	end	Return to privileged EXEC mode.
Step 6	show ip bgp neighbor show ip bgp network	Verify the configuration.
Step 7	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Configuring BGP Route Reflectors

BGP requires that all of the IBGP speakers be fully meshed. When a router receives a route from an external neighbor, it must advertise it to all internal neighbors. To prevent a routing information loop, all IBPG speakers must be connected. The internal neighbors do not send routes learned from internal neighbors to other internal neighbors.

With route reflectors, all IBGP speakers need not be fully meshed because another method is used to pass learned routes to neighbors. When you configure an internal BGP peer to be a route reflector, it is responsible for passing IBGP learned routes to a set of IBGP neighbors. The internal peers of the route reflector are divided into two groups: client peers and nonclient peers (all the other routers in the autonomous system). A route reflector reflects routes between these two groups. The route reflector and its client peers form a cluster. The nonclient peers must be fully meshed with each other, but the client peers need not be fully meshed. The clients in the cluster do not communicate with IBGP speakers outside their cluster.

When the route reflector receives an advertised route, it takes one of these actions, depending on the neighbor:

•A route from an external BGP speaker is advertised to all clients and nonclient peers.

•A route from a nonclient peer is advertised to all clients.

•A route from a client is advertised to all clients and nonclient peers. Hence, the clients need not be fully meshed.

Usually a cluster of clients have a single route reflector, and the cluster is identified by the route reflector router ID. To increase redundancy and to avoid a single point of failure, a cluster might have more than one route reflector. In this case, all route reflectors in the cluster must be configured with the same 4-byte cluster ID so that a route reflector can recognize updates from route reflectors in the same cluster. All the route reflectors serving a cluster should be fully meshed and should have identical sets of client and nonclient peers.

Beginning in privileged EXEC mode, use these commands to configure a route reflector and clients:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router bgp autonomous-system	Enter BGP router configuration mode.
Step 3	neighbor ip-address | peer-group-name route-reflector-client	Configure the local router as a BGP route reflector and the specified neighbor as a client.
Step 4	bgp cluster-id cluster-id	(Optional) Configure the cluster ID if the cluster has more than one route reflector.
Step 5	no bgp client-to-client reflection	(Optional) Disable client-to-client route reflection. By default, the routes from a route reflector client are reflected to other clients. However, if the clients are fully meshed, the route reflector does not need to reflect routes to clients.
Step 6	end	Return to privileged EXEC mode.
Step 7	show ip bgp	Verify the configuration. Display the originator ID and the cluster-list attributes.
Step 8	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Configuring Route Dampening

Route flap dampening is a BGP feature designed to minimize the propagation of flapping routes across an internetwork. A route is considered to be flapping when it is repeatedly available, then unavailable, then available, then unavailable, and so on. When route dampening is enabled, a numeric penalty value is assigned to a route when it flaps. When a route's accumulated penalties reach a configurable limit, BGP suppresses advertisements of the route, even if the route is running. The reuse limit is a configurable value that is compared with the penalty. If the penalty is less than the reuse limit, a suppressed route that is up is advertised again.

Dampening is not applied to routes that are learned by IBGP. This policy prevents the IBGP peers from having a higher penalty for routes external to the AS.

Beginning in privileged EXEC mode, use these commands to configure BGP route dampening:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	router bgp autonomous-system	Enter BGP router configuration mode.
Step 3	bgp dampening	Enable BGP route dampening.
Step 4	bgp dampening half-life reuse suppress max-suppress [route-map map]	(Optional) Change the default values of route dampening factors.
Step 5	end	Return to privileged EXEC mode.
Step 6	show ip bgp flap-statistics [{regexp regexp} | {filter-list list} | {address mask [longer-prefix]}]	(Optional) Monitor the flaps of all paths that are flapping. The statistics are deleted when the route is not suppressed and is stable.
Step 7	show ip bgp dampened-paths	(Optional) Display the dampened routes, including the time remaining before they are suppressed.
Step 8	clear ip bgp flap-statistics [{regexp regexp} | {filter-list list} | {address mask [longer-prefix]}	(Optional) Clear BGP flap statistics to make it less likely that a route will be dampened.
Step 9	clear ip bgp dampening	(Optional) Clear route dampening information, and unsuppress the suppressed routes.
Step 10	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable flap dampening, use the no bgp dampening router configuration command without keywords. To set dampening factors back to the default values, use the no bgp dampening router configuration command with values.

Monitoring and Maintaining BGP

You can remove all contents of a particular cache, table, or database. This might be necessary when the contents of the particular structure have become or are suspected to be invalid.

You can display specific statistics, such as the contents of BGP routing tables, caches, and databases. You can use the information to determine resource utilization and solve network problems. You can also display information about node reachability and discover the routing path your device's packets are taking through the network.

Table 34-8 lists the privileged EXEC commands for clearing and displaying BGP. For explanations of the display fields, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.

Table 34-11  IP BGP Clear and Show Commands  

Command	Purpose
clear ip bgp address	Reset a particular BGP connection.
clear ip bgp *	Reset all BGP connections.
clear ip bgp peer-group tag	Remove all members of a BGP peer group.
show ip bgp prefix	Display peer groups and peers not in peer groups to which the prefix has been advertised. Also display prefix attributes such as the next hop and the local prefix.
show ip bgp cidr-only	Display all BGP routes that contain subnet and supernet network masks.
show ip bgp community [community-number] [exact]	Display routes that belong to the specified communities.
show ip bgp community-list community-list-number [exact-match]	Display routes that are permitted by the community list.
show ip bgp filter-list access-list-number	Display routes that are matched by the specified AS path access list.
show ip bgp inconsistent-as	Display the routes with inconsistent originating autonomous systems.
show ip bgp regexp regular-expression	Display the routes that have an AS path that matches the specified regular expression entered on the command line.
show ip bgp	Display the contents of the BGP routing table.
show ip bgp neighbors [address]	Display detailed information on the BGP and TCP connections to individual neighbors.
show ip bgp neighbors [address] [advertised-routes | dampened-routes | flap-statistics | paths regular-expression | received-routes | routes]	Display routes learned from a particular BGP neighbor.
show ip bgp paths	Display all BGP paths in the database.
show ip bgp peer-group [tag] [summary]	Display information about BGP peer groups.
show ip bgp summary	Display the status of all BGP connections.

You can also enable the logging of messages generated when a BGP neighbor resets, comes up, or goes down by using the bgp log-neighbor changes router configuration command.

Configuring ISO CLNS Routing

The International Organization for Standardization (ISO) Connectionless Network Service (CLNS) protocol is a standard for the network layer of the Open System Interconnection (OSI) model. Addresses in the ISO network architecture are referred to as network service access point (NSAP) addresses and network entity titles (NETs). Each node in an OSI network has one or more NETs. In addition, each node has many NSAP addresses.

When you enable connectionless routing on the switch by using the clns routing global configuration command, the switch makes only forwarding decisions, with no routing-related functionality. For dynamic routing, you must also enable a routing protocol. The switch supports the Intermediate System-to-Intermediate System (IS-IS) dynamic routing protocols for ISO CLNS networks:  

When dynamically routing, you use IS-IS. This routing protocol supports the concept of areas. Within an area, all routers know how to reach all the system IDs. Between areas, routers know how to reach the proper area. IS-IS supports two levels of routing: station routing (within an area) and area routing (between areas).

The key difference between the ISO IGRP and IS-IS NSAP addressing schemes is in the definition of area addresses. Both use the system ID for Level 1 routing (routing within an area). However, they differ in the way addresses are specified for area routing. An ISO IGRP NSAP address includes three separate fields for routing: the domain, area, and system ID. An IS-IS address includes two fields: a single continuous area field (comprising the domain and area fields) and the system ID.

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Note For more detailed information about ISO CLNS, see the Cisco IOS Apollo Domain, Banyan VINES, DECnet, ISO CLNS and XNS Configuration Guide, Release 12.2. For complete syntax and usage information for the commands used in this chapter, see the Cisco IOS Apollo Domain, Banyan VINES, DECnet, ISO CLNS and XNS Command Reference, Release 12.2, use the IOS command reference master index, or search online.

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Configuring IS-IS Dynamic Routing

IS-IS is an ISO dynamic routing protocol (described in ISO 105890). Unlike other routing protocols, enabling IS-IS requires that you create an IS-IS routing process and assign it to a specific interface, rather than to a network. You can specify more than one IS-IS routing process per Layer 3 switch or router by using the multiarea IS-IS configuration syntax. You then configure the parameters for each instance of the IS-IS routing process.

Small IS-IS networks are built as a single area that includes all the routers in the network. As the network grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2 routers from all areas, which is in turn connected to local areas. Within a local area, routers know how to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone routers know how to reach other areas.

Routers establish Level 1 adjacencies to perform routing within a local area (station routing). Routers establish Level 2 adjacencies to perform routing between Level 1 areas (area routing).

A single Cisco router can participate in routing in up to 29 areas and can perform Level 2 routing in the backbone. In general, each routing process corresponds to an area. By default, the first instance of the routing process configured performs both Level 1and Level 2 routing. You can configure additional router instances, which are automatically treated as Level 1 areas. You must configure the parameters for each instance of the IS-IS routing process individually.

For IS-IS multiarea routing, you can configure only one process to perform Level 2 routing, although you can define up to 29 Level 1 areas for each Cisco unit. If Level 2 routing is configured on any process, all additional processes are automatically configured as Level 1. You can configure this process to perform Level 1 routing at the same time. If Level 2 routing is not desired for a router instance, remove the Level 2 capability using the is-type global configuration command. Use the is-type command also to configure a different router instance as a Level 2 router.

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Note For more detailed information about IS-IS, see the "IP Routing Protocols" chapter of the Cisco IOS IP Configuration Guide, Release 12.2. For complete syntax and usage information for the commands used in this section, see the Cisco IOS IP Command Reference, Release 12.2.

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This section briefly describes how to configure IS-IS routing. It includes this information:

•Default IS-IS Configuration

•Enabling IS-IS Routing

•Configuring IS-IS Global Parameters

•Configuring IS-IS Interface Parameters

Default IS-IS Configuration

Table 34-12 shows the default IS-IS configuration.

Table 34-12 Default IS-IS Configuration 

Feature	Default Setting
Ignore link-state PDU (LSP) errors	Enabled.
IS-IS type	Conventional IS-IS: the router acts as both a Level 1 (station) and a Level 2 (area) router. Multiarea IS-IS: the first instance of the IS-IS routing process is a Level 1-2 router. Remaining instances are Level 1 routers.
Default-information originate	Disabled.
Log IS-IS adjacency state changes.	Disabled.
LSP generation throttling timers	Maximum interval between two consecutive occurrences: 5 seconds. Initial LSP generation delay: 50 ms. Hold time between the first and second LSP generation: 5000 ms.
LSP maximum lifetime (without a refresh)	1200 seconds (20 minutes) before t.he LSP packet is deleted.
LSP refresh interval	Send LSP refreshes every 900 seconds (15 minutes).
Maximum LSP packet size	1497 bytes.
NSF Awareness1	Enabled2 . Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes.
Partial route computation (PRC) throttling timers	Maximum PRC wait interval: 5 seconds. Initial PRC calculation delay after a topology change: 2000 ms. Hold time between the first and second PRC calculation: 5000 ms.
Partition avoidance	Disabled.
Password	No area or domain password is defined, and authentication is disabled.
Set-overload-bit	Disabled. When enabled, if no arguments are entered, the overload bit is set immediately and remains set until you enter the no set-overload-bit command.
Shortest path first (SPF) throttling timers	Maximum interval between consecutive SFPs: 10 seconds. Initial SFP calculation after a topology change: 5500 ms. Holdtime between the first and second SFP calculation: 5500 ms.
Summary-address	Disabled.

1 NSF = Nonstop Forwarding 2 IS-IS NSF awareness is enabled for IPv4 on switches running Cisco IOS Release 12.2(25)SEG or later.

Nonstop Forwarding Awareness

The integrated IS-IS NSF Awareness feature is supported for IPv4, beginning with Cisco IOS Release 12.2(25)SEG. The feature allows customer premises equipment (CPE) routers that are NSF-aware to help NSF-capable routers perform nonstop forwarding of packets. The local router is not necessarily performing NSF, but its awareness of NSF allows the integrity and accuracy of the routing database and link-state datbase on the neighboring NSF-capable router to be maintained during the switchover process.

This feature is automatically enabled and requires no configuration. For more information on this feature, see the Integrated IS-IS Nonstop Forwarding (NSF) Awareness Feature Guide at this URL: http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_white_paper09186a00801541c7.shtml

Enabling IS-IS Routing

To enable IS-IS, you specify a name and NET for each routing process. You then enable IS-IS routing on the interface and specify the area for each instance of the routing process.

Beginning in privileged EXEC mode, follow these steps to enable IS-IS and specify the area for each instance of the IS-IS routing process:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	clns routing	Enable ISO connectionless routing on the switch.
Step 3	router isis [area tag]	Enable the IS-IS routing for the specified routing process and enter IS-IS routing configuration mode. (Optional) Use the area tag argument to identify the area to which the IS-IS router is assigned. You must enter a value if you are configuring multiple IS-IS areas. The first IS-IS instance configured is Level 1-2 by default. Later instances are automatically Level 1. You can change the level of routing by using the is-type global configuration command.
Step 4	net network-entity-title	Configure the NETs for the routing process. If you are configuring multiarea IS-IS, specify a NET for each routing process. You can specify a name for a NET and for an address.
Step 5	is-type {level-1 | level-1-2 | level-2-only}	(Optional) You can configure the router to act as a Level 1 (station) router, a Level 2 (area) router for multi-area routing, or both (the default): •level-1—act as a station router only •level-1-2—act as both a station router and an area router •level 2—act as an area router only
Step 6	exit	Return to global configuration mode.
Step 7	interface interface-id	Specify an interface to route IS-IS, and enter interface configuration mode. If the interface is not already configured as a Layer 3 interface, enter the no switchport command to put it into Layer 3 mode.
Step 8	ip router isis [area tag]	Configure an IS-IS routing process for ISO CLNS on the interface and attach an area designator to the routing process.
Step 9	clns router isis [area tag]	Enable ISO CLNS on the interface.
Step 10	ip address ip-address-mask	Define the IP address for the interface. An IP address is required on all interfaces in an area enabled for IS-IS if any one interface is configured for IS-IS routing.
Step 11	end	Return to privileged EXEC mode.
Step 12	show isis [area tag] database detail	Verify your entries.
Step 13	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable IS-IS routing, use the no router isis area-tag router configuration command.

This example shows how to configure three routers to run conventional IS-IS as an IP routing protocol. In conventional IS-IS, all routers act as Level 1 and Level 2 routers (by default).

Router A

Switch(config)# clns routing

Switch(config)# router isis

Switch(config-router)# net 49.0001.0000.0000.000a.00

Switch(config-router)# exit

Switch(config)# interface gigabitethernet1/0/1

Switch(config-if)# ip router isis

Switch(config-if)# clns router isis

Switch(config)# interface gigabitethernet1/0/2

Switch(config-if)# ip router isis

Switch(config-if)# clns router isis

Switch(config-router)# exit

Router B

Switch(config)# clns routing

Switch(config)# router isis

Switch(config-router)# net 49.0001.0000.0000.000b.00

Switch(config-router)# exit

Switch(config)# interface gigabitethernet1/0/1

Switch(config-if)# ip router isis

Switch(config-if)# clns router isis

Switch(config)# interface gigabitethernet1/0/2

Switch(config-if)# ip router isis

Switch(config-if)# clns router isis

Switch(config-router)# exit

Router C

Switch(config)# clns routing

Switch(config)# router isis

Switch(config-router)# net 49.0001.0000.0000.000c.00

Switch(config-router)# exit

Switch(config)# interface gigabitethernet1/0/1

Switch(config-if)# ip router isis

Switch(config-if)# clns router isis

Switch(config)# interface gigabitethernet1/0/2

Switch(config-if)# ip router isis

Switch(config-if)# clns router isis

Switch(config-router)# exit

Configuring IS-IS Global Parameters

These are some optional IS-IS global parameters that you can configure:

•You can force a default route into an IS-IS routing domain by configuring a default route controlled by a route map. You can also specify other filtering options configurable under a route map.

•You can configure the router to ignore IS-IS LSPs that are received with internal checksum errors or to purge corrupted LSPs, which causes the initiator of the LSP to regenerate it.

•You can assign passwords to areas and domains.

•You can create aggregate addresses that are represented in the routing table by a summary address (route-summarization). Routes learned from other routing protocols can also be summarized. The metric used to advertise the summary is the smallest metric of all the specific routes.

•You can set an overload bit.

•You can configure the LSP refresh interval and the maximum time that an LSP can remain in the router database without a refresh

•You can set the throttling timers for LSP generation, shortest path first computation, and partial route computation.

•You can configure the switch to generate a log message when an IS-IS adjacency changes state (up or down).

•If a link in the network has a maximum transmission unit (MTU) size of less than 1500 bytes, you can lower the LSP MTU so that routing will still occur.

•The partition avoidance router configuration command prevents an area from becoming partitioned when full connectivity is lost among a Level1-2 border router, adjacent Level 1 routers, and end hosts.

Beginning in privileged EXEC mode, follow these steps to configure IS-IS parameters:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	clns routing	Enable ISO connectionless routing on the switch.
Step 3	router isis	Specify the IS-IS routing protocol and enter router configuration mode.
Step 4	default-information originate [route-map map-name]	(Optional) Force a default route into the IS-IS routing domain.If you enter route-map map-name, the routing process generates the default route if the route map is satisfied.
Step 5	ignore-lsp-errors	(Optional) Configure the router to ignore LSPs with internal checksum errors, instead of purging the LSPs. This command is enabled by default (corrupted LSPs are dropped). To purge the corrupted LSPs, enter the no ignore-lsp-errors router configuration command.
Step 6	area-password password	(Optional Configure the area authentication password, which is inserted in Level 1 (station router level) LSPs.
Step 7	domain-password password	(Optional) Configure the routing domain authentication password, which is inserted in Level 2 (area router level) LSPs.
Step 8	summary-address address mask [level-1 | level-1-2 | level-2]	(Optional) Create a summary of addresses for a given level.
Step 9	set-overload-bit [on-startup {seconds | wait-for-bgp}]	(Optional) Set an overload bit (a hippity bit) to allow other routers to ignore the router in their shortest path first (SPF) calculations if the router is having problems. •(Optional) on-startup—sets the overload bit only on startup. If on-startup is not specified, the overload bit is set immediately and remains set until you enter the no set-overload-bit command. If on-startup is specified, you must enter a number of seconds or wait-for-bgp. •seconds—When the on-startup keyword is configured, causes the overload bit to be set upon system startup and remain set for this number of seconds. The range is from 5 to 86400 seconds. •wait-for-bgp—When the on-startup keyword is configured, causes the overload bit to be set upon system startup and remain set until BGP has converged. If BGP does not signal IS-IS that it is converged, IS-IS will turn off the overload bit after 10 minutes.
Step 10	lsp-refresh-interval seconds	(Optional) Set an LSP refresh interval in seconds. The range is from 1 to 65535 seconds. The default is to send LSP refreshes every 900 seconds (15 minutes).
Step 11	max-lsp-lifetime seconds	(Optional) Set the maximum time that LSP packets remain in the router database without being refreshed. The range is from 1 to 65535 seconds. The default is 1200 seconds (20 minutes). After the specified time interval, the LSP packet is deleted.
Step 12	lsp-gen-interval [level-1 | level-2] lsp-max-wait [lsp-initial-wait lsp-second-wait]	(Optional) Set the IS-IS LSP generation throttling timers: •lsp-max-wait—the maximum interval (in seconds) between two consecutive occurrences of an LSP being generated. The range is 1 to 120, the default is 5. •lsp-initial-wait—the initial LSP generation delay (in milliseconds). The range is 1 to 10000; the default is 50. •lsp-second-wait—the hold time between the first and second LSP generation (in milliseconds). The range is 1 to 10000; the default is 5000.
Step 13	spf-interval [level-1 | level-2] spf-max-wait [spf-initial-wait spf-second-wait]	(Optional) Sets IS-IS shortest path first (SPF) throttling timers. •spf-max-wait—the maximum interval between consecutive SFPs (in seconds). The range is 1 to 120, the default is 10. •spf-initial-wait—the initial SFP calculation after a topology change (in milliseconds). The range is 1 to 10000; the default is 5500. •spf-second-wait—the holdtime between the first and second SFP calculation (in milliseconds). The range is 1 to 10000; the default is 5500.
Step 14	prc-interval prc-max-wait [prc-initial-wait prc-second-wait]	(Optional) Sets IS-IS partial route computation (PRC) throttling timers. •prc-max-wait—the maximum interval (in seconds) between two consecutive PRC calculations. The range is 1 to 120; the default is 5. •prc-initial-wait—the initial PRC calculation delay (in milliseconds) after a topology change. The range is 1 to 10,000; the default is 2000. •prc-second-wait—the hold time between the first and second PRC calculation (in milliseconds). The range is 1 to 10,000; the default is 5000.
Step 15	log-adjacency-changes [all]	(Optional) Set the router to log IS-IS adjacency state changes. Enter all to include all changes generated by events that are not related to the Intermediate System-to-Intermediate System Hellos, including End System-to-Intermediate System PDUs and link state packets (LSPs).
Step 16	lsp-mtu size	(Optional) Specify the maximum LSP packet size in bytes. The range is 128 to 4352; the default is 1497 bytes. Note If any link in the network has a reduced MTU size, you must change the LSP MTU size on all routers in the network.
Step 17	partition avoidance	(Optional) Causes an IS-IS Level 1-2 border router to stop advertising the Level 1 area prefix into the Level 2 backbone when full connectivity is lost among the border router, all adjacent level 1 routers, and end hosts.
Step 18	end	Return to privileged EXEC mode.
Step 19	show clns	Verify your entries.
Step 20	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable default route generation, use the no default-information originate router configuration command. Use the no area-password or no domain-password router configuration command to disable passwords. To disable LSP MTU settings, use the no lsp mtu router configuration command. To return to the default conditions for summary addressing, LSP refresh interval, LSP lifetime, LSP timers, SFP timers, and PRC timers, use the no form of the commands. Use the no partition avoidance router configuration command to disable the output format.

Configuring IS-IS Interface Parameters

You can optionally configure certain interface-specific IS-IS parameters, independently from other attached routers. However, if you change some values from the defaults, such as multipliers and time intervals, it makes sense to also change them on multiple routers and interfaces. Most of the interface parameters can be configured for level 1, level 2, or both.

These are some interface level parameters you can configure:

•The default metric on the interface, which is used as a value for the IS-IS metric and assigned when there is no quality of service (QoS) routing performed.

•The hello interval (length of time between hello packets sent on the interface) or the default hello packet multiplier used on the interface to determine the hold time sent in IS-IS hello packets. The hold time determines how long a neighbor waits for another hello packet before declaring the neighbor down. This determines how quickly a failed link or neighbor is detected so that routes can be recalculated. Change the hello-multiplier in circumstances where hello packets are lost frequently and IS-IS adjacencies are failing unnecessarily. You can raise the hello multiplier and lower the hello interval correspondingly to make the hello protocol more reliable without increasing the time required to detect a link failure.

•Other time intervals:

–Complete sequence number PDU (CSNP) interval. CSNPs are sent by the designated router to maintain database synchronization

–Retransmission interval. This is the time between retransmission of IS-IS LSPs for point-to-point links.

–IS-IS LSP retransmission throttle interval. This is the maximum rate (number of milliseconds between packets) at which IS-IS LSPs are re-sent on point-to-point links This interval is different from the retransmission interval, which is the time between successive retransmissions of the same LSP

•Designated router election priority, which allows you to reduce the number of adjacencies required on a multiaccess network, which in turn reduces the amount of routing protocol traffic and the size of the topology database.

•The interface circuit type, which is the type of adjacency desired for neighbors on the specified interface

•Password authentication for the interface

Beginning in privileged EXEC mode, follow these steps to configure IS-IS interface parameters:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interface interface-id	Specify the interface to be configured and enter interface configuration mode. If the interface is not already configured as a Layer 3 interface, enter the no switchport command to put it into Layer 3 mode.
Step 3	isis metric default-metric [level-1 | level-2]	(Optional) Configure the metric (or cost) for the specified interface. The range is from 0 to 63. The default is 10. If no level is entered, the default is to apply to both Level 1 and Level 2 routers.
Step 4	isis hello-interval {seconds | minimal} [level-1 | level-2]	(Optional) Specify the length of time between hello packets sent by the switch. By default, a value three times the hello interval seconds is advertised as the holdtime in the hello packets sent. With smaller hello intervals, topological changes are detected faster, but there is more routing traffic. •minimal—causes the system to compute the hello interval based on the hello multiplier so that the resulting hold time is 1 second. •seconds—the range is from 1 to 65535. The default is 10 seconds.
Step 5	isis hello-multiplier multiplier [level-1 | level-2]	(Optional) Specify the number of IS-IS hello packets a neighbor must miss before the router should declare the adjacency as down. The range is from 3 to 1000. The default is 3. Using a smaller hello-multiplier causes fast convergence, but can result in more routing instability.
Step 6	isis csnp-interval seconds [level-1 | level-2]	(Optional) Configure the IS-IS complete sequence number PDU (CSNP) interval for the interface. The range is from 0 to 65535. The default is 10 seconds.
Step 7	isis retransmit-interval seconds	(Optional) Configure the number of seconds between retransmission of IS-IS LSPs for point-to-point links. The value you specify should be an integer greater than the expected round-trip delay between any two routers on the network. The range is from 0 to 65535. The default is 5 seconds.
Step 8	isis retransmit-throttle-interval milliseconds	(Optional) Configure the IS-IS LSP retransmission throttle interval, which is the maximum rate (number of milliseconds between packets) at which IS-IS LSPs will be re-sent on point-to-point links. The range is from 0 to 65535. The default is determined by the isis lsp-interval command.
Step 9	isis priority value [level-1 | level-2]	(Optional) Configure the priority to use for designated router election. The range is from 0 to 127. The default is 64.
Step 10	isis circuit-type {level-1 | level-1-2 | level-2-only}	(Optional) Configure the type of adjacency desired for neighbors on the specified interface (specify the interface circuit type). •level-1—a Level 1 adjacency is established if there is at least one area address common to both this node and its neighbors. •level-1-2—a Level 1 and 2 adjacency is established if the neighbor is also configured as both Level 1 and Level 2 and there is at least one area in common. If there is no area in common, a Level 2 adjacency is established. This is the default. •level 2—a Level 2 adjacency is established. If the neighbor router is a Level 1 router, no adjacency is established.
Step 11	isis password password [level-1 | level-2]	(Optional) Configure the authentication password for an interface. By default, authentication is disabled. Specifying Level 1 or Level 2 enables the password only for Level 1 or Level 2 routing, respectively. If you do not specify a level, the default is Level 1 and Level 2.
Step 12	end	Return to privileged EXEC mode.
Step 13	show clns interface interface-id	Verify your entries.
Step 14	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return to the default settings, use the no forms of the commands.

Monitoring and Maintaining ISO IGRP and IS-IS

You can remove all contents of a CLNS cache or remove information for a particular neighbor or route. You can display specific CLNS or IS-IS statistics, such as the contents of routing tables, caches, and databases. You can also display information about specific interfaces, filters, or neighbors.

Table 34-13 lists the privileged EXEC commands for clearing and displaying ISO CLNS and IS-IS routing. For explanations of the display fields, see the Cisco IOS Apollo Domain, Banyan VINES, DECnet, ISO CLNS and XNS Command Reference, Release 12.2, use the Cisco IOS command reference master index, or search online.

Table 34-13 ISO CLNS and IS-IS Clear and Show Commands

Command	Purpose
clear clns cache	Clear and reinitialize the CLNS routing cache.
clear clns es-neighbors	Remove end system (ES) neighbor information from the adjacency database.
clear clns is-neighbors	Remove intermediate system (IS) neighbor information from the adjacency database.
clear clns neighbors	Remove CLNS neighbor information from the adjacency database.
clear clns route	Remove dynamically derived CLNS routing information.
show clns	Display information about the CLNS network.
show clns cache	Display the entries in the CLNS routing cache.
show clns es-neighbors	Display ES neighbor entries, including the associated areas.
show clns filter-expr	Display filter expressions.
show clns filter-set	Display filter sets.
show clns interface [interface-id]	Display the CLNS-specific or ES-IS information about each interface.
show clns neighbor	Display information about IS-IS neighbors.
show clns protocol	List the protocol-specific information for each IS-IS or ISO IGRP routing process in this router.
show clns route	Display all the destinations to which this router knows how to route CLNS packets.
show clns traffic	Display information about the CLNS packets this router has seen.
show ip route isis	Display the current state of the ISIS IP routing table.
show isis database	Display the IS-IS link-state database.
show isis routes	Display the IS-IS Level 1 routing table.
show isis spf-log	Display a history of the shortest path first (SPF) calculations for IS-IS.
show isis topology	Display a list of all connected routers in all areas.
show route-map	Display all route maps configured or only the one specified.
trace clns destination	Discover the paths taken to a specified destination by packets in the network.
which-route {nsap-address | clns-name}	Display the routing table in which the specified CLNS destination is found.

Configuring Multi-VRF CE

Virtual Private Networks (VPNs) provide a secure way for customers to share bandwidth over a service-provider backbone network. A VPN is a collection of sites sharing a common routing table. A customer site is connected to the service-provider network by one or more interfaces, and the service provider associates each interface with a VPN routing table, called a VPN routing/forwarding (VRF) table. In a multiprotocol label switching (MPLS) VPN deployment, all customers have their own customer-edge (CE) device), and every CE has a dedicated line connected to a provider-edge (PE) router.

With multiple VPN routing/forwarding (multi-VRF), multiple customers can share one CE, and only one physical link is needed between the CE and the PE. This is accomplished by extending some PE functionality to the shared CE devices. The shared CE maintains separate VRF tables for each customer and switches or routes packets for each customer based on the CE routing table. Multi-VRF CE allows a service provider to support two or more VPNs with overlapping IP addresses.

This section includes these topics:

•Understanding Multi-VRF CE

•Default Multi-VRF CE Configuration

•Multi-VRF CE Configuration Guidelines

•Configuring VRFs

•Configuring a VPN Routing Session

•Configuring BGP PE to CE Routing Sessions

•Multi-VRF CE Configuration Example

•Displaying Multi-VRF CE Status

Understanding Multi-VRF CE

Multi-VRF CE is a feature that allows a service provider to support two or more VPNs, where IP addresses can be overlapped among the VPNs. Multi-VRF CE uses input interfaces to distinguish routes for different VPNs and forms virtual packet-forwarding tables by associating one or more Layer 3 interfaces with each VRF. Interfaces in a VRF can be either physical, such as Ethernet ports, or logical, such as VLAN SVIs, but an interface cannot belong to more than one VRF at any time.

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Note Multi-VRF CE interfaces must be Layer 3 interfaces.

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Multi-VRF CE includes these devices:

•CE devices provide customers access to the service-provider network over a data link to one or more PE routers. The CE device advertises the site's local routes to the router and learns the remote VPN routes from it. A Catalyst 3750 Metro switch can be a CE.

•PE routers exchange routing information with CE devices by using static routing or a routing protocol such as BGP, RIPv2, OSPF, IS-IS, or EIGRP. The PE is only required to maintain VPN routes for those VPNs to which it is directly attached, eliminating the need for the PE to maintain all of the service-provider VPN routes. Each PE router maintains a VRF for each of its directly connected sites. Multiple interfaces on a PE router can be associated with a single VRF if all of these sites participate in the same VPN. Each VPN is mapped to a specified VRF. After learning local VPN routes from CEs, a PE router exchanges VPN routing information with other PE routers by using internal BGP (IBPG). A Catalyst 3750 Metro switch would typically be used as a PE.

•Provider routers or core routers are any routers in the service-provider network that do not attach to CE devices.

With multi-VRF CE, multiple customers can share one CE, and only one physical link is used between the CE and the PE. The shared CE maintains separate VRF tables for each customer and switches or routes packets for each customer based on its own routing table. Multi-VRF CE extends limited PE functionality to a CE device, giving it the ability to maintain separate VRF tables to extend the privacy and security of a VPN to the branch office.

Figure 34-5 shows a configuration using Catalyst 3750 Metro switches as multiple virtual CEs. This scenario is suited for customers who have low bandwidth requirements for their VPN service, for example, small companies. In this case, multi-VRF CE support is required in the Catalyst 3750 Metro switches. Because multi-VRF CE is a Layer 3 feature, each interface in a VRF must be a Layer 3 interface.

Figure 34-5 Catalyst 3750 Metro Switches Acting as Multiple Virtual CEs

When the CE switch receives a command to add a Layer 3 interface to a VRF, it sets up the appropriate mapping between the VLAN ID and the policy label in multi-VRF-CE-related data structures and adds the VLAN ID and policy label to the VLAN database.

When the multi-VRF CE is configured, the Layer 3 forwarding table is conceptually partitioned into two sections:

•The multi-VRF CE routing section contains the routes from different VPNs.

•The global routing section contains routes to non-VPN networks, such as the Internet.

VLAN IDs from different VRFs are mapped into different policy labels, which are used to distinguish the VRFs during processing. If no route is found in the multi-VRF CE section of the Layer 3 forwarding table, the global routing section is used to determine the forwarding path. For each new VPN route learned, the Layer 3 setup function retrieves the policy label by using the VLAN ID of the ingress port and inserts the policy label and new route to the multi-VRF CE routing section. If the packet is received from a routed port, the port internal VLAN ID number is used; if the packet is received from an SVI, the VLAN number is used.

This is the packet-forwarding process in a multi-VRF-CE-enabled network:

•When the switch receives a packet from a VPN, the switch looks up the routing table, based on the input policy label number. When a route is found, the switch forwards the packet to the PE.

•When the ingress PE receives a packet from the CE, it performs a VRF lookup. When a route is found, the router adds a corresponding MPLS label to the packet and sends it to the MPLS network.

•When an egress PE receives a packet from the network, it removes the label and uses the label to identify the correct VPN routing table. Then it performs the normal route lookup. When a route is found, it forwards the packet to the correct adjacency.

•When a CE receives a packet from an egress PE, it uses the input policy label to look up the correct VPN routing table. When a route is found, it forwards the packet within the VPN.

To configure VRF, you create a VRF table and specify the Layer 3 interface associated with the VRF. Then configure the routing protocols in the VPN and between the CE and the PE. BGP is the preferred routing protocol used to distribute VPN routing information across the provider's backbone.

The multi-VRF CE network has three major components:

•VPN route target communities—lists of all other members of a VPN community. You need to configure VPN route targets for each VPN community member.

•Multiprotocol BGP peering of VPN community PE routers—propagates VRF reachability information to all members of a VPN community. You need to configure BGP peering in all PE routers within a VPN community.

•VPN forwarding—transports all traffic between all VPN community members across a VPN service-provider network.

Default Multi-VRF CE Configuration

Table 34-14 shows the default VRF configuration.

Table 34-14 Default VRF Configuration 

Feature	Default Setting
VRF	Disabled. No VRFs are defined.
Maps	No import maps, export maps, or route maps are defined.
VRF maximum routes	8000 (total number of routes supported in hardware).
Forwarding table	The default for an interface is the global routing table.

Multi-VRF CE Configuration Guidelines

These are considerations when configuring VRF in your network:

•A switch with multi-VRF CE is shared by multiple customers, and each customer has its own routing table.

•Because customers use different VRF tables, the same IP addresses can be reused. Overlapped IP addresses are allowed in different VPNs.

•Multi-VRF CE lets multiple customers share the same physical link between the PE and the CE. Trunk ports with multiple VLANs separate packets among customers. Each customer has its own VLAN.

•Multi-VRF CE does not support all MPLS-VRF functionality. It does not support label exchange, LDP adjacency, or labeled packets.

•