- Preface
- Software Licensing
- The Cisco IOS command-line interface (CLI)
- Configuring Interfaces
- Switch Alarms
- Initial Switch Configuration (IP address assignments and DHCP autoconfiguration)
- How to Setup and Use the Cisco Configuration Engine
- How to Create and Manage Switch Clusters
- Performing Switch Administration
- Configuring Precision Time Protocol (PTP)
- Configuring PROFINET
- Common Industrial Protocol (CIP)
- Configuring SDM Templates
- Configuring Switch-Based Authentication
- Configuring IEEE 802.1x Port-Based Authentication
- MACsec
- Web-Based Authentication
- Configuring Smartports Macros
- Configuring SGACL Monitor Mode and SGACL Logging
- Configuring SGT Exchange Protocol over TCP (SXP) and Layer 3 Transport
- Configuring VLANs
- VLAN Trunking Protocol (VTP)
- Configuring Voice VLAN
- How to Configure Spanning Tree Protocol (STP)
- Configuring MSTP
- Configuring Optional Spanning-Tree Features
- Configuring Resilient Ethernet Protocol
- Configuring the FlexLinks and the MAC Address-Table Move Update
- Configuring DHCP
- Dynamic Address Resolution Protocol (ARP)
- Configuring IP Source Guard
- How to Configure Internet Group Management Protocol (IGMP) and Multicast VLAN Registration (MVR)
- Configuring Port-Based Traffic Control
- Configuring LLDP, LLDP-MED, and Wired Location Service
- Configuring SPAN and RSPAN
- One-to-one (1:1) Layer 2 Network Address Translation (NAT)
- How to Configure CDP
- Configuring UniDirectional Link Detection (UDLD)
- Configuring RMON
- Configuring System Message Logging
- Configuring Simple Network Management Protocol (SNMP)
- Network Security with ACLs
- Configuring Quality of Service (QoS)
- Configuring Static IP Unicast Routing
- Configuring IPv6 Host Functions
- Configuring Link State Tracking
- Configuring IP multicast routing
- Configuring Multicast Source Discovery Protocol (MSDP)
- Configuring Multicast Listener Discovery (MLD) snooping
- Configuring HSRP and VRRP
- Configuring IPv6 access control lists (ACLs)
- Configuring Embedded Event Manager (EEM)
- IP Unicast Routing
- IPv6 Unicast Routing
- Unicast Routing Overview
- Configuring Cisco IOS IP SLAs Operations
- Configuring Dying-Gasp
- How to Configure Enhanced Object Tracking
- Configuring MODBUS TCP
- Configuring Ethernet CFM
- Working with the Flash File System
- How to Configure EtherChannels
- Troubleshooting
- How to use a Secure Digital (SD) flash memory module (SD card)
- Information About IP Routing
- Prerequisites
- Guidelines and Limitations
- Configuring IP Addressing
- Enabling IPv4 Unicast Routing
- Configuring RIP
- Configuring OSPF
- Configuring EIGRP
- Configuring BGP
- 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 BGP Filtering By Access Lists
- 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 BFD
- 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
- Controlling Route Redistribution
- Configuring Policy-Based Routing
- Filtering Routing Information
- Managing Authentication Keys
Configuring IP Unicast Routing
This chapter describes how to configure IP Version 4 (IPv4) unicast routing on the Cisco Industrial Ethernet Switches, hereafter referred to as switch.
Note: Dynamic routing protocols are only supported on switches running IP Services feature set. Static routing is supported on Lan Base feature set.
For more detailed IPv4 unicast configuration information and complete syntax and usage information for the commands used in this chapter, see documents listed in the Related Documents.
This chapter includes the following sections:
■
Enabling IPv4 Unicast Routing
■Configuring Protocol-Independent Features
Information About IP Routing
In an IP network, each subnetwork is mapped to an individual VLAN. 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 99 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 99 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, finds 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.
Types of Routing
Routers and Layer 3 switches can route packets in the following ways:
■By using default routing—sending traffic with a destination unknown to the router to a default outlet or destination.
■By using preprogrammed static routes for the traffic
Static unicast routing forwards packets from predetermined ports through a single path into and out of a network. Static routing does not automatically respond to changes in the network and therefore, might result in unreachable destinations.
■By dynamically calculating routes by using a routing protocol
Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding traffic. Routing protocols supported by the switch are Routing Information Protocol (RIP), Border Gateway Protocol (BGP), Open Shortest Path First (OSPF) protocol, Enhanced IGRP (EIGRP), System-to-Intermediate System (IS-IS), and Bidirectional Forwarding Detection (BFD).
Prerequisites
■In order to use dynamic routing protocols, an IP Services License is needed.
■To support VLAN interfaces, create and configure VLANs on the switch, and assign VLAN membership to Layer 2 interfaces.
■By default, IPv4 routing is disabled on the switch, and you must enable it before routing can take place. See Enabling IPv4 Unicast Routing.
■We recommend that you configure the BFD interval parameters on an interface before configuring the routing protocol commands, especially when using EIGRP. For information about BFD, see Configuring BFD.
Guidelines and Limitations
■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.
■The switch does not support tunnel interfaces for unicast routed traffic.
■All Layer 3 interfaces on which routing will occur must have IP addresses assigned to them. See Assigning IP Addresses to Network Interfaces.
■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 support IPv4 routing, use the sdm prefer default global configuration command to set the Switch Database Management (sdm) feature to balance resources. For more information on the SDM templates, see the sdm prefer command in the command reference listed in the Related Documents.
Steps for Configuring Routing
Configuring IPv4 routing consists of several main procedures:
■Configure Layer 3 interfaces.
■Enable IPv4 routing on the switch.
■Assign IPv4 addresses to the Layer 3 interfaces.
Configuring IP Addressing
IP routing requires that Layer 3 network interfaces are assigned IP addresses to enable the interfaces and to allow communication with the hosts on 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
Assigning IP Addresses to Network Interfaces
An IP address identifies a location to which IP packets can be sent. 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.
BEFORE YOU BEGIN
To receive an assigned network number, contact your Internet service provider.
DETAILED STEPS
EXAMPLE
Enabling Subnet Zero
Enabling subnet zero provides the ability to configure and route to subnet 0 subnets.
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.
BEFORE YOU BEGIN
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.
DETAILED STEPS
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Enable the use of subnet zero for interface addresses and routing updates. |
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Use the no ip subnet-zero global configuration command to restore the default and disable the use of subnet zero.
EXAMPLE
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 100, 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 100 IP Classless Routing
In Figure 101, 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 101 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.
BEFORE YOU BEGIN
Review the Information About IP Routing.
DETAILED STEPS
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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.
EXAMPLE
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. To communicate with a device on Ethernet, the software must learn the MAC address of the device. The process of learning the MAC address from an IP address is called address resolution . The process of learning 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 learns 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 learn 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.
The switch also uses 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 IP Addressing: ARP Configuration Guide, Cisco IOS Release 15M&T.
You can perform these tasks to configure address resolution:
Defining 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.
BEFORE YOU BEGIN
Review the Configuring Address Resolution Methods.
DETAILED STEPS
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.
EXAMPLE
Setting 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.
BEFORE YOU BEGIN
The encapsulation type specified in this procedure should match the encapsulation type specified in the Defining a Static ARP Cache.
DETAILED STEPS
To disable an encapsulation type, use the no arp arpa or no arp snap interface configuration command.
EXAMPLE
Enabling Proxy ARP
By default, the switch uses proxy ARP to help hosts learn MAC addresses of hosts on other networks or subnets. Follow these steps to enable proxy ARP if it has been disabled.
BEFORE YOU BEGIN
Review the Configuring Address Resolution Methods.
DETAILED STEPS
To disable proxy ARP on the interface, use the no ip proxy-arp interface configuration command.
EXAMPLE
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
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 learn 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 Enabling Proxy ARP. 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.
DETAILED STEPS
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Display the address of the default gateway router to verify the setting. |
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Use the no ip default-gateway global configuration command to disable this function.
EXAMPLE
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) routing 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.
BEFORE YOU BEGIN
■The ip irdp multicast 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.
■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.
DETAILED STEPS
Use the no ip irdp interface configuration command to disable IRDP routing.
EXAMPLE
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.
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.
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. The switch supports several addressing schemes for forwarding broadcast messages.
■Enabling Directed Broadcast-to-Physical Broadcast Translation
■Forwarding UDP Broadcast Packets and Protocols
Enabling Directed Broadcast-to-Physical Broadcast Translation
By default, IP-directed broadcasts are not forwarded; they are dropped to make 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.
BEFORE YOU BEGIN
You can specify an access list to control which broadcasts are forwarded. Only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts.
DETAILED STEPS
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.
EXAMPLE
The following example enables forwarding of IP directed broadcasts on Ethernet interface 0. The ip forward-protocol command using the udp keyword without specifying any port numbers allows forwarding of UDP packets on the default ports.
Forwarding UDP Broadcast Packets and Protocols
User Datagram Protocol (UDP) is an IP host-to-host layer protocol that 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 find 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 configure 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.
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.
BEFORE YOU BEGIN
See the description for the ip forward-protocol interface configuration command in the Cisco IOS IP Application Services Command Reference for the list of ports that are forwarded by default if you do not specify any UDP ports.
DETAILED STEPS
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.
EXAMPLE
The following example defines a helper address and uses the ip forward-protocol command. Using the udp keyword without specifying any port numbers will allow forwarding of UDP packets on the default ports.
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.
DETAILED STEPS
To restore the default IP broadcast address, use the no ip broadcast-address interface configuration command.
EXAMPLE
The following example specifies an IP broadcast address of 0.0.0.0:
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, the interface can receive broadcasts but it never forwards the 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 so it 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.
BEFORE YOU BEGIN
Ensure that bridging is configured on each interface that is to participate in the flooding.
DETAILED STEPS
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Use the bridging spanning-tree database to flood UDP datagrams. |
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Use the no ip forward-protocol spanning-tree global configuration command to disable the flooding of IP broadcasts.
EXAMPLE
The following example permits IP broadcasts to be flooded through the internetwork in a controlled fashion:
Speeding up STP-Based UDP Flooding
In the 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.
BEFORE YOU BEGIN
Enable the flooding of IP broadcasts as described in the Flooding IP Broadcasts.
DETAILED STEPS
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Use the spanning-tree database to speed up flooding of UDP datagrams. |
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To disable this feature, use the no ip forward-protocol turbo-flood global configuration command.
EXAMPLE
The following example shows how to speed up the flooding of UDP packets using the spanning-tree algorithm:
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.
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Remove one or all entries from the hostname and the address cache. |
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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.
Enabling IPv4 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.
BEFORE YOU BEGIN
Review the Guidelines and Limitations.
DETAILED STEPS
Use the no ip routing global configuration command to disable routing.
EXAMPLE
This example shows how to enable IP routing using RIP as the routing protocol:
Configuring RIP
The Routing Information Protocol (RIP) is an interior gateway protocol (IGP) used in small, homogeneous networks. It is a distance-vector routing protocol that uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. 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, but is treated by RIP as a network to implement default routing. 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 includes the following topics:
■Configuring Basic RIP Parameters
■Configuring RIP Authentication
Default RIP Configuration
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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. RIP configuration commands are ignored until you configure the network number.
BEFORE YOU BEGIN
Complete the RIP network strategy and planning for your network. For example, you must decide whether to receive and send only RIP Version 1 or RIP Version 2 packets and whether to use RIP authentication. (RIP Version 1 does not support authentication.)
DETAILED STEPS
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.
EXAMPLE
In the following example, RIP updates are sent to all interfaces on network 10.108.0.0 except Ethernet interface 1. However, in this case, a neighbor router configuration command is included. This command permits the sending of routing updates to specific neighbors. One copy of the routing update is generated per neighbor.
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 specifies 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.
The switch supports two modes of authentication on interfaces for which RIP authentication is enabled: plain text and MD5. The default is plain text.
BEFORE YOU BEGIN
Configure RIP as described in the Configuring Basic RIP Parameters.
DETAILED STEPS
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.
EXAMPLE
The following example configures the interface to accept and send any key belonging to the key chain named trees and configures the interface to use MD5 authentication:
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 when links are broken.
BEFORE YOU BEGIN
In general, Cisco does not recommend disabling split horizon unless you are certain that your application requires disabling it to properly advertise routes.
DETAILED STEPS
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Enter interface configuration mode, and specify the interface to configure. |
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Enable the interface if necessary. By default, UNIs and ENIs are disabled and NNIs are enabled. |
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To enable the split horizon mechanism, use the ip split-horizon interface configuration command.
EXAMPLE
The following simple example disables split horizon on a serial link:
Configuring Summary Addresses
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.
Note: If split horizon is enabled, neither autosummary nor interface IP summary addresses are advertised.
BEFORE YOU BEGIN
If the interface is in Layer 2 mode (the default), you must enter a no switchport interface configuration command before entering the ip address interface configuration command.
DETAILED STEPS
To disable IP summarization, use the no ip summary-address rip router configuration command.
EXAMPLE
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 interface Gigabit Ethernet port 2, and 10.0.0.0 is not advertised.
Configuring OSPF
Open Shortest Path First (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.
This section briefly describes how to configure OSPF. For a complete description of the OSPF commands, see the OSPF documents listed in the Related Documents.
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. The switch supports all these network types.
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.
■Not-so-stubby-areas (NSSAs) per RFC 1587 are 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 includes the following topics:
■Configuring OSPF Network Types
■Configuring OSPF Area Parameters
■Configuring Other OSPF Parameters
■Configuring a Loopback Interface
Default OSPF Configuration
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Cost: No default cost predefined. Retransmit interval: 5 seconds. |
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Disabled. When enabled, the default metric setting is 10, and the external route type default is Type 2. |
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Built-in, automatic metric translation, as appropriate for each routing protocol. |
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dist1 (all routes within an area): 110. |
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Disabled. All outgoing link-state advertisements (LSAs) are flooded to the interface. |
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NSF1 awareness |
Enabled2. Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes. |
No area ID or router ID defined. Retransmit interval: 5 seconds. |
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2.OSPF NSF awareness is enabled for IPv4 on switches running the IP services image. |
Nonstop Forwarding Awareness
The OSPF NSF Awareness feature is supported for IPv4 in the IP services image. 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 about this feature, see the “Configuring Nonstop Forwarding” chapter in the High Availability Configuration Guide, Cisco IOS Release 15S.
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.
BEFORE YOU BEGIN
Complete the OSPF network strategy and planning for your network. For example, you must decide whether multiple areas are required.
DETAILED STEPS
To terminate an OSPF routing process, use the no router ospf process-id global configuration command.
EXAMPLE
This example shows how to configure an OSPF routing process and assign it a process number of 109:
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.
Note: The ip ospf interface configuration commands are all optional.
BEFORE YOU BEGIN
If you modify these parameters, be sure all routers in the network have compatible values.
DETAILED STEPS
Use the no form of these commands to remove the configured parameter value or return to the default value.
EXAMPLE
The following example specifies a cost of 65 and sets the interval between link-state advertisement (LSA) retransmissions to 1 second:
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).
BEFORE YOU BEGIN
Complete the OSPF network strategy and planning for your network.
DETAILED STEPS
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.
EXAMPLE
The following example declares a router at address 192.168.3.4 on a nonbroadcast network, with a priority of 1 and a poll interval of 180 seconds:
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.
BEFORE YOU BEGIN
Complete the OSPF network strategy and planning for your network.
DETAILED STEPS
Use the no form of the ip ospf network command to return to the default network type for the media.
EXAMPLE
The following example sets your OSPF network as a broadcast network:
The following example illustrates a point-to-multipoint network with broadcast:
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.
Note: The OSPF area router configuration commands are all optional.
BEFORE YOU BEGIN
Evaluate the following considerations before you implement this feature:
■You can set a Type 7 default route that can be used to reach external destinations. When configured, the router generates a Type 7 default into the NSSA or the NSSA ABR.
■Every router within the same area must agree that the area is NSSA; otherwise, the routers will not be able to communicate.
DETAILED STEPS
Use the no form of these commands to remove the configured parameter value or to return to the default value.
EXAMPLE
The following example mandates authentication for areas 0 and 10.0.0.0 of OSPF routing process 201. Authentication keys are also provided.
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, 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 specified 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.
BEFORE YOU BEGIN
Complete the OSPF network strategy and planning for your network.
DETAILED STEPS
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(Optional) Specify an address and IP subnet mask for redistributed routes so that only one summary route is advertised. |
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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 Configuring OSPF Interfaces for parameter definitions and the Default OSPF Configuration for virtual link defaults. |
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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. |
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(Optional) Configure DNS name lookup. The default is disabled. |
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(Optional) Specify an address range for which a single route will be advertised. Use this command only with area border routers. |
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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. |
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(Optional) Suppress the sending of hello packets through the specified interface. |
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(Optional) Configure route calculation timers. ■ ■ ■ |
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(Optional) Send syslog message when a neighbor state changes. |
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Display lists of information related to the OSPF database for a specific router. For some of the keyword options, see Monitoring OSPF. |
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EXAMPLE
In the following example, the summary address 10.1.0.0 includes address 10.1.1.0, 10.1.2.0, 10.1.3.0, and so on. Only the address 10.1.0.0 is advertised in an external link-state advertisement.
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.
BEFORE YOU BEGIN
Do not change the packet pacing timers unless all other options to meet OSPF packet flooding requirements have been exhausted. Specifically, network operators should prefer summarization, stub area usage, queue tuning, and buffer tuning before changing the default flooding timers. Furthermore, there are no guidelines for changing timer values; each OSPF deployment is unique and should be considered on a case-by-case basis. The network operator assumes the risks associated with changing the default timer values.
DETAILED STEPS
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To return to the default value, use the no timers pacing lsa-group router configuration command.
EXAMPLE
The following example configures OSPF group packet-pacing updates between LSA groups to occur in 60-second intervals for OSPF routing process 1:
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.
BEFORE YOU BEGIN
The IP address for the loopback interface must be unique and not in use by another interface.
DETAILED STEPS
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Create a loopback interface, and enter interface configuration mode. |
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Use the no interface loopback 0 global configuration c ommand to disable the loopback interface.
EXAMPLE
Monitoring OSPF
You can display specific statistics such as the contents of IP routing tables, caches, and databases.
Following are some of the privileged EXEC commands for displaying OSPF statistics. For more show ip ospf database privileged EXEC command options and for explanations of fields in the resulting display, see Cisco IOS IP Routing: OSPF Command Reference.
Configuring EIGRP
Enhanced IGRP (EIGRP) is a Cisco proprietary enhanced version of the Interior Gateway Routing Protocol (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.
EIGRP 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 by periodically sending small hello packets. As long as hello packets are received, the neighbor is alive and functioning. When this status is determined, the neighboring routers 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, 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. To ensure low convergence time, the reliable transport sends multicast packets quickly when there are unacknowledged packets pending.
■The DUAL finite state machine handles the decision process for all route computations. It tracks all routes advertised by all neighbors and 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 to determine a new successor. The amount of time it takes to recompute the route affects the convergence time. When a topology change occurs, DUAL tests for feasible successors 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. Routing decisions are stored in the IP routing table. EIGRP also redistributes routes learned by other IP routing protocols.
This section includes the following topics:
■Configuring Basic EIGRP Parameters
■Configuring EIGRP Route Authentication
■Configuring EIGRP Stub Routing
■Monitoring and Maintaining EIGRP
Default EIGRP Configuration
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Enabled. Subprefixes are summarized to the classful network boundary when crossing classful network boundaries. |
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Exterior routes are accepted and default information is passed between EIGRP processes when doing redistribution. |
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Only connected routes and interface static routes can be redistributed without a default metric. The metric includes: ■ ■ ■ ■ ■ |
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For low-speed nonbroadcast multiaccess (NBMA) networks: 60 seconds; all other networks: 5 seconds. |
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For low-speed NBMA networks: 180 seconds; all other networks: 15 seconds. |
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NSF3 Awareness |
Enabled4. Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes. |
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4.EIGRP NSF awareness is enabled for IPv4 on switches running the IP services image. |
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.
Nonstop Forwarding Awareness
The EIGRP NSF Awareness feature is supported for IPv4 in the IP services image. 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 “Configuring Nonstop Forwarding” chapter in the High Availability Configuration Guide, Cisco IOS Release 15S.
Configuring Basic EIGRP Parameters
In this procedure, configuring the routing process is required; other steps are optional.
BEFORE YOU BEGIN
Complete the EIGRP network strategy and planning for your network.
DETAILED STEPS
Use the no forms of these commands to disable the feature or return the setting to the default value.
EXAMPLE
The following example configures EIGRP autonomous system 1 and establishes neighbors through networks 172.16.0.0 and 192.168.0.0:
Configuring EIGRP Interfaces
Other optional EIGRP parameters can be configured on an interface basis.
BEFORE YOU BEGIN
Enable EIGRP as described in the Configuring Basic EIGRP Parameters.
DETAILED STEPS
Use the no forms of these commands to disable the feature or return the setting to the default value.
EXAMPLE
The following example allows EIGRP to use up to 75 percent (42 kbps) of a 56-kbps serial link in autonomous system 209:
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.
BEFORE YOU BEGIN
Enable EIGRP as described in the Configuring Basic EIGRP Parameters.
DETAILED STEPS
Use the no forms of these commands to disable the feature or to return the setting to the default value.
EXAMPLE
The following example configures EIGRP to apply authentication to address-family autonomous system 1 and identifies a key chain named SITE1:
Configuring EIGRP Stub Routing
The EIGRP stub routing feature reduces resource utilization by moving routed traffic closer to the end user. In a network using EIGRP stub routing, the only allowable route for IP traffic to the user is through a switch that is configured with EIGRP stub routing. The switch sends the routed traffic to interfaces that are configured as user interfaces or are connected to other devices.
When using EIGRP stub routing, you need to configure the distribution and remote routers to use EIGRP and to configure only the switch as a stub. Only specified routes are propagated from the switch. The switch responds to all queries for summaries, connected routes, and routing updates.
Note: EIGRP stub routing only advertises connected or summary routes from the routing tables to other switches in the network. The switch uses EIGRP stub routing at the access layer to eliminate the need for other types of routing advertisements. If you try to configure multi-VRF-CE and EIGRP stub routing at the same time, the configuration is not allowed.
Any neighbor that receives a packet informing it of the stub status does not query the stub router for any routes, and a router that has a stub peer does not query that peer. The stub router depends on the distribution router to send the proper updates to all peers.
In Figure 102, switch B is configured as an EIGRP stub router. Switches A and C are connected to the rest of the WAN. Switch B advertises connected, static, redistribution, and summary routes to switch A and C. Switch B does not advertise any routes learned from switch A (and the reverse).
Figure 102 EIGRP Stub Router Configuration
For more information about EIGRP stub routing, see IP Routing: EIGRP Configuration Guide, Cisco IOS Release 15M&T.
BEFORE YOU BEGIN
Complete the EIGRP network strategy and planning for your network.
DETAILED STEPS
Enter the show ip eigrp neighbor detail privileged EXEC command from the distribution router to verify the configuration.
EXAMPLE
In the following example, the eigrp stub command is used to configure the router as a stub that advertises connected and summary routes:
Monitoring and Maintaining EIGRP
You can delete neighbors from the neighbor table. You can also display various EIGRP routing statistics.
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show ip eigrp topology [ autonomous-system-number ] | [[ ip-address ] mask ]] |
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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 for loop-free exchanges 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.
For details about BGP configuration and commands, see the BGP documents listed in Related Documents.
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 103 shows a network that is running both EBGP and IBGP.
Figure 103 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 103, 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 Configuring BGP Decision Attributes 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.
This section includes the following topics:
■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
■Monitoring and Maintaining BGP
Default BGP Configuration
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100. The range is 0 to 4294967295 with the higher value preferred. |
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Disabled by default. When enabled: ■ |
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The IP address of a loopback interface if one is configured or the highest IP address configured for a physical interface on the router. |
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Default information originate (protocol or network redistribution) |
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■ ■ ■ |
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■ ■ |
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■ |
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■ ■ ■ ■ ■ |
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■ ■ ■ ■ ■ ■ ■ ■ ■ ■ |
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NSF5 Awareness |
Disabled6. Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes. |
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6.BGP NSF Awareness can be enabled for IPv4 on switches with the IP services image by enabling Graceful Restart. |
Nonstop Forwarding Awareness
The BGP NSF Awareness feature is supported for IPv4 in the IP services image. 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. For more information, see IP Routing: BGP Configuration Guide, Cisco IOS Release 15M&T.
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 must pass traffic through it from another AS to a third AS, it is important to be consistent about the routes it advertises. If BGP advertises a route before all routers in the network learn about the route through the IGP, the AS might receive traffic that some routers can 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.
BEFORE YOU BEGIN
You should know your network design and how you want traffic to flow through it before configuring BGP. Gather the network requirements you need, which should include the following:
■Whether you need to run IBGP for internal connectivity
■External connectivity to the service provider network
■Configuration parameters such as neighbor IP addresses and their AS number, and which networks you will advertise through BGP
DETAILED STEPS
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.
EXAMPLE
These examples show how to configure BGP on the routers in Figure 103.
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:
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 specify where to send updates.
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. The switch supports a soft reset without any prior configuration when both BGP peers 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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
EXAMPLE
In the following example, an outbound soft reset is initiated for sessions with all routers in the autonomous system numbered 35700:
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. The decision is based on the value of attributes that the update contains and other BGP-configurable factors. The selected path is entered into the BGP routing table and propagated to its neighbors.
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, 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. 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 these 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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
Use the no form of each command to return to the default state.
EXAMPLE
The following example forces all updates destined for 10.108.1.1 to advertise this router as the next hop:
In the following example, the local BGP routing process is configured to compare the MED from alternative paths, regardless of the autonomous system from which the paths are received:
Configuring BGP Filtering with Route Maps
Within BGP, you can use route maps to control and to modify routing information and to define the conditions by which routes are redistributed between routing domains. See Using Route Maps to Redistribute Routing Information for more information about route maps. Each route map has a name that identifies the route map ( map tag) and an optional sequence number.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
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.
EXAMPLE
In the following example, the inbound route map named rmap sets the next hop:
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 Controlling Advertising and Processing in Routing Updates 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-lis t 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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
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.
EXAMPLE
The following router configuration mode example applies list 39 to incoming advertisements from neighbor172.16.4.1. List 39 permits the advertisement of network 10.109.0.0.
Configuring BGP Filtering By Access Lists
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. (See Using Regular Expressions in BGP for more information on forming regular expressions.) To use this method, define an autonomous system path access list, and apply it to updates to and from particular neighbors.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
EXAMPLE
In the following example, an autonomous system path access list (number 500) is defined to configure the router to not advertise any path through or from autonomous system 65535 to the 10.20.2.2 neighbor:
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. 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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
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.
EXAMPLE
In the following example, a prefix list is configured to deny the default route 0.0.0.0/0:
In the following example, a prefix list is configured to permit traffic from the 172.16.1.0/24 subnet:
In the following example, a prefix list is configured to permit routes from the 10.0.0.0/8 network that have a mask length that is less than or equal to 24 bits:
In the following example, a prefix list is configured to deny routes from the 10.0.0.0/8 network that have a mask length that is greater than or equal to 25 bits:
In the following example, a prefix list is configured to permit routes from any network that have a mask length from 8 to 24 bits:
In the following example, a prefix list is configured to deny any route with any mask length from the 10.0.0.0/8 network:
Configuring BGP Community Filtering
One way that BGP controls the distribution of routing information based on the value of the COMMUNITIES attribute. 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.
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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
EXAMPLE
In the following example, a standard community list is configured that permits routes from network 10 in autonomous system 50000:
In the following router configuration mode example, the router belongs to autonomous system 109 and is configured to send the communities attribute to its neighbor at IP address 172.16.70.23:
In the following example, a router that uses the 32-bit number community format is upgraded to use the AA:NN format:
The following sample output shows how BGP community numbers are displayed when the ip bgp-community new-format command is enabled:
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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
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.
EXAMPLE
The following example configures a peer group and sets the minimum time between sending BGP routing updates to 10 seconds for the peer group:
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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
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.
EXAMPLE
In the following example, an aggregate BGP address is created in router configuration mode. The path advertised for this route will be an AS_SET consisting of all elements contained in all paths that are being summarized.
In the following example, a route map called MAP-ONE is created to match on an AS-path access list. The path advertised for this route will be an AS_SET consisting of elements contained in paths that are matched in the route map.
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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
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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. |
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EXAMPLE
In the following example, the routing domain is divided into autonomous systems 50001, 50002, 50003, 50004, 50005, and 50006 and is identified by the confederation identifier 50007. Neighbor 10.2.3.4 is a peer inside of the routing domain confederation. Neighbor 10.4.5.6 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 50007.
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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
EXAMPLE
In the following router configuration mode example, the local router is a route reflector. It passes learned IBGP routes to the neighbor at 172.16.70.24.
Configuring Route Dampening
Route flap dampening minimizes 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.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing.
DETAILED STEPS
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.
EXAMPLE
In the following example, BGP dampening is applied to prefixes filtered through the route-map named BLUE:
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 get 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.
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. 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.
For more detailed information about ISO CLNS, see the ISO CLNS documents listed in the Related Documents.
Configuring IS-IS Dynamic Routing
IS-IS is an ISO dynamic routing protocol. 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.
This section briefly describes how to configure IS-IS routing. For more detailed information about IS-IS, see the IS-IS documents listed in the Related Documents.
This section includes the following topics:
Default IS-IS Configuration
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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. |
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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. |
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NSF7 Awareness |
Enabled8. Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes. |
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. |
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No area or domain password is defined, and authentication is disabled. |
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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. |
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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. |
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8.IS-IS NSF awareness is enabled for IPv4 on switches running the IP services image. |
Nonstop Forwarding Awareness
The integrated IS-IS NSF Awareness feature is supported for IPv4 in the IP services image. 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 database 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 “Configuring Nonstop Forwarding” chapter in the High Availability Configuration Guide, Cisco IOS Release 15S.
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.
BEFORE YOU BEGIN
You should know your network design and how you want traffic to flow through it before configuring IS-IS. Define areas, prepare an addressing plan for the devices (including defining the NETs), and determine the interfaces that will run integrated IS-IS. To facilitate verification, a matrix of adjacencies should be prepared before you configure your devices, showing what neighbors should be expected in the adjacencies table.
DETAILED STEPS
To disable IS-IS routing, use the no router isis area-tag router configuration command.
EXAMPLE
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).
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 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.
BEFORE YOU BEGIN
Enable IS-IS routing as described in the Enabling IS-IS Routing.
DETAILED STEPS
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.
EXAMPLE
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.
–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.
BEFORE YOU BEGIN
Enable IS-IS routing as described in the Enabling IS-IS Routing.
DETAILED STEPS
To return to the default settings, use the no forms of the commands.
EXAMPLE
The following configuration example for an IS-IS routing process called area1 sets a global default metric of 111 for the IS-IS interfaces:
Monitoring and Maintaining 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.
Configuring BFD
The Bidirectional Forwarding Detection (BFD) Protocol quickly detects forwarding-path failures for a variety of media types, encapsulations, topologies, and routing protocols. It operates in a unicast, point-to-point mode on top of any data protocol being forwarded between two systems to track IPv4 connectivity between directly connected neighbors. BFD packets are encapsulated in UDP packets with a destination port number of 3784 or 3785.
In EIGRP, IS-IS, and OSPF deployments, the closest alternative to BFD is the use of modified failure-detection mechanisms. Although reducing the EIGRP, IS-IS, and OSPF timers can result in a failure-detection rate of 1 to 2 seconds, BFD can provide failure detection in less than 1 second. BFD can be less CPU-intensive than the reduced timers and, because it is not tied to any particular routing protocol, it can be used as a generic and consistent failure detection mechanism for multiple routing protocols.
To create a BFD session, you must configure BFD on both systems (BFD peers). Enabling BFD at the interface and routing protocol level on BFD peers creates a BFD session. BFD timers are negotiated and the BFD peers send control packets to each other at the negotiated intervals. If the neighbor is not directly connected, BFD neighbor registration is rejected.
Figure 104 shows a simple network with two routers running OSPF and BFD. When OSPF discovers a neighbor (1), it sends a request to the BFD process to initiate a BFD neighbor session with the neighbor OSPF router (2), establishing the BFD neighbor session (3).
Figure 104 Establishing a BFD Session
Figure 105 shows what happens when a failure occurs in the network (1). The BFD neighbor session with the OSPF neighbor closes (2). BFD notifies the OSPF process that the BFD neighbor is no longer reachable, and the OSPF process breaks the OSPF neighbor relationship (4). If an alternative path is available, the routers start converging on it.
Figure 105 Breaking an OSPF Neighbor Relationship
BFD clients are routing protocols that register neighbors with BFD. The switch supports IS-IS, OSPF v1 and v2, BGP, EIGRP, and HSRP clients. You can use one BFD session for multiple client protocols. For example, if a network is running OSPF and EIGRP across the same link to the same peer, you need to create only one BFD session, and information is shared with both routing protocols.
The switch supports BFD version 0 and version 1. BFD neighbors automatically negotiate the version and the protocol always runs at the higher version. The default version is version 1.
By default, BFD neighbors exchange both control packets and echo packets for detecting forwarding failures. The switch sends echo packets at the configured BFD interval rate (from 50 to 999 ms), and control packets at the BFD slow-timer rate (from 1000 to 3000 ms).
Failure-rate detection can be faster in BFD echo mode, which is enabled by default when you configure BFD session. In this mode, the switch sends echo packets from the BFD software layer, and the BFD neighbor responds to the echo packets through its fast-switching layer. The echo packets do not reach the BFD neighbor software layer, but are reflected back over the forwarding path for failure detection. You configure the rate at which each BFD interface sends BFD echo packets by entering the bfd interval interface configuration command.
To reduce bandwidth consumption, you can disable the sending of echo packets by entering the no bfd echo interface configuration command. When echo mode is disabled, control packets are used to detect forwarding failures. Control packets are exchanged at the configured slow-timer rate, which could result in longer failure-detection time. You configure this rate by entering the bfd slow-timer global configuration command. The range is from 1000 to 3000 ms; the default rate is every 1000 ms.
You can enable or disable echo processing at a switch interface independent of the BFD neighbor configuration. Disabling echo mode only disables the sending of echo packets by the interface. The fast-switching layer that receives an echo packet always reflects it back to the sender.
To run BFD on a switch, you need to configure basic BFD interval parameters on BFD interfaces, enable routing on the switch, and enable one or more one routing protocol clients for BFD. You also need to confirm that Cisco Express Forwarding (CEF) is enabled (the default) on participating switches.
For more information on the configuration and commands, see the BFD documents listed in the Related Documents.
This section includes the following topics:
■Default BFD Configuration Guidelines
■Configuring BFD Session Parameters on an Interface
■Enabling BFD Routing Protocol Clients
Default BFD Configuration
■No BFD sessions are configured. BFD is disabled on all interfaces.
■When configured, BFD version 1 is the default, but switches negotiate for version. Version 0 is also supported.
■Standby BFD (for HSRP) is enabled by default.
■Asynchronous BFD echo mode is enabled when a BFD session is configured.
Default BFD Configuration Guidelines
The switch supports a maximum of 28 BFD sessions at one time.
■Configure basic BFD interval parameters on each interface over which you want to run BFD sessions.
■Enable routing on the switch. You can configure BFD without enabling routing, but BFD sessions do not become active unless routing is enabled on the switch and on the BFD interfaces.
■Enable one or more one routing protocol clients for BFD. You should implement fast convergence for the routing protocol that you are using.
Note: We recommend that you configure the BFD interval parameters on an interface before configuring the routing protocol commands, especially when using EIGRP.
Confirm that CEF is enabled on participating switches (the default) as well as IP routing.
BFD is supported on physical interfaces that are configured as routing interfaces. It is not supported on Layer 2 interfaces, pseudowires, static routes, SVI interfaces, or port channels.
Although you can configure BFD interface commands on a Layer 2 port, BFD sessions do not operate on the interface unless it is configured as a Layer 3 interface (no switchport) and assigned an IP address.
In HSRP BFD, standby BFD is enabled globally by default and on all interfaces. If you disable it on an interface, you then must disable and reenable it globally for BFD sessions to be active.
When using BFD echo mode (the default), you should disable sending of ICMP redirect messages by entering the no ip redirects interface configuration command on the BFD interface.
Configuring BFD Session Parameters on an Interface
Before you can start a BFD session on an interface, you must put the interface into Layer 3 mode and set the baseline BFD parameters on it.
Note: Although you can configure BFD on Layer 2 interfaces, a BFD session cannot start until both interfaces are in Layer 3 mode and routing is enabled on the switch.
BEFORE YOU BEGIN
DETAILED STEPS
To remove the BFD parameter configuration, enter the no bfd interval interface configuration command.
Enabling BFD Routing Protocol Clients
After you configure BFD parameters on an interface, you can start a BFD session for one or more routing protocols. You must first enable routing by entering the ip routing global configuration command on the switch. Note that there can be more than one way to start a BFD session on an interface, depending on the routing protocol.
Configuring BFD for OSPF
When you start BFD sessions for OSPF, OSPF must be running on all participating devices. You can enable BFD support for OSPF by enabling it globally on all OSPF interfaces or by enabling it on one or more interfaces.
Configuring BFD for OSPF Globally
BEFORE YOU BEGIN
■Configure BFD parameters as described in the Configuring BFD Session Parameters on an Interface.
■Configure OSPF as described in the Configuring OSPF.
DETAILED STEPS
To disable OSPF BFD on all interfaces, enter the no bfd all-interfaces router configuration command.To disable it on an interface, enter the no ip osfp bfd or the ip ospf bfd disable interface configuration command on the interface.
If you want to run OSPF BFD on only one or a few interfaces, you can enter the ip ospf bfd interface configuration command on those interfaces instead of enabling it globally. See the next procedure.
Note: If you try to configure OSPF BFD on a Layer 2 interface, the configuration is not recognized.
EXAMPLE
This is an example of enabling BFD for OSPF on all OSPF interfaces:
Configuring BFD for OSPF on an Interface
BEFORE YOU BEGIN
■Configure BFD parameters on the interface as described in the Configuring BFD Session Parameters on an Interface.
■Configure OSPF as described in the Configuring OSPF.
DETAILED STEPS
To disable OSPF BFD on an interface, enter the no ip osfp bfd or the ip ospf bfd disable interface configuration command on the interface.
EXAMPLE
This is an example of enabling BFD for OSPF on a single interface:
Configuring BFD for IS-IS
When you start BFD sessions for IS-IS, IS-IS must be running on all devices participating in BFD. You can enable BFD support for IS-IS by enabling it globally on all IS-IS interfaces or by enabling it on one or more interfaces.
Configuring BFD for IS-IS Globally
BEFORE YOU BEGIN
■Configure BFD parameters on the interface as described in the Configuring BFD Session Parameters on an Interface.
■Configure IS-IS as described in the Configuring IS-IS Dynamic Routing.
DETAILED STEPS
To disable IS-IS BFD on all interfaces, enter the no bfd all-interfaces router configuration command. To disable it on the specified interface, enter the no isis bfd or the isis bfd disable interface configuration command on the interface.
If you only want to run IS-IS BFD on a few interfaces, instead of enabling it globally, you can enter the isis bfd interface configuration command on those interfaces. See the next procedure.
Note: Although IS-IS BFD operates only on Layer 3 interfaces, you can configure it on interfaces in Layer 2 or Layer 3 mode. When you enable it, you see this message:
%ISIS BFD is reverting to router mode configuration, and remains disabled.
EXAMPLE
This is an example of setting fast convergence and enabling BFD for IS-IS on all IS-IS interfaces:
Configuring BFD for IS-IS on an Interface
BEFORE YOU BEGIN
■Configure BFD parameters on the interface as described in the Configuring BFD Session Parameters on an Interface.
■Configure IS-IS as described in the Configuring IS-IS Dynamic Routing.
DETAILED STEPS
To disable IS-IS BFD on an interface, enter the no isis bfd or the isis bfd disable interface configuration command on the interface.
EXAMPLE
This is an example of enabling BFD for IS-IS on a single interface:
Configuring BFD for BGP
When you start BFD sessions for BGP, BGP must be running on all participating devices. You enter the IP address of the BFD neighbor to enable BFD for BGP.
BEFORE YOU BEGIN
■Configure BFD parameters on the interface as described in the Configuring BFD Session Parameters on an Interface.
■Configure BGP as described in the Configuring BGP.
DETAILED STEPS
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Specify a BGP autonomous system, and enter router configuration mode. |
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Verify the configuration. |
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To disable BGP BFD, enter the no neighbor ip-address fall-over bfd router configuration command.
Configuring BFD for EIGRP
When you start BFD sessions for EIGRP, EIGRP must be running on all participating devices.You can enable BFD support for EIGRP by globally enabling it on all EIGRP interfaces or by enabling it on one or more interfaces.
BEFORE YOU BEGIN
■Configure BFD parameters on the interface as described in the Configuring BFD Session Parameters on an Interface.
■Configure EIGRP as described in the Configuring EIGRP.
DETAILED STEPS
To disable EIGRP BFD on all interfaces, enter the no bfd all-interfaces router configuration command. To disable it on an interface, enter the no bfd interface interface-id router configuration command.
Configuring BFD for HSRP
HSRP supports BFD by default; it is globally enabled on all interfaces. If HSRP support has been manually disabled, you can reenable it in interface or global configuration mode.
BEFORE YOU BEGIN
■Configure BFD parameters on the interface as described in the Configuring BFD Session Parameters on an Interface.
■Ensure that all participating devices have HSRP enabled and CEF enabled (the default).
DETAILED STEPS
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Specify an interface for a BFD session, and enter interface configuration mode. Only physical interfaces support BFD. |
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Configure the IP address and IP subnet mask for the interface. |
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To disable HSRP support for BFD on all interfaces, enter the no standby bfd all-interfaces global configuration command. To disable it on an interface, enter the no standby bfd interface configuration command.
Note: If you disable standby BFD on an interface by entering the no standby bfd interface configuration command, to activate BFD sessions on other interfaces, you must disable and reenable it globally by entering the no standby bfd all-interfaces global configuration command followed by the standby bfd all-interfaces global configuration command.
EXAMPLE
The following example shows how to reenable HSRP BFD peering if it has been disabled on a switch:
Disabling BFD Echo Mode
When you configure a BFD session, BFD echo mode is enabled by default on BFD interfaces. You can disable echo mode on an interface so it sends no echo packets and but only sends back echo packets received from a neighbor. When echo mode is disabled, control packets are used to detect forwarding failures. You can configure slow timers to reduce the frequency of BFD control packets.
BEFORE YOU BEGIN
Configure BFD parameters on the interface as described in the Configuring BFD Session Parameters on an Interface.
DETAILED STEPS
To reenable echo mode on the switch, enter the bfd echo global configuration command.
EXAMPLE
The following example disables echo mode between BFD neighbors:
Configuring Multi-VRF CE
Virtual Private Networks (VPNs) provide a secure way for customers to share bandwidth over an ISP 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.
The switch supports multiple VPN routing/forwarding (multi-VRF) instances in customer edge (CE) devices (multi-VRF CE). With multi-VRF CE, a service provider can support two or more VPNs with overlapping IP addresses.
Note: The switch does not use Multiprotocol Label Switching (MPLS) to support VPNs. For information about MPLS VRF, refer to the MPLS: Layer 3 VPNs Configuration Guide, Cisco IOS Release 15M&T.
■Information About Multi-VRF CE
■Default Multi-VRF CE Configuration
■Multi-VRF CE Configuration Guidelines
■Configuring VRF-Aware Services
■Configuring a VPN Routing Session
■Configuring BGP PE to CE Routing Sessions
■Displaying Multi-VRF CE Status
Information About Multi-VRF CE
Multi-VRF CE 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.
Note: Multi-VRF CE interfaces must be Layer 3 interfaces.
Multi-VRF CE includes these devices:
■Customer edge (CE) devices provide customers access to the service-provider network over a data link to one or more provider edge routers. The CE device advertises the site local routes to the router and learns the remote VPN routes from it. The Cisco Connected Grid switch can be a CE.
■Provider edge (PE) routers exchange routing information with CE devices by using static routing or a routing protocol such as BGP, RIPv2, OSPF, 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).
■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 106 shows a configuration using Cisco Connected Grid 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 Cisco Connected Grid switches. Because multi-VRF CE is a Layer 3 feature, each interface in a VRF must be a Layer 3 interface.
Figure 106 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 (PL) in multi-VRF-CE-related data structures and adds the VLAN ID and PL to the VLAN database.
When 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 strips 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. If 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
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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.
■For the PE router, there is no difference between using multi-VRF CE or using multiple CEs. In Figure 106, multiple virtual Layer 3 interfaces are connected to the multi-VRF CE device.
■The switch supports configuring VRF by using physical ports, VLAN SVIs, or a combination of both. The SVIs can be connected through an access port or a trunk port.
■A customer can use multiple VLANs as long as they do not overlap with those of other customers. A customer’s VLANs are mapped to a specific routing table ID that is used to identify the appropriate routing tables stored on the switch.
■The switch supports one global network and up to 26 VRFs.
■Most routing protocols (BGP, OSPF, RIP, EIGRP, and static routing) can be used between the CE and the PE. However, we recommend using external BGP (EBGP) for these reasons:
–BGP does not require multiple algorithms to communicate with multiple CEs.
–BGP is designed for passing routing information between systems run by different administrations.
–BGP makes it easy to pass attributes of the routes to the CE.
■Multi-VRF CE does not affect the packet switching rate.
■If no VRFs are configured, up to 105 policies can be configured.
■If even one VRF is configured than 41 policies can be configured.
■If more than 41 policies are configured then VRF cannot be configured.
■VRF and private VLANs are mutually exclusive. You cannot enable VRF on a private VLAN. Similarly, you cannot enable private VLAN on a VLAN with VRF configured on the VLAN interface.
■VRF and policy-based routing (PBR) are mutually exclusive on a switch interface. You cannot enable VRF when PBR is enabled on an interface. In contrast, you cannot enable PBR when VRF is enabled on an interface.
Configuring VRFs
Follow the steps in this procedure to configure one or more VRFs.
BEFORE YOU BEGIN
DETAILED STEPS
Use the no ip vrf vrf-name global configuration command to delete a VRF and to remove all interfaces from it. Use the no ip vrf forwarding interface configuration command to remove an interface from the VRF.
EXAMPLE
The following example shows how to import a route map to a VRF instance named VPN1:
Configuring VRF-Aware Services
IP services can be configured on global interfaces, and these services run within the global routing instance. IP services are enhanced to run on multiple routing instances; they are VRF-aware. Any configured VRF in the system can be specified for a VRF-aware service.
VRF-aware services are implemented in platform-independent modules. VRF means multiple routing instances in Cisco IOS. Each platform has its own limit on the number of VRFs it supports.
VRF-aware services have the following characteristics:
■The user can ping a host in a user-specified VRF.
■ARP entries are learned in separate VRFs. The user can display Address Resolution Protocol (ARP) entries for specific VRFs.
■Simple Network Management Protocol (SNMP)
■Hot Standby Router Protocol (HSRP)
Note: VRF-aware services are not supported for Unicast Reverse Path Forwarding (uRPF).
User Interface for ARP
Use the arp command in global configuration mode to add a VRF to the ARP cache.
BEFORE YOU BEGIN
Configure a VRF as described in the Configuring VRFs.
DETAILED STEPS
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arp vrf vrf-name hardware-address encap-type [ interface-type ] [alias] |
Add a VRF instance. The vrf-name argument is the name of the VRF table. |
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EXAMPLE
User Interface for PING
To check if a configured VRF is working, you can use the ping vrf command.
When attempting to ping from a provider edge (PE) router to a customer edge (CE) router, or from a PE router to PE router, the standard ping command will not usually work. The ping vrf command allows you to ping the IP addresses of LAN interfaces on CE routers.
If you are on a PE router, be sure to indicate the specific VRF (VPN) name, as shown in the “Examples” section.
If all required information is not provided at the command line, the system will enter the interactive dialog (extended mode) for ping.
BEFORE YOU BEGIN
Configure a VRF as described in the Configuring VRFs.
DETAILED STEPS
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Tests a connection in the context of a specific VPN connection. |
EXAMPLE
In the following example, the target host in the domain 209.165.201.1 is pinged (using IP/ICMP) in the context of the “CustomerA” VPN connection:
User Interface for SNMP
Follow the steps in this procedure to configure configure VRF-aware services for SNMP.
BEFORE YOU BEGIN
Configure a VRF as described in the Configuring VRFs.
DETAILED STEPS
EXAMPLE
The following example specifies the SNMP engine ID and configures the VRF name traps-vrf for SNMP communications with the remote device at 172.16.20.3:
The following example shows how to send all SNMP notifications to example.com over the VRF named trap-vrf using the community string public:
User Interface for HSRP
Hot Standby Router Protocol (HSRP) support for VRFs ensures that HSRP virtual IP addresses are added to the correct IP routing table.
BEFORE YOU BEGIN
Configure a VRF as described in the Configuring VRFs.
DETAILED STEPS
EXAMPLE
User Interface for Syslog
Follow the steps in this procedure to configure VRF-aware services for Syslog.
BEFORE YOU BEGIN
Configure a VRF as described in the Configuring VRFs.
DETAILED STEPS
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Enable or temporarily disable logging of storage router event message. |
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Specify the host address of the syslog server where logging messages are to be sent. |
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EXAMPLE
The following example specifies a VRF that connects to the syslog server host:
User Interface for Traceroute
Follow the steps in this procedure to find the destination address in a VRF.
BEFORE YOU BEGIN
Configure a VRF as described in the Configuring VRFs.
DETAILED STEPS
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Specify the name of a VPN VRF in which to find the destination address. |
EXAMPLE
The following example displays output of the traceroute command with the vrf keyword. Output includes the incoming VRF name/tag and the outgoing VRF name/tag.
User Interface for FTP and TFTP
FTP and TFTP are VRF-aware, which means that file transfer is supported across an interface within a VRF instance. To specify a VRF as a source for FTP or TFTP connections, the VRF must be associated with the same interface that you configure with the ip ftp source-interface command. In this configuration, FTP looks for the destination IP address for file transfer in the specified VRF table. If the specified source interface is not up, Cisco IOS software selects the address of the interface closest to the destination as the source address.
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To specify the IP address of an interface as the source address for TFTP connections, use the ip tftp source-interface show mode command. To return to the default, use the no form of this command.
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EXAMPLE
The following example shows how to configure the switch to use the VRF table named vpn1 to look for the destination IP address for the transfer of FTP packets:
User Interface for VRF-Aware RADIUS
To configure VRF-aware RADIUS, you must first enable AAA on a RADIUS server. The switch supports the ip vrf forwarding vrf-name server-group configuration and the ip radius source-interface global configuration commands.
Configuring a VPN Routing Session
Routing within the VPN can be configured with any supported routing protocol (RIP, OSPF, EIGRP, or BGP) or with static routing. The configuration shown here is for OSPF, but the process is the same for other protocols.
Note: To configure an EIGRP routing process to run within a VRF instance, you must configure an autonomous-system number by entering the autonomous-system a utonomous-system-number address-family configuration mode command.
BEFORE YOU BEGIN
Configure a VRF as described in the Configuring VRFs.
DETAILED STEPS
Use the no router ospf process-id vrf vrf-name global configuration command to disassociate the VPN forwarding table from the OSPF routing process.
EXAMPLE
This example shows a basic OSPF configuration using the router ospf command to configure OSPF VRF processes for the VRFs first, second, and third:
Configuring BGP PE to CE Routing Sessions
BEFORE YOU BEGIN
■Complete the BGP network strategy and planning for your network.
■Configure OSPF as described in the Configuring OSPF.
■Configure a VRF as described in the Configuring VRFs.
DETAILED STEPS
Use the no router bgp autonomous-system-number global configuration command to delete the BGP routing process. Use the command with keywords to delete routing characteristics.
EXAMPLE
The following example configures BGP for CE to PE routing:
Displaying Multi-VRF CE Status
You can use the following privileged EXEC commands to display information about multi-VRF CE configuration and status.
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show ip route vrf vrf-name [ connected ] [ protocol [ as-number ]] [ list ] [ mobile ] [ odr ] [ profile ] [ static ] [ summary ] [ supernets-only ] |
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Configuring Protocol-Independent Features
This section describes how to configure IP routing protocol-independent features. For a complete description of the IP routing protocol-independent commands in this chapter, see the Cisco IOS IP Routing: Protocol-Independent Command Reference.
This section includes the following topics:
■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
■Filtering Routing Information
Configuring Cisco Express Forwarding
Cisco Express Forwarding (CEF) is a Layer 3 IP switching technology used to optimize network performance. CEF implements an advanced IP look-up and forwarding algorithm to deliver maximum Layer 3 switching performance. CEF is less CPU-intensive than fast switching route caching, allowing more CPU processing power to be dedicated to packet forwarding. In dynamic networks, fast switching cache entries are frequently invalidated because of routing changes, which can cause traffic to be process switched using the routing table, instead of fast switched using the route cache. CEF uses the Forwarding Information Base (FIB) lookup table to perform destination-based switching of IP packets.
The two main components in CEF are the distributed FIB and the distributed adjacency tables.
■The FIB is similar to a routing table or information base and maintains a mirror image of the forwarding information in the IP routing table. When routing or topology changes occur in the network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table. Because the FIB contains all known routes that exist in the routing table, CEF eliminates route cache maintenance, is more efficient for switching traffic, and is not affected by traffic patterns.
■Nodes in the network are said to be adjacent if they can reach each other with a single hop across a link layer. CEF uses adjacency tables to prepend Layer 2 addressing information. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries.
Because the switch uses Application Specific Integrated Circuits (ASICs) to achieve Gigabit-speed line rate IP traffic, CEF forwarding applies only to the software-forwarding path, that is, traffic that is forwarded by the CPU.
CEF is enabled globally by default. If for some reason it is disabled, you can re-enable it by using the ip cef global configuration command.
The default configuration is CEF enabled on all Layer 3 interfaces. Entering the no ip route-cache cef interface configuration command disables CEF for traffic that is being forwarded by software. This command does not affect the hardware forwarding path. Disabling CEF and using the debug ip packet detail privileged EXEC command can be useful to debug software-forwarded traffic. To enable CEF on an interface for the software-forwarding path, use the ip route-cache cef interface configuration command.
Caution: Although the no ip route-cache cef interface configuration command to disable CEF on an interface is visible in the CLI, we strongly recommend that you do not disable CEF on interfaces except for debugging purposes.
BEFORE YOU BEGIN
■Cisco Express Forwarding requires a software image that includes Cisco Express Forwarding and IP routing enabled on the switch.
■If you enable Cisco Express Forwarding and then create an access list that uses the log keyword, the packets that match the access list are not Cisco Express Forwarding switched. They are process switched. Logging disables Cisco Express Forwarding.
DETAILED STEPS
EXAMPLE
Configuring the Number of Equal-Cost Routing Paths
When a router has two or more routes to the same network with the same metrics, these routes can be thought of as having an equal cost. The term parallel path is another way to see occurrences of equal-cost routes in a routing table. If a router has two or more equal-cost paths to a network, it can use them concurrently. Parallel paths provide redundancy in case of a circuit failure and also enable a router to load balance packets over the available paths for more efficient use of available bandwidth.
Although the router automatically learns about and configures equal-cost routes, you can control the maximum number of parallel paths supported by an IP routing protocol in its routing table.
DETAILED STEPS
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Set the maximum number of parallel paths for the protocol routing table. The range is from 1 to 8; the default is 4 for most IP routing protocols, but only 1 for BGP. |
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Use the no maximum-paths router configuration command to restore the default value.
EXAMPLE
The following example shows how to allow a maximum of 16 paths to a destination in an OSPF routing process:
Configuring Static Unicast Routes
Static unicast routes are user-defined routes that cause packets moving between a source and a destination to take a specified path. Static routes can be important if the router cannot build a route to a particular destination and are useful for specifying a gateway of last resort to which all unroutable packets are sent.
The switch retains static routes until you remove them. However, you can override static routes with dynamic routing information by assigning administrative distance values. Each dynamic routing protocol has a default administrative distance, as listed in Table 2. If you want a static route to be overridden by information from a dynamic routing protocol, set the administrative distance of the static route higher than that of the dynamic protocol.
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Static routes that point to an interface are advertised through RIP, IGRP, and other dynamic routing protocols, whether or not static redistribute router configuration commands were specified for those routing protocols. These static routes are advertised because static routes that point to an interface are considered in the routing table to be connected and hence lose their static nature. However, if you define a static route to an interface that is not one of the networks defined in a network command, no dynamic routing protocols advertise the route unless a redistribute static command is specified for these protocols.
When an interface goes down, all static routes through that interface are removed from the IP routing table. When the software can no longer find a valid next hop for the address specified as the forwarding router's address in a static route, the static route is also removed from the IP routing table.
DETAILED STEPS
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Display the current state of the routing table to verify the configuration. |
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Use the no ip route prefix mask { address | interface } global configuration command to remove a static route.
EXAMPLE
The following example shows how to choose an administrative distance of 110. In this case, packets for network 10.0.0.0 will be routed to a router at 172.31.3.4 if dynamic information with an administrative distance less than 110 is not available.
Specifying Default Routes and Networks
A router might not be able to learn the routes to all other networks. To provide complete routing capability, you can use some routers as smart routers and give the remaining routers default routes to the smart router. (Smart routers have routing table information for the entire internetwork.) These default routes can be dynamically learned or can be configured in the individual routers. Most dynamic interior routing protocols include a mechanism for causing a smart router to generate dynamic default information that is then forwarded to other routers.
If a router has a directly connected interface to the specified default network, the dynamic routing protocols running on that device generate a default route. In RIP, it advertises the pseudonetwork 0.0.0.0.s
A router that is generating the default for a network also might need a default of its own. One way a router can generate its own default is to specify a static route to the network 0.0.0.0 through the appropriate device.
When default information is passed through a dynamic routing protocol, no further configuration is required. The system periodically scans its routing table to choose the optimal default network as its default route. In IGRP networks, there might be several candidate networks for the system default. Cisco routers use administrative distance and metric information to set the default route or the gateway of last resort.
If dynamic default information is not being passed to the system, candidates for the default route are specified with the ip default-network global configuration command. If this network appears in the routing table from any source, it is flagged as a possible choice for the default route. If the router has no interface on the default network, but does have a path to it, the network is considered as a possible candidate, and the gateway to the best default path becomes the gateway of last resort.
BEFORE YOU BEGIN
The ip default-network command is a classful command. It is effective only if the network mask of the network that you wish to configure as a candidate route for computing the gateway of last resort matches the network mask in the Routing Information Base (RIB).
For example, if you configure ip default-network 10.0.0.0, then the mask considered by the routing protocol is 10.0.0.0/8, as it is a Class A network. The gateway of last resort is set only if the RIB contains a 10.0.0.0/8 route.
If you need to use the ip default-network command, ensure that the RIB contains a network route that matches the major mask of the network class.
DETAILED STEPS
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Display the selected default route in the gateway of last resort display. |
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Use the no ip default-network network number global configuration command to remove the route.
EXAMPLE
The following example defines a static route to network 10.0.0.0 as the static default route:
Using Route Maps to Redistribute Routing Information
The switch can run multiple routing protocols simultaneously, and it can redistribute information from one routing protocol to another. Redistributing information from one routing protocol to another applies to all supported IP-based routing protocols.
You can also conditionally control the redistribution of routes between routing domains by defining enhanced packet filters or route maps between the two domains. The match and set route-map configuration commands define the condition portion of a route map. The match command specifies that a criterion must be matched. The set command specifies an action to be taken if the routing update meets the conditions defined by the match command. Although redistribution is a protocol-independent feature, some of the match and set route-map configuration commands are specific to a particular protocol.
One or more match commands and one or more set commands follow a route-map command. If there are no match commands, everything matches. If there are no set commands, nothing is done, other than the match. Therefore, you need at least one match or set command.
Note: A route map with no set route-map configuration commands is sent to the CPU, which causes high CPU utilization.
You can also identify route-map statements as permit or deny. If the statement is marked as a deny, the packets meeting the match criteria are sent back through the normal forwarding channels (destination-based routing). If the statement is marked as permit, set clauses are applied to packets meeting the match criteria. Packets that do not meet the match criteria are forwarded through the normal routing channel.
You can use the BGP route map continue clause to execute additional entries in a route map after an entry is executed with successful match and set clauses. You can use the continue clause to configure and organize more modular policy definitions so that specific policy configurations need not be repeated within the same route map. The switch supports the continue clause for outbound policies. For more information about using the route map continue clause, see the “BGP Route-Map Continue” section in the IP Routing: BGP Configuration Guide, Cisco IOS Release 15M&T.
Note: Although each of Steps 3 through 14 in the following section is optional, you must enter at least one match route-map configuration command and one set route-map configuration command.
BEFORE YOU BEGIN
You should know your network design and how you want traffic to flow through it before configuring route redistribution or policy-based routing.
DETAILED STEPS
To delete an entry, use the no route-map map tag global configuration command or the no match or no set route-map configuration commands.
EXAMPLE
The following example shows how to redistribute Routing Information Protocol (RIP) routes with a hop count equal to 1 to Open Shortest Path First (OSPF). These routes will be redistributed to OSPF as external link-state advertisements (LSAs) with a metric of 5, metric type of Type 1, and a tag equal to 1.
Controlling Route Redistribution
You can distribute routes from one routing domain into another and control route distribution. Note that the keywords in this procedure are the same as defined in the previous procedure.
The metrics of one routing protocol do not necessarily translate into the metrics of another. In these situations, an artificial metric is assigned to the redistributed route. Uncontrolled exchanging of routing information between different routing protocols can create routing loops and seriously degrade network operation.
If you have not defined a default redistribution metric that replaces metric conversion, some automatic metric translations occur between routing protocols:
■RIP can automatically redistribute static routes. It assigns static routes a metric of 1 (directly connected).
■Any protocol can redistribute other routing protocols if a default mode is in effect.
BEFORE YOU BEGIN
Review the usage guidelines and additional examples for the redistribute command in the Cisco IOS IP Routing: Protocol-Independent Command Reference.
DETAILED STEPS
EXAMPLE
Given the following configuration, a RIP-learned route for network 160.89.0.0 and an ISO IGRP-learned route with prefix 49.0001.0002 will be redistributed into an IS-IS Level 2 link-state PDU with metric 5:
Configuring Policy-Based Routing
You can use policy-based routing (PBR) to configure a defined policy for traffic flows. By using PBR, you can have more control over routing by reducing the reliance on routes derived from routing protocols. PBR can specify and implement routing policies that allow or deny paths based on:
■Identity of a particular end system
You can use PBR to provide equal-access and source-sensitive routing, routing based on interactive versus batch traffic, or routing based on dedicated links. For example, you could transfer stock records to a corporate office on a high-bandwidth, high-cost link for a short time while transmitting routine application data such as e-mail over a low-bandwidth, low-cost link.
With PBR, you classify traffic using access control lists (ACLs) and then make traffic go through a different path. PBR is applied to incoming packets. All packets received on an interface with PBR enabled are passed through route maps. Based on the criteria defined in the route maps, packets are forwarded (routed) to the appropriate next hop.
■If packets do not match any route map statements, all set clauses are applied.
■If a statement is marked as permit and the packets do not match any route-map statements, the packets are sent through the normal forwarding channels, and destination-based routing is performed.
■For PBR, route-map statements marked as deny are not supported.
For more information about configuring route maps, see Using Route Maps to Redistribute Routing Information.
You can use standard IP ACLs to specify match criteria for a source address or extended IP ACLs to specify match criteria based on an application, a protocol type, or an end station. The process proceeds through the route map until a match is found. If no match is found, normal destination-based routing occurs. There is an implicit deny at the end of the list of match statements.
If match clauses are satisfied, you can use a set clause to specify the IP addresses identifying the next hop router in the path.
For details about PBR commands and keywords, see IP Routing: Protocol-Independent Configuration Guide, Cisco IOS Release 15M&T .
PBR Configuration Guidelines
Before configuring PBR, you should be aware of this information:
■Multicast traffic is not policy-routed. PBR applies to only to unicast traffic.
■You can enable PBR on a routed port or an SVI.
■The switch does not support route-map deny statements for PBR.
■You can apply a policy route map to an EtherChannel port channel in Layer 3 mode, but you cannot apply a policy route map to a physical interface that is a member of the EtherChannel. If you try to do so, the command is rejected. When a policy route map is applied to a physical interface, that interface cannot become a member of an EtherChannel.
■You can define a maximum of 246 IP policy route maps on the switch.
■You can define a maximum of 512 access control entries (ACEs) for PBR on the switch.
■When configuring match criteria in a route map, follow these guidelines:
–Do not match ACLs that permit packets destined for a local address. PBR would forward these packets, which could cause ping or Telnet failure or route protocol flapping.
–Do not match ACLs with deny ACEs. Packets that match a deny ACE are sent to the CPU, which could cause high CPU utilization.
■To use PBR, you must first enable the default template by using the sdm prefer default global configuration command. PBR is not supported with the Layer 2 template.
■VRF and PBR are mutually-exclusive on a switch interface. You cannot enable VRF when PBR is enabled on an interface. In contrast, you cannot enable PBR when VRF is enabled on an interface.
■The number of TCAM entries used by PBR depends on the route map itself, the ACLs used, and the order of the ACLs and route-map entries.
■Policy-based routing based on packet length, IP precedence and TOS, set interface, set default next hop, or set default interface are not supported. Policy maps with no valid set actions or with set action set to Don’t Fragment are not supported.
Enabling PBR
By default, PBR is disabled on the switch. To enable PBR, you must create a route map that specifies the match criteria and the resulting action if all of the match clauses are met. Then, you must enable PBR for that route map on an interface. All packets arriving on the specified interface matching the match clauses are subject to PBR.
PBR can be fast-switched or implemented at speeds that do not slow down the switch. Fast-switched PBR supports most match and set commands. PBR must be enabled before you enable fast-switched PBR. Fast-switched PBR is disabled by default.
Packets that are generated by the switch, or local packets, are not normally policy-routed. When you globally enable local PBR on the switch, all packets that originate on the switch are subject to local PBR. Local PBR is disabled by default.
BEFORE YOU BEGIN
DETAILED STEPS
Use the no route-map map-tag global configuration command or the no match or no set route-map configuration commands to delete an entry. Use the no ip policy route-map map-tag interface configuration command to disable PBR on an interface. Use the no ip route-cache policy interface configuration command to disable fast-switching PBR. Use the no ip local policy route-map map-tag global configuration command to disable policy-based routing on packets originating on the switch.
EXAMPLE
The following example sends packets with the destination IP address of 172.21.16.18 to a router at IP address 172.30.3.20:
Filtering Routing Information
You can filter routing protocol information by performing the tasks described in this section.
Note: When routes are redistributed between OSPF processes, no OSPF metrics are preserved.
Setting Passive Interfaces
To prevent other routers on a local network from dynamically learning about routes, you can use the passive-interface router configuration command to keep routing update messages from being sent through a router interface. When you use this command in the OSPF protocol, the interface address you specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received through the specified router interface.
In networks with many interfaces, to avoid having to manually set them as passive, you can set all interfaces to be passive by default by using the passive-interface default router configuration command and manually setting interfaces where adjacencies are desired.
BEFORE YOU BEGIN
You should know your network design and how you want traffic to flow through it before filtering routing information.
DETAILED STEPS
Use a network monitoring privileged EXEC command such as show ip ospf interface to verify the interfaces that you enabled as passive, or use the show ip interface privileged EXEC command to verify the interfaces that you enabled as active.
To re-enable the sending of routing updates, use the no passive-interface interface-id router configuration command.
EXAMPLE
The following example sends EIGRP updates to all interfaces on network 10.108.0.0 except Ethernet interface 1:
The following example sets all interfaces as passive and then activates Ethernet interface 0:
Controlling Advertising and Processing in Routing Updates
You can use the distribute-list router configuration command with access control lists to suppress routes from being advertised in routing updates and to prevent other routers from learning one or more routes. When used in OSPF, this feature applies to only external routes, and you cannot specify an interface name.
You can also use a distribute-list router configuration command to avoid processing certain routes listed in incoming updates. (This feature does not apply to OSPF.)
BEFORE YOU BEGIN
Configure an access list defining which networks are to be sent or received and which are to be suppressed in routing updates.
DETAILED STEPS
Use the no distribute-list in router configuration command to change or cancel a filter. To cancel suppression of network advertisements in updates, use the no distribute-list out router configuration command.
EXAMPLE
In the following example, a prefix list and distribute list are defined to configure the BGP routing process to accept traffic from only network 10.1.1.0/24, network 192.168.1.0, and network 10.108.0.0. An inbound route refresh is initiated to activate the distribute-list.
Filtering Sources of Routing Information
Because some routing information might be more accurate than others, you can use filtering to prioritize information coming from different sources. An administrative distance is a rating of the trustworthiness of a routing information source, such as a router or group of routers. In a large network, some routing protocols can be more reliable than others. By specifying administrative distance values, you enable the router to intelligently discriminate between sources of routing information. The router always picks the route whose routing protocol has the lowest administrative distance.
Because each network has its own requirements, there are no general guidelines for assigning administrative distances.
BEFORE YOU BEGIN
■Always set the administrative distance from the least to the most specific network.
■Review the usage guidelines and additional examples for the distance command in the Cisco IOS IP Routing: Protocol-Independent Command Reference.
DETAILED STEPS
To remove a distance definition, use the no distance router configuration command.
EXAMPLE
In the following example, the routereigrp global configuration command sets up EIGRP routing in autonomous system number 109. The network router configuration commands specify EIGRP routing on networks 192.168.7.0 and 172.16.0.0. The first distance command sets the administrative distance to 90 for all routers on the Class C network 192.168.7.0. The second distance command sets the administrative distance to 120 for the router with the address 172.16.1.3.
In the following example, the set distance is from the least to the most specific network:
Managing Authentication Keys
Key management is a method of controlling authentication keys used by routing protocols. Not all protocols can use key management. Authentication keys are available for EIGRP and RIP Version 2.
To manage authentication keys, define a key chain, identify the keys that belong to the key chain, and specify how long each key is valid. Each key has its own key identifier (specified with the key number key chain configuration command), which is stored locally. The combination of the key identifier and the interface associated with the message uniquely identifies the authentication algorithm and Message Digest 5 (MD5) authentication key in use.
You can configure multiple keys with life times. Only one authentication packet is sent, regardless of how many valid keys exist. The software examines the key numbers in order from lowest to highest, and uses the first valid key it encounters. The lifetimes allow for overlap during key changes. Note that the router must know these lifetimes.
BEFORE YOU BEGIN
Before you manage authentication keys, you must enable authentication. See the appropriate protocol section to see how to enable authentication for that protocol.
DETAILED STEPS
To remove the key chain, use the no key chain name-of-chain global configuration command.
EXAMPLE
The following example configures a key chain named chain1. The key named key1 will be accepted from 1:30 p.m. to 3:30 p.m. and be sent from 2:00 p.m. to 3:00 p.m. The key named key2 will be accepted from 2:30 p.m. to 4:30 p.m. and be sent from 3:00 p.m. to 4:00 p.m. The overlap allows for migration of keys or a discrepancy in the set time of the router. There is a 30-minute leeway on each side to handle time differences.
Verifying Configuration
You can remove all contents of a particular cache, table, or database. You can also display specific statistics.
Related Documents
■ Cisco IOS Master Command List, All Releases
■ IP Addressing: ARP Configuration Guide, Cisco IOS Release 15M&T
■ Cisco IOS IP Routing: RIP Command Reference
■ IP Routing: RIP Configuration Guide, Cisco IOS Release 15M&T
■ Cisco IOS IP Routing: OSPF Command Reference
■ IP Routing: OSPF Configuration Guide, Cisco IOS Release 15M&T
■ Cisco IOS IP Routing: EIGRP Command Reference
■ IP Routing: EIGRP Configuration Guide, Cisco IOS Release 15M&T
■ Cisco IOS IP Routing: BGP Command Reference
■ IP Routing: BGP Configuration Guide, Cisco IOS Release 15M&T
■ Cisco IOS ISO CLNS Command Reference
■ ISO CLNS Configuration Guide, Cisco IOS Release 15M&T
■ Cisco IOS IP Routing: ISIS Command Reference
■ IP Routing: ISIS Configuration Guide, Cisco IOS Release 15M&T
■ High Availability Configuration Guide, Cisco IOS Release 15S
■ IP Routing: BFD Configuration Guide, Cisco IOS Release 15M&T
■ Cisco IOS IP Routing: Protocol-Independent Command Reference
■ IP Routing: Protocol-Independent Configuration Guide, Cisco IOS Release 15M&T
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