Software Configuration Guide, Cisco IOS Release 15.2(3)E and Later (Catalyst 3750-X and 3560-X Switches)

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Information About Configuring IP Unicast Routing

This module describes how to configure IP Version 4 (IPv4) unicast routing on the switch.

A switch stack operates and appears as a single router to the rest of the routers in the network. Basic routing functions like static routing are available with .

Note

In addition to IPv4 traffic, you can also enable IP Version 6 (IPv6) unicast routing and configure interfaces to forward IPv6 traffic.

Information About IP Routing

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

Figure 1. Routing Topology Example. This figure 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.

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 these ways:

By using default routing
By using preprogrammed static routes for the traffic
By dynamically calculating routes by using a routing protocol

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

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

Beginning with Cisco IOS release 12.2(58)SE, switches running the LAN base feature set support 16 user-configured static routes, in addition to any default routes used for the management interface. The LAN base image supports static routing only on SVIs and only when the switch is running the default SDM template.

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

Routers using distance-vector protocols maintain routing tables with distance values of networked resources, and periodically pass these tables to their neighbors. Distance-vector protocols use one or a series of metrics for calculating the best routes. These protocols are easy to configure and use.
Routers using link-state protocols maintain a complex database of network topology, based on the exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in the network, which speeds up the convergence time or time required to respond to these changes. Link-state protocols respond quickly to topology changes, but require greater bandwidth and more resources than distance-vector protocols.

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

Note

On a switch or switch stack, the supported protocols are determined by the software running on the active switch. If the switch is running the LAN base feature set, you can configure 16 static routes on SVIs. All other routing protocols require the IP services feature set.

IP Routing and Switch Stacks

A Switch stack appears to the network as a single Switch, regardless of which Switch in the stack is connected to a routing peer.

The stack master performs these functions:

It initializes and configures the routing protocols.
It sends routing protocol messages and updates to other routers.
It processes routing protocol messages and updates received from peer routers.
It generates, maintains, and distributes the distributed Cisco Express Forwarding (dCEF) database to all stack members. The routes are programmed on all Switch in the stack bases on this database.
The MAC address of the stack master is used as the router MAC address for the whole stack, and all outside devices use this address to send IP packets to the stack.
All IP packets that require software forwarding or processing go through the CPU of the stack master.

Stack members perform these functions:

They act as routing standby Switch, ready to take over in case they are elected as the new stack master if the stack master fails.
They program the routes into hardware.

If a stack master fails, the stack detects that the stack master is down and elects one of the stack members to be the new stack master. During this period, except for a momentary interruption, the hardware continues to forward packets with no active protocols.

However, even though the Switch stack maintains the hardware identification after a failure, the routing protocols on the router neighbors might flap during the brief interruption before the stack master restarts. Routing protocols such as OSPF and EIGRP need to recognize neighbor transitions.

The router uses two levels of nonstop forwarding (NSF) to detect a Switch over, to continue forwarding network traffic, and to recover route information from peer devices:

NSF-aware routers tolerate neighboring router failures. After the neighbor router restarts, an NSF-aware router supplies information about its state and route adjacencies on request.
NSF-capable routers support NSF. When they detect a stack master change, they rebuild routing information from NSF-aware or NSF-capable neighbors and do not wait for a restart.

The Switch stack supports NSF-capable routing for OSPF and EIGRP.

Upon election, the new stack master performs these functions:

It starts generating, receiving, and processing routing updates.
It builds routing tables, generates the CEF database, and distributes it to stack members.

It uses its MAC address as the router MAC address. To notify its network peers of the new MAC address, it periodically (every few seconds for 5 minutes) sends a gratuitous ARP reply with the new router MAC address.

Note

If you configure the persistent MAC address feature on the stack and the stack master changes, the stack MAC address does not change for the configured time period. If the previous stack master rejoins the stack as a member Switch during that time period, the stack MAC address remains the MAC address of the previous stack master.

It attempts to determine the reachability of every proxy ARP entry by sending an ARP request to the proxy ARP IP address and receiving an ARP reply. For each reachable proxy ARP IP address, it generates a gratuitous ARP reply with the new router MAC address. This process is repeated for 5 minutes after a new stack master election.

Note

When a stack master is running the IP services feature set, the stack can run all supported protocols, including Open Shortest Path First (OSPF), and Enhanced IGRP (EIGRP) and Border Gateway Protocol (BGP). If the stack master fails and the new elected stack master is running the IP base or LAN base feature set, these protocols will no longer run in the stack.

Caution

Partitioning of the Switch stack into two or more stacks might lead to undesirable behavior in the network.

How to Configure IP Routing

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

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.

Note

If the Switch is running the LAN base feature set, static routes are supported only on SVIs.

An EtherChannel port channel in Layer 3 mode: a port-channel logical interface created by using the interface port-channel port-channel-number global configuration command and binding the Ethernet interface into the channel group. For more information, see the “Configuring Layer 3 EtherChannels” chapter in the Layer 2 Configuration Guide.

Note

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.

Note

A Layer 3 switch can have an IP address assigned to each routed port and SVI.

The number of routed ports and SVIs that you can configure is not limited by software. However, the interrelationship between this number and the number and volume of features being implemented might have an impact on CPU utilization because of hardware limitations. To optimize system memory for routing, use the sdm prefer routing global configuration command when running the IP base or IP services feature set.

Configuring routing consists of several main procedures:

To support VLAN interfaces, create and configure VLANs on the Switch or switch stack, and assign VLAN membership to Layer 2 interfaces. For more information, see the "Configuring VLANs” chapter in the VLAN Configuration Guide.
Configure Layer 3 interfaces.
Enable IP routing on the switch.
Assign IP addresses to the Layer 3 interfaces.
Enable selected routing protocols on the switch.
Configure routing protocol parameters (optional).

How to Configure IP Addressing

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

Note

On switches running the LAN base feature set, you can only assign an IP address to an SVI and configure a static unicast route on the interface. Other configurations are not supported.

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 IP Addressing Configuration

Table 1. Default Addressing Configuration
Feature	Default Setting
IP address	None defined.
ARP	No permanent entries in the Address Resolution Protocol (ARP) cache. Encapsulation: Standard Ethernet-style ARP. Timeout: 14400 seconds (4 hours).
IP broadcast address	255.255.255.255 (all ones).
IP classless routing	Enabled.
IP default gateway	Disabled.
IP directed broadcast	Disabled (all IP directed broadcasts are dropped).
IP domain	Domain list: No domain names defined. Domain lookup: Enabled. Domain name: Enabled.
IP forward-protocol	If a helper address is defined or User Datagram Protocol (UDP) flooding is configured, UDP forwarding is enabled on default ports. Any-local-broadcast: Disabled. Spanning Tree Protocol (STP): Disabled. Turbo-flood: Disabled.
IP helper address	Disabled.
IP host	Disabled.
IRDP	Disabled. Defaults when enabled: Broadcast IRDP advertisements. Maximum interval between advertisements: 600 seconds. Minimum interval between advertisements: 0.75 times max interval Preference: 0.
IP proxy ARP	Enabled.
IP routing	Disabled.
IP subnet-zero	Disabled.

Assigning IP Addresses to Network Interfaces

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

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

Procedure

Command or Action Purpose

Step 1

enable

Example:


Switch> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 3

interface interface-id

Example:


Switch(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Note

Only SVI interfaces are supported on switches running the LAN base feature set.

Step 4

no switchport

Example:


Switch(config-if)# no switchport

Removes the interface from Layer 2 configuration mode (if it is a physical interface).

Step 5

ip address ip-address subnet-mask

Example:


Switch(config-if)# ip address 10.1.5.1 255.255.255.0

Configures the IP address and IP subnet mask.

Step 6

no shutdown

Example:


Switch(config-if)# no shutdown

Enables the physical interface.

Step 7

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 8

show ip route

Example:


Switch# show ip route

Verifies your entries.

Step 9

show ip interface [interface-id]

Example:


Switch# show ip interface gigabitethernet 1/0/1

Verifies your entries.

Step 10

show running-config

Example:


Switch# show running-config

Verifies your entries.

Step 11

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

Step 12

On switches running the LAN base image, you can only assign an IP address to an SVI and then configure a static route for the SVI. The switch supports 16 user-configured static routes. No other routing configurations are supported.

Using Subnet Zero

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

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

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

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	ip subnet-zero Example: `Switch(config)# ip subnet-zero`	Enables the use of subnet zero for interface addresses and routing updates.
Step 4	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 5	show running-config Example: `Switch# show running-config`	Verifies your entries.
Step 6	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

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 the figure, 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.

In the figure , 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.

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

Disabling 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.

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	no ip classless Example: `Switch(config)#no ip classless`	Disables classless routing behavior.
Step 4	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 5	show running-config Example: `Switch# show running-config`	Verifies your entries.
Step 6	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.
Step 7		To restore the default and have the switch forward packets destined for a subnet of a network with no network default route to the best supernet route possible, use the ip classless global configuration command.

Configuring Address Resolution Methods

You can perform the following tasks to configure address resolution.

Address Resolution

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.

Note

In a switch stack, network communication uses a single MAC address and the IP address of the stack.

The local address or MAC address is known as a data link address because it is contained in the data link layer (Layer 2) section of the packet header and is read by data link (Layer 2) devices. To communicate with a device on Ethernet, the software must 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 the Cisco IOS Configuration Fundamentals Configuration Guide

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.

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.

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	arp `ip-address hardware-address type` Example: `Switch(config)# ip 10.1.5.1 c2f3.220a.12f4 arpa`	Associates an IP address with a MAC (hardware) address in the ARP cache, and specifies encapsulation type as one of these: arpa —ARP encapsulation for Ethernet interfaces snap —Subnetwork Address Protocol encapsulation for Token Ring and FDDI interfaces sap —HP’s ARP type
Step 4	arp `ip-address hardware-address type` [alias ] Example: `Switch(config)# ip 10.1.5.3 d7f3.220d.12f5 arpa alias`	(Optional) Specifies that the switch respond to ARP requests as if it were the owner of the specified IP address.
Step 5	interface `interface-id` Example: `Switch(config)# interface gigabitethernet 1/0/1`	Enters interface configuration mode, and specifies the interface to configure.
Step 6	arp `timeout seconds` Example: `Switch(config-if)# arp 20000`	(Optional) Sets the length of time an ARP cache entry will stay in the cache. The default is 14400 seconds (4 hours). The range is 0 to 2147483 seconds.
Step 7	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 8	show interfaces [`interface-id`] Example: `Switch# show interfaces gigabitethernet 1/0/1`	Verifies the type of ARP and the timeout value used on all interfaces or a specific interface.
Step 9	show arp Example: `Switch# show arp`	Views the contents of the ARP cache.
Step 10	show ip arp Example: `Switch# show ip arp`	Views the contents of the ARP cache.
Step 11	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

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.

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

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	interface `interface-id` Example: `Switch(config)# interface gigabitethernet 1/0/2`	Enters interface configuration mode, and specifies the Layer 3 interface to configure.
Step 4	arp {arpa \| snap } Example: `Switch(config-if)# arp arpa`	Specifies the ARP encapsulation method: arpa —Address Resolution Protocol snap —Subnetwork Address Protocol
Step 5	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 6	show interfaces [`interface-id`] Example: `Switch# show interfaces`	Verifies ARP encapsulation configuration on all interfaces or the specified interface.
Step 7	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Enabling Proxy ARP

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

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

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	interface `interface-id` Example: `Switch(config)# interface gigabitethernet 1/0/2`	Enters interface configuration mode, and specifies the Layer 3 interface to configure.
Step 4	ip proxy-arp Example: `Switch(config-if)# ip proxy-arp`	Enables proxy ARP on the interface.
Step 5	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 6	show ip interface [`interface-id`] Example: `Switch# show ip interface gigabitethernet 1/0/2`	Verifies the configuration on the interface or all interfaces.
Step 7	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Routing Assistance When IP Routing is Disabled

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

Proxy ARP
Default Gateway
ICMP Router Discovery Protocol (IRDP)

Proxy ARP

Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no routing information to communicate with hosts on other networks or subnets. The host assumes that all hosts are on the same local Ethernet and that they can use ARP to 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

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

Default Gateway

Another method for locating routes is to define a default router or default gateway. All non-local 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.

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

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	ip default-gateway `ip-address` Example: `Switch(config)# ip default gateway 10.1.5.1`	Sets up a default gateway (router).
Step 4	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 5	show ip redirects Example: `Switch# show ip redirects`	Displays the address of the default gateway router to verify the setting.
Step 6	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

ICMP Router Discovery Protocol

Router discovery allows the Switch to dynamically learn about routes to other networks using ICMP router discovery protocol (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.

ICMP Router Discovery Protocol (IRDP)

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

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

Procedure

Command or Action Purpose

Step 1

enable

Example:


Switch> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 3

interface interface-id

Example:


Switch(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 4

ip irdp

Example:


Switch(config-if)# ip irdp

Enables IRDP processing on the interface.

Step 5

ip irdp multicast

Example:


Switch(config-if)# ip irdp multicast

(Optional) Sends IRDP advertisements to the multicast address (224.0.0.1) instead of IP broadcasts.

Note

This command allows for compatibility with Sun Microsystems Solaris, which requires IRDP packets to be sent out as multicasts. Many implementations cannot receive these multicasts; ensure end-host ability before using this command.

Step 6

ip irdp holdtime seconds

Example:


Switch(config-if)# ip irdp holdtime 1000

(Optional) Sets the IRDP period for which advertisements are valid. The default is three times the maxadvertinterval value. It must be greater than maxadvertinterval and cannot be greater than 9000 seconds. If you change the maxadvertinterval value, this value also changes.

Step 7

ip irdp maxadvertinterval seconds

Example:


Switch(config-if)# ip irdp maxadvertinterval 650

(Optional) Sets the IRDP maximum interval between advertisements. The default is 600 seconds.

Step 8

ip irdp minadvertinterval seconds

Example:


Switch(config-if)# ip irdp minadvertinterval 500

(Optional) Sets the IRDP minimum interval between advertisements. The default is 0.75 times the maxadvertinterval . If you change the maxadvertinterval , this value changes to the new default (0.75 of maxadvertinterval ).

Step 9

ip irdp preference number

Example:


Switch(config-if)# ip irdp preference 2

(Optional) Sets a device IRDP preference level. The allowed range is –231 to 231. The default is 0. A higher value increases the router preference level.

Step 10

ip irdp address address [number]

Example:


Switch(config-if)# ip irdp address 10.1.10.10

(Optional) Specifies an IRDP address and preference to proxy-advertise.

Step 11

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 12

show ip irdp

Example:


Switch# show ip irdp

Verifies settings by displaying IRDP values.

Step 13

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

Configuring Broadcast Packet Handling

Perform the tasks in these sections to enable these schemes:

Enabling Directed Broadcast-to-Physical Broadcast Translation
Forwarding UDP Broadcast Packets and Protocols
Establishing an IP Broadcast Address
Flooding IP Broadcasts

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. Many implementations, including the one in the Switch, support several addressing schemes for forwarding broadcast messages.

Enabling Directed Broadcast-to-Physical Broadcast Translation

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

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

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

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.

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	interface `interface-id` Example: `Switch(config)# interface gigabitethernet 1/0/2`	Enters interface configuration mode, and specifies the interface to configure.
Step 4	ip directed-broadcast [`access-list-number`] Example: `Switch(config-if)# ip directed-broadcast 103`	Enables directed broadcast-to-physical broadcast translation on the interface. You can include an access list to control which broadcasts are forwarded. When an access list, only IP packets permitted by the access list can be translated.
Step 5	exit Example: `Switch(config-if)# exit`	Returns to global configuration mode.
Step 6	ip forward-protocol {udp [`port`] \| nd \| sdns } Example: `Switch(config)# ip forward-protocol nd`	Specifies which protocols and ports the router forwards when forwarding broadcast packets. udp —Forward UPD datagrams. port: (Optional) Destination port that controls which UDP services are forwarded. nd —Forward ND datagrams. sdns —Forward SDNS datagrams
Step 7	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 8	show ip interface [`interface-id`] Example: `Switch# show ip interface`	Verifies the configuration on the interface or all interfaces
Step 9	show running-config Example: `Switch# show running-config`	Verifies your entries.
Step 10	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

UDP Broadcast Packets and Protocols

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

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

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

Forwarding UDP Broadcast Packets and Protocols

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.

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.

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	interface `interface-id` Example: `Switch(config)# interface gigabitethernet 1/0/1`	Enters interface configuration mode, and specifies the Layer 3 interface to configure.
Step 4	ip helper-address `address` Example: `Switch(config-if)# ip helper address 10.1.10.1`	Enables forwarding and specifies the destination address for forwarding UDP broadcast packets, including BOOTP.
Step 5	exit Example: `Switch(config-if)# exit`	Returns to global configuration mode.
Step 6	ip forward-protocol {udp [`port`] \| nd \| sdns } Example: `Switch(config)# ip forward-protocol sdns`	Specifies which protocols the router forwards when forwarding broadcast packets.
Step 7	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 8	show ip interface [`interface-id`] Example: `Switch# show ip interface gigabitethernet 1/0/1`	Verifies the configuration on the interface or all interfaces.
Step 9	show running-config Example: `Switch# show running-config`	Verifies your entries.
Step 10	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

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.

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

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	interface `interface-id` Example: `Switch(config)# interface gigabitethernet 1/0/1`	Enters interface configuration mode, and specifies the interface to configure.
Step 4	ip broadcast-address `ip-address` Example: `Switch(config-if)# ip broadcast-address 128.1.255.255`	Enters a broadcast address different from the default, for example 128.1.255.255.
Step 5	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 6	show ip interface [`interface-id`] Example: `Switch# show ip interface`	Verifies the broadcast address on the interface or all interfaces.
Step 7	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

IP Broadcast Flooding

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

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

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

The packet must be a MAC-level broadcast.
The packet must be an IP-level broadcast.
The packet must be a TFTP, DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP specified by the ip forward-protocol udp global configuration command.
The time-to-live (TTL) value of the packet must be at least two.

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

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

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

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.

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

Flooding IP Broadcasts

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	ip forward-protocol spanning-tree Example: `Switch(config)# ip forward-protocol spanning-tree`	Uses the bridging spanning-tree database to flood UDP datagrams.
Step 4	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 5	show running-config Example: `Switch# show running-config`	Verifies your entries.
Step 6	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.
Step 7	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 8	ip forward-protocol turbo-flood Example: `Switch(config)# ip forward-protocol turbo-flood`	Uses the spanning-tree database to speed up flooding of UDP datagrams.
Step 9	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 10	show running-config Example: `Switch# show running-config`	Verifies your entries.
Step 11	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

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. The Table lists the commands for clearing contents.

Table 2. Commands to Clear Caches, Tables, and Databases

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

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

Table 3. Commands to Display Caches, Tables, and Databases

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

Enabling IP Unicast Routing

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

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

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	ip routing Example: `Switch(config)# ip routing`	Enables IP routing.
Step 4	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 5	show running-config Example: `Switch# show running-config`	Verifies your entries.
Step 6	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Example of Enabling IP Routing

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


Switch# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)# ip routing

Switch(config-router)# end

Information About RIP

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

Note

RIP is supported in the .

Note

RIP is the only routing protocol supported by the IP base feature set; other routing protocols require the switch or stack master to be running the IP services feature set.

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

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

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

Default RIP Configuration

Table 4. Default RIP Configuration
Feature	Default Setting
Auto summary	Enabled.
Default-information originate	Disabled.
Default metric	Built-in; automatic metric translations.
IP RIP authentication key-chain	No authentication. Authentication mode: clear text.
IP RIP triggered	Disabled
IP RIP receive version	According to the version router configuration command.
IP RIP send version	According to the version router configuration command.
IP RIP triggered	According to the version router configuration command.
IP split horizon	Varies with media.
Neighbor	None defined.
Network	None specified.
Offset list	Disabled.
Output delay	0 milliseconds.
Timers basic	Update: 30 seconds. Invalid: 180 seconds. Hold-down: 180 seconds. Flush: 240 seconds.
Validate-update-source	Enabled.
Version	Receives RIP Version 1 and 2 packets; sends Version 1 packets.

Configuring Basic RIP Parameters

To configure RIP, you enable RIP routing for a network and optionally configure other parameters. On the Switch, RIP configuration commands are ignored until you configure the network number.

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.

Procedure

Command or Action Purpose

Step 1

enable

Example:


Switch> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 3

ip routing

Example:


Switch(config)# ip routing

Enables IP routing. (Required only if IP routing is disabled.)

Step 4

router rip

Example:


Switch(config)# router rip

Enables a RIP routing process, and enter router configuration mode.

Step 5

network network number

Example:


Switch(config)# network 12

Associates a network with a RIP routing process. You can specify multiple network commands. RIP routing updates are sent and received through interfaces only on these networks.

Note

You must configure a network number for the RIP commands to take effect.

Step 6

neighbor ip-address

Example:


Switch(config)# neighbor 10.2.5.1

(Optional) Defines a neighboring router with which to exchange routing information. This step allows routing updates from RIP (normally a broadcast protocol) to reach nonbroadcast networks.

Step 7

offset-list [access-list number | name] {in | out } offset [type number]

Example:


Switch(config)# offset-list 103 in 10

(Optional) Applies an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through RIP. You can limit the offset list with an access list or an interface.

Step 8

timers basic update invalid holddown flush

Example:


Switch(config)# timers basic 45 360 400 300

(Optional) Adjusts routing protocol timers. Valid ranges for all timers are 0 to 4294967295 seconds.

update —The time between sending routing updates. The default is 30 seconds.
invalid —The timer after which a route is declared invalid. The default is 180 seconds.
holddown —The time before a route is removed from the routing table. The default is 180 seconds.
flush —The amount of time for which routing updates are postponed. The default is 240 seconds.

Step 9

version {1 | 2 }

Example:


Switch(config)# version 2

(Optional) Configures the switch to receive and send only RIP Version 1 or RIP Version 2 packets. By default, the switch receives Version 1 and 2 but sends only Version 1.  You can also use the interface commands ip rip {send | receive } version 1 | 2 | 1 2 } to control what versions are used for sending and receiving on interfaces.

Step 10

no auto summary

Example:


Switch(config)# no auto summary

(Optional) Disables automatic summarization. By default, the switch summarizes subprefixes when crossing classful network boundaries. Disable summarization (RIP Version 2 only) to advertise subnet and host routing information to classful network boundaries.

Step 11

no validate-update-source

Example:


Switch(config)# no validdate-update-source

(Optional) Disables validation of the source IP address of incoming RIP routing updates. By default, the switch validates the source IP address of incoming RIP routing updates and discards the update if the source address is not valid. Under normal circumstances, disabling this feature is not recommended. However, if you have a router that is off-network and you want to receive its updates, you can use this command.

Step 12

output-delay delay

Example:


Switch(config)# output-delay 8

(Optional) Adds interpacket delay for RIP updates sent. By default, packets in a multiple-packet RIP update have no delay added between packets. If you are sending packets to a lower-speed device, you can add an interpacket delay in the range of 8 to 50 milliseconds.

Step 13

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 14

show ip protocols

Example:


Switch# show ip protocols

Verifies your entries.

Step 15

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

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.

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

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.

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	interface `interface-id` Example: `Switch(config)# interface gigabitethernet 1/0/1`	Enters interface configuration mode, and specifies the interface to configure.
Step 4	ip rip authentication key-chain `name-of-chain` Example: `Switch(config-if)# ip rip authentication key-chain trees`	Enables RIP authentication.
Step 5	ip rip authentication mode {text \| md5 } Example: `Switch(config-if)# ip rip authentication mode md5`	Configures the interface to use plain text authentication (the default) or MD5 digest authentication.
Step 6	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 7	show running-config Example: `Switch# show running-config`	Verifies your entries.
Step 8	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Summary Addresses and Split Horizon

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

Configuring Summary Addresses and Split Horizon

Note

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

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

Note

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

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

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	interface `interface-id` Example: `Switch(config)# interface gigabitethernet 1/0/1`	Enters interface configuration mode, and specifies the Layer 3 interface to configure.
Step 4	ip address `ip-address subnet-mask` Example: `Switch(config-if)# ip address 10.1.1.10 255.255.255.0`	Configures the IP address and IP subnet.
Step 5	ip summary-address rip ip address `ip-network mask` Example: `Switch(config-if)# ip summary-address rip ip address 10.1.1.30 255.255.255.0`	Configures the IP address to be summarized and the IP network mask.
Step 6	no ip split horizon Example: `Switch(config-if)# no ip split horizon`	Disables split horizon on the interface.
Step 7	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 8	show ip interface `interface-id` Example: `Switch# show ip interface gigabitethernet 1/0/1`	Verifies your entries.
Step 9	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Configuring Split Horizon

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

Note

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

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

Procedure

	Command or Action	Purpose
Step 1	enable Example: `Switch> enable`	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 3	interface `interface-id` Example: `Switch(config)# interface gigabitethernet 1/0/1`	Enters interface configuration mode, and specifies the interface to configure.
Step 4	ip address `ip-address subnet-mask` Example: `Switch(config-if)# ip address 10.1.1.10 255.255.255.0`	Configures the IP address and IP subnet.
Step 5	no ip split-horizon Example: `Switch(config-if)# no ip split-horizon`	Disables split horizon on the interface.
Step 6	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 7	show ip interface `interface-id` Example: `Switch# show ip interface gigabitethernet 1/0/1`	Verifies your entries.
Step 8	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Configuration Example for Summary Addresses and Split Horizon

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. In the example, if the interface is still in Layer 2 mode (the default), you must enter a no switchport interface configuration command before entering the ip address interface configuration command.

Note

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


Switch(config)# router rip
Switch(config-router)# interface gigabitethernet1/0/2
Switch(config-if)# ip address 10.1.5.1 255.255.255.0
Switch(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0
Switch(config-if)# no ip split-horizon
Switch(config-if)# exit
Switch(config)# router rip
Switch(config-router)# network 10.0.0.0
Switch(config-router)# neighbor 2.2.2.2 peer-group mygroup
Switch(config-router)# end

Information About OSPF

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

Note

OSPF classifies different media into broadcast, nonbroadcast, and point-to-point networks. The switch supports broadcast (Ethernet, Token Ring, and FDDI) and point-to-point networks (Ethernet interfaces configured as point-to-point links).

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

Definition of stub areas is supported.
Routes learned through any IP routing protocol can be redistributed into another IP routing protocol. At the intradomain level, this means that OSPF can import routes learned through EIGRP and RIP. OSPF routes can also be exported into RIP.
Plain text and MD5 authentication among neighboring routers within an area is supported.
Configurable routing interface parameters include interface output cost, retransmission interval, interface transmit delay, router priority, router dead and hello intervals, and authentication key.
Virtual links are supported.
Not-so-stubby-areas (NSSAs) per RFC 1587are supported.

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

How to Configure OSPF

Default OSPF Configuration

Table 5. Default OSPF Configuration

Feature

Default Setting

Interface parameters

Cost: No default cost predefined

Retransmit interval: 5 seconds.

Transmit delay: 1 second.

Priority: 1.

Hello interval: 10 seconds.

Dead interval: 4 times the hello interval.

No authentication.

No password specified.

MD5 authentication disabled.

Area

Authentication type: 0 (no authentication).

Default cost: 1.

Range: Disabled.

Stub: No stub area defined.

NSSA: No NSSA area defined.

Auto cost

100 Mb/s.

Default-information originate

Disabled. When enabled, the default metric setting is 10, and the external route type default is Type 2.

Default metric

Built-in, automatic metric translation, as appropriate for each routing protocol.

Distance OSPF

dist1 (all routes within an area): 110.

 dist2 (all routes from one area to another): 110. 

dist3 (routes from other routing domains): 110.

OSPF database filter

Disabled. All outgoing link-state advertisements (LSAs) are flooded to the interface.

IP OSPF name lookup

Disabled.

Log adjacency changes

Enabled.

Neighbor

None specified.

Neighbor database filter

Disabled. All outgoing LSAs are flooded to the neighbor.

Network area

Disabled.

Nonstop Forwarding (NSF) awareness

Enabled. Allows Layer 3 Switch to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes.

NSF capability

Disabled.

Note

The Switch stack supports OSPF NSF-capable routing for IPv4.

Router ID

No OSPF routing process defined.

Summary address

Disabled.

Timers LSA group pacing

240 seconds.

Timers shortest path first (spf)

spf delay: 5 seconds.; spf-holdtime: 10 seconds.

Virtual link

No area ID or router ID defined.

Hello interval: 10 seconds.

Retransmit interval: 5 seconds.

Transmit delay: 1 second.

Dead interval: 40 seconds.

Authentication key: no key predefined.

Message-digest key (MD5): no key predefined.

OSPF for Routed Access

With Cisco IOS Release 12.2(55)SE, the IP Base image supports OSPF for routed access. The IP services image is required if you need multiple OSPFv2 and OSPFv3 instances without route restrictions. Additionally, the IP services image is required to enable the multi-VRF-CE feature.

OSPF for Routed Access is specifically designed so that you can extend Layer 3 routing capabilities to the wiring closet.

Note

OSPF for Routed Access supports only one OSPFv2 and one OSPFv3 instance with a combined total of 200 dynamically learned routes. The IP Base image provides OSPF for routed access.

However, these restrictions are not enforced in this release.

With the typical topology (hub and spoke) in a campus environment, where the wiring closets (spokes) are connected to the distribution switch (hub) that forwards all nonlocal traffic to the distribution layer, the wiring closet Switch need not hold a complete routing table. A best practice design, where the distribution Switch sends a default route to the wiring closet Switch to reach interarea and external routes (OSPF stub or totally stub area configuration) should be used when OSPF for Routed Access is used in the wiring closet.

For more details, see the “High Availability Campus Network Design—Routed Access Layer using EIGRP or OSPF” document.

OSPF Nonstop Forwarding

The Switch or switch stack supports two levels of nonstop forwarding (NSF):

OSPF NSF Awareness
OSPF NSF Capability

OSPF NSF Awareness

The IP-services feature set supports OSPF NSF Awareness supported for IPv4. 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.

OSPF NSF Capability

The IP services feature set supports the OSPFv2 NSF IETF format in addition to the OSPFv2 NSF Cisco format that is supported in earlier releases. For information about this feature, see : NSF—OSPF (RFC 3623 OSPF Graceful Restart).

The IP-services feature set also supports OSPF NSF-capable routing for IPv4 for better convergence and lower traffic loss following a stack master change. When a stack master change occurs in an OSPF NSF-capable stack, the new stack master must do two things to resynchronize its link-state database with its OSPF neighbors:

Release the available OSPF neighbors on the network without resetting the neighbor relationship.
Reacquire the contents of the link-state database for the network.

After a stack master change, the new master sends an OSPF NSF signal to neighboring NSF-aware devices. A device recognizes this signal to mean that it should not reset the neighbor relationship with the stack. As the NSF-capable stack master receives signals from other routes on the network, it begins to rebuild its neighbor list.

When the neighbor relationships are reestablished, the NSF-capable stack master resynchronizes its database with its NSF-aware neighbors, and routing information is exchanged between the OSPF neighbors. The new stack master uses this routing information to remove stale routes, to update the routing information database (RIB), and to update the forwarding information base (FIB) with the new information. The OSPF protocols then fully converge.

Note

OSPF NSF requires that all neighbor networking devices be NSF-aware. If an NSF-capable router discovers non-NSF aware neighbors on a network segment, it disables NSF capabilities for that segment. Other network segments where all devices are NSF-aware or NSF-capable continue to provide NSF capabilities.

Use the nsf OSPF routing configuration command to enable OSPF NSF routing. Use the show ip ospf privileged EXEC command to verify that it is enabled.

For more information, see Cisco Nonstop Forwarding: http://www.cisco.com/en/US/docs/ios/ha/configuration/guide/ha-nonstp_fwdg.html

Note

NSF is not supported on interfaces configured for Hot Standby Router Protocol (HSRP).

Configuring Basic OSPF Parameters

To enable OSPF, create an OSPF routing process, specify the range of IP addresses to associate with the routing process, and assign area IDs to be associated with that range. For switches running the IP services image, you can configure either the Cisco OSPFv2 NSF format or the IETF OSPFv2 NSF format.

Procedure

Command or Action Purpose

Step 1

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 2

router ospf process-id

Example:


Switch(config)# router ospf 15

Enables OSPF routing, and enter router configuration mode. The process ID is an internally used identification parameter that is locally assigned and can be any positive integer. Each OSPF routing process has a unique value.

Note

OSPF for Routed Access supports only one OSPFv2 and one OSPFv3 instance with a maximum number of 200 dynamically learned routes.

Step 3

nsf cisco [enforce global ]

Example:


Switch(config)# nsf cisco enforce global

(Optional) Enables Cisco NSF operations for OSPF. The enforce global keyword cancels NSF restart when non-NSF-aware neighboring networking devices are detected.

Note

Enter the command in Step 3 or Step 4, and go to Step 5.

Step 4

nsf ietf [restart-interval seconds]

Example:


Switch(config)# nsf ietf restart-interval 60

(Optional) Enables IETF NSF operations for OSPF. The restart-interval keyword specifies the length of the graceful restart interval, in seconds. The range is from 1 to 1800. The default is 120.

Note

Enter the command in Step 3 or Step 4, and go to Step 5.

Step 5

network address wildcard-mask area area-id

Example:


Switch(config)# network 10.1.1.1 255.240.0.0 area 20

Define an interface on which OSPF runs and the area ID for that interface. You can use the wildcard-mask to use a single command to define one or more multiple interfaces to be associated with a specific OSPF area. The area ID can be a decimal value or an IP address.

Step 6

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 7

show ip protocols

Example:


Switch# show ip protocols

Verifies your entries.

Step 8

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

Example: Configuring Basic OSPF Parameters

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


Switch(config)# router ospf 109
Switch(config-router)# network 131.108.0.0 255.255.255.0 area 24

Configuring OSPF Interfaces

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

Note

The ip ospf interface configuration commands are all optional.

Procedure

	Command or Action	Purpose
Step 1	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 2	interface interface-id Example: `Switch(config)# interface gigabitethernet 1/0/1`	Enters interface configuration mode, and specifies the Layer 3 interface to configure.
Step 3	ip ospf cost Example: `Switch(config-if)# ip ospf 8`	(Optional) Explicitly specifies the cost of sending a packet on the interface.
Step 4	ip ospf retransmit-interval seconds Example: `Switch(config-if)# ip ospf transmit-interval 10`	(Optional) Specifies the number of seconds between link state advertisement transmissions. The range is 1 to 65535 seconds. The default is 5 seconds.
Step 5	ip ospf transmit-delay seconds Example: `Switch(config-if)# ip ospf transmit-delay 2`	(Optional) Sets the estimated number of seconds to wait before sending a link state update packet. The range is 1 to 65535 seconds. The default is 1 second.
Step 6	ip ospf priority number Example: `Switch(config-if)# ip ospf priority 5`	(Optional) Sets priority to help find the OSPF designated router for a network. The range is from 0 to 255. The default is 1.
Step 7	ip ospf hello-interval seconds Example: `Switch(config-if)# ip ospf hello-interval 12`	(Optional) Sets the number of seconds between hello packets sent on an OSPF interface. The value must be the same for all nodes on a network. The range is 1 to 65535 seconds. The default is 10 seconds.
Step 8	ip ospf dead-interval seconds Example: `Switch(config-if)# ip ospf dead-interval 8`	(Optional) Sets the number of seconds after the last device hello packet was seen before its neighbors declare the OSPF router to be down. The value must be the same for all nodes on a network. The range is 1 to 65535 seconds. The default is 4 times the hello interval.
Step 9	ip ospf authentication-key key Example: `Switch(config-if)# ip ospf authentication-key password`	(Optional) Assign a password to be used by neighboring OSPF routers. The password can be any string of keyboard-entered characters up to 8 bytes in length. All neighboring routers on the same network must have the same password to exchange OSPF information.
Step 10	ip ospf message digest-key keyid md5 key Example: `Switch(config-if)# ip ospf message digest-key 16 md5 your1pass`	(Optional) Enables MDS authentication. `keyid` —An identifier from 1 to 255. `key` —An alphanumeric password of up to 16 bytes.
Step 11	ip ospf database-filter all out Example: `Switch(config-if)# ip ospf database-filter all out`	(Optional) Block flooding of OSPF LSA packets to the interface. By default, OSPF floods new LSAs over all interfaces in the same area, except the interface on which the LSA arrives.
Step 12	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 13	show ip ospf interface [`interface-name`] Example: `Switch# show ip ospf interface`	Displays OSPF-related interface information.
Step 14	show ip ospf neighbor detail Example: `Switch# show ip ospf neighbor detail`	Displays NSF awareness status of neighbor switch. The output matches one of these examples: `Options is 0x52` `LLS Options is 0x1 (LR)` When both of these lines appear, the neighbor switch is NSF aware. `Options is 0x42` —This means the neighbor switch is not NSF aware.
Step 15	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

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.

Configuring OSPF Area Parameters

Before you begin

Note

The OSPF area router configuration commands are all optional.

Procedure

	Command or Action	Purpose
Step 1	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 2	router ospf process-id Example: `Switch(config)# router ospf 109`	Enables OSPF routing, and enter router configuration mode.
Step 3	area `area-id` authentication Example: `Switch(config-router)# area 1 authentication`	(Optional) Allow password-based protection against unauthorized access to the identified area. The identifier can be either a decimal value or an IP address.
Step 4	area `area-id` authentication message-digest Example: `Switch(config-router)# area 1 authentication message-digest`	(Optional) Enables MD5 authentication on the area.
Step 5	area `area-id` stub [no-summary ] Example: `Switch(config-router)# area 1 stub`	(Optional) Define an area as a stub area. The no-summary keyword prevents an ABR from sending summary link advertisements into the stub area.
Step 6	area `area-id` nssa [no-redistribution ] [default-information-originate ] [no-summary ] Example: `Switch(config-router)# area 1 nssa default-information-originate`	(Optional) Defines an area as a not-so-stubby-area. Every router within the same area must agree that the area is NSSA. Select one of these keywords: no-redistribution —Select when the router is an NSSA ABR and you want the redistribute command to import routes into normal areas, but not into the NSSA. default-information-originate —Select on an ABR to allow importing type 7 LSAs into the NSSA. no-redistribution —Select to not send summary LSAs into the NSSA.
Step 7	area `area-id` range `address mask` Example: `Switch(config-router)# area 1 range 255.240.0.0`	(Optional) Specifies an address range for which a single route is advertised. Use this command only with area border routers.
Step 8	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 9	show ip ospf [`process-id`] Example: `Switch# show ip ospf`	Displays information about the OSPF routing process in general or for a specific process ID to verify configuration.
Step 10	show ip ospf [`process-id` [`area-id`]] database Example: `Switch# show ip osfp database`	Displays lists of information related to the OSPF database for a specific router.
Step 11	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Other OSPF Parameters

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

Route summarization: When redistributing routes from other protocols. 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.

Configuring Other OSPF Parameters

Procedure

	Command or Action	Purpose
Step 1	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 2	router ospf `process-id` Example: `Switch(config)# router ospf 10`	Enables OSPF routing, and enter router configuration mode.
Step 3	summary-address `address mask` Example: `Switch(config)# summary-address 10.1.1.1 255.255.255.0`	(Optional) Specifies an address and IP subnet mask for redistributed routes so that only one summary route is advertised.
Step 4	area `area-id` virtual-link `router-id` [hello-interval `seconds`] [retransmit-interval `seconds`] [trans ] [[authentication-key `key`] \| message-digest-key `keyid` md5 `key`]] Example: `Switch(config)# area 2 virtual-link 192.168.255.1 hello-interval 5`	(Optional) Establishes a virtual link and set its parameters.
Step 5	default-information originate [always ] [metric `metric-value`] [metric-type `type-value`] [route-map `map-name`] Example: `Switch(config)# default-information originate metric 100 metric-type 1`	(Optional) Forces the ASBR to generate a default route into the OSPF routing domain. Parameters are all optional.
Step 6	ip ospf name-lookup Example: `Switch(config)# ip ospf name-lookup`	(Optional) Configures DNS name lookup. The default is disabled.
Step 7	ip auto-cost reference-bandwidth `ref-bw` Example: `Switch(config)# ip auto-cost reference-bandwidth 5`	(Optional) Specifies an address range for which a single route will be advertised. Use this command only with area border routers.
Step 8	distance ospf {[inter-area `dist1`] [inter-area `dist2`] [external `dist3`]} Example: `Switch(config)# distance ospf inter-area 150`	(Optional) Changes the OSPF distance values. The default distance for each type of route is 110. The range is 1 to 255.
Step 9	passive-interface `type number` Example: `Switch(config)# passive-interface gigabitethernet 1/0/6`	(Optional) Suppresses the sending of hello packets through the specified interface.
Step 10	timers throttle spf `spf-delay spf-holdtime spf-wait` Example: `Switch(config)# timers throttle spf 200 100 100`	(Optional) Configures route calculation timers. `spf-delay` —Delay between receiving a change to SPF calculation. The range is from 1 to 600000 miliseconds. `spf-holdtime` —Delay between first and second SPF calculation. The range is form 1 to 600000 in milliseconds. `spf-wait` —Maximum wait time in milliseconds for SPF calculations. The range is from 1 to 600000 in milliseconds.
Step 11	ospf log-adj-changes Example: `Switch(config)# ospf log-adj-changes`	(Optional) Sends syslog message when a neighbor state changes.
Step 12	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 13	show ip ospf [`process-id` [`area-id`]] database Example: `Switch# show ip ospf database`	Displays lists of information related to the OSPF database for a specific router.
Step 14	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

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.

Changing LSA Group Pacing

Procedure

	Command or Action	Purpose
Step 1	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 2	router ospf `process-id` Example: `Switch(config)# router ospf 25`	Enables OSPF routing, and enter router configuration mode.
Step 3	timers lsa-group-pacing `seconds` Example: `Switch(config-router)# timers lsa-group-pacing 15`	Changes the group pacing of LSAs.
Step 4	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 5	show running-config Example: `Switch# show running-config`	Verifies your entries.
Step 6	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Loopback Interfaces

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.

Configuring a Loopback Interface

Procedure

	Command or Action	Purpose
Step 1	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 2	interface loopback 0 Example: `Switch(config)# interface loopback 0`	Creates a loopback interface, and enter interface configuration mode.
Step 3	ip address address mask Example: `Switch(config-if)# ip address 10.1.1.5 255.255.240.0`	Assign an IP address to this interface.
Step 4	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 5	show ip interface Example: `Switch# show ip interface`	Verifies your entries.
Step 6	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

Monitoring OSPF

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

Table 6. Show IP OSPF Statistics Commands

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

Information About EIGRP

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

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

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

EIGRP Features

EIGRP offers these features:

Fast convergence.
Incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table, minimizing the bandwidth required for EIGRP packets.
Less CPU usage because full update packets need not be processed each time they are received.
Protocol-independent neighbor discovery mechanism to learn about neighboring routers.
Variable-length subnet masks (VLSMs).
Arbitrary route summarization.
EIGRP scales to large networks.

EIGRP Components

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 with low overhead by periodically sending small hello packets. As long as hello packets are received, the Cisco IOS software can learn that a neighbor is alive and functioning. When this status is determined, the neighboring routers can exchange routing information.
The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP packets must be sent reliably, and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multiaccess network that has multicast capabilities (such as Ethernet), it is not necessary to send hellos reliably to all neighbors individually. Therefore, EIGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets (such as updates) require acknowledgment, which is shown in the packet. The reliable transport has a provision to send multicast packets quickly when there are unacknowledged packets pending. Doing so helps ensure that convergence time remains low in the presence of varying speed links.
The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses the distance information (known as a metric) to select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors, but there are neighbors advertising the destination, a recomputation must occur. This is the process whereby a new successor is determined. The amount of time it takes to recompute the route affects the convergence time. Recomputation is processor-intensive; it is advantageous to avoid recomputation if it is not necessary. When a topology change occurs, DUAL tests for feasible successors. If there are feasible successors, it uses any it finds to avoid unnecessary recomputation.

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

Note

To enable EIGRP, the Switch or stack master must be running the IP services feature set.

How to Configure EIGRP

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.

Note

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

Default EIGRP Configuration

Table 7. Default EIGRP Configuration

Feature

Default Setting

Auto summary

Disabled.

Subprefixes are summarized to the classful network boundary when crossing classful network boundaries.

Default-information

Exterior routes are accepted and default information is passed between EIGRP processes when doing redistribution.

Default metric

Only connected routes and interface static routes can be redistributed without a default metric. The metric includes:

Bandwidth: 0 or greater kb/s.
Delay (tens of microseconds): 0 or any positive number that is a multiple of 39.1 nanoseconds.
Reliability: any number between 0 and 255 (255 means 100 percent reliability).
Loading: effective bandwidth as a number between 0 and 255 (255 is 100 percent loading).
MTU: maximum transmission unit size of the route in bytes. 0 or any positive integer.

Distance

Internal distance: 90.

External distance: 170.

EIGRP log-neighbor changes

Disabled. No adjacency changes logged.

IP authentication key-chain

No authentication provided.

IP authentication mode

No authentication provided.

IP bandwidth-percent

50 percent.

IP hello interval

For low-speed nonbroadcast multiaccess (NBMA) networks: 60 seconds; all other networks: 5 seconds.

IP hold-time

For low-speed NBMA networks: 180 seconds; all other networks: 15 seconds.

IP split-horizon

Enabled.

IP summary address

No summary aggregate addresses are predefined.

Metric weights

tos: 0; k1 and k3: 1; k2, k4, and k5: 0

Network

None specified.

Nonstop Forwarding (NSF) Awareness

Enabled for IPv4 on switches running the IP services feature set. Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes.

NSF capability

Disabled.

Note

The Switch supports EIGRP NSF-capable routing for IPv4.

Offset-list

Disabled.

Router EIGRP

Disabled.

Set metric

No metric set in the route map.

Traffic-share

Distributed proportionately to the ratios of the metrics.

Variance

1 (equal-cost load-balancing).

EIGRP Nonstop Forwarding

The Switch stack supports two levels of EIGRP nonstop forwarding:

EIGRP NSF Awareness
EIGRP NSF Capability

EIGRP NSF Awareness

The IP-services feature set supports EIGRP NSF Awareness for IPv4. When the neighboring router is NSF-capable, the Layer 3 Switch continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in a router failing and the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software upgrade.

This feature cannot be disabled. For more information on this feature, see the “EIGRP Nonstop Forwarding (NSF) Awareness” section of the Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4.

EIGRP NSF Capability

The IP services feature set supports EIGRP Cisco NSF routing to speed up convergence and to eliminate traffic loss after a stack master change. For details about this NSF capability, see the “Configuring Nonstop Forwarding” chapter in the High Availability Configuration Guide, Cisco IOS XE Release 3S.

The IP-services feature set also supports EIGRP NSF-capable routing for IPv4 for better convergence and lower traffic loss following a stack master change. When an EIGRP NSF-capable stack master restarts or a new stack master starts up and NSF restarts, the Switch has no neighbors, and the topology table is empty. The Switch must bring up the interfaces, reacquire neighbors, and rebuild the topology and routing tables without interrupting the traffic directed toward the Switch stack. EIGRP peer routers maintain the routes learned from the new stack master and continue forwarding traffic through the NSF restart process.

To prevent an adjacency reset by the neighbors, the new stack master uses a new Restart (RS) bit in the EIGRP packet header to show the restart. When the neighbor receives this, it synchronizes the stack in its peer list and maintains the adjacency with the stack. The neighbor then sends its topology table to the stack master with the RS bit set to show that it is NSF-aware and is aiding the new stack master.

If at least one of the stack peer neighbors is NSF-aware, the stack master receives updates and rebuilds its database. Each NSF-aware neighbor sends an end of table (EOT) marker in the last update packet to mark the end of the table content. The stack master recognizes the convergence when it receives the EOT marker, and it then begins sending updates. When the stack master has received all EOT markers from its neighbors or when the NSF converge timer expires, EIGRP notifies the routing information database (RIB) of convergence and floods its topology table to all NSF-aware peers.

Note

NSF is not supported on interfaces configured for Hot Standby Router Protocol (HSRP).

Use the nsf EIGRP routing configuration command to enable EIGRP NSF routing. Use the show ip protocols privileged EXEC command to verify that NSF is enabled on the device. See the command reference for this release for information about the nsf command.

Configuring Basic EIGRP Parameters

Procedure

Command or Action Purpose

Step 1

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 2

router eigrp autonomous-system

Example:


Switch(config)# router eigrp 10

Enables an EIGRP routing process, and enter router configuration mode. The AS number identifies the routes to other EIGRP routers and is used to tag routing information.

Step 3

nsf

Example:


Switch(config)# nsf

(Optional) Enables EIGRP NSF. Enter this command on the stack master and on all of its peers.

Step 4

network network-number

Example:


Switch(config)# network 192.168.0.0

Associate networks with an EIGRP routing process. EIGRP sends updates to the interfaces in the specified networks.

Step 5

eigrp log-neighbor-changes

Example:


Switch(config)# eigrp log-neighbor-changes

(Optional) Enables logging of EIGRP neighbor changes to monitor routing system stability.

Step 6

metric weights tos k1 k2 k3 k4 k5

Example:


Switch(config)# metric weights 0 2 0 2 0 0

(Optional) Adjust the EIGRP metric. Although the defaults have been carefully set to provide excellent operation in most networks, you can adjust them.

Caution

Setting metrics is complex and is not recommended without guidance from an experienced network designer.

Step 7

offset-list [access-list number | name] {in | out } offset [type number]

Example:


Switch(config)# offset-list 21 out 10

(Optional) Applies an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through EIGRP. You can limit the offset list with an access list or an interface.

Step 8

auto-summary

Example:


Switch(config)# auto-summary

(Optional) Enables automatic summarization of subnet routes into network-level routes.

Step 9

ip summary-address eigrp autonomous-system-number address mask

Example:


Switch(config)# ip summary-address eigrp 1 192.168.0.0 255.255.0.0

(Optional) Configures a summary aggregate.

Step 10

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 11

show ip protocols

Example:


Switch# show ip protocols

Verifies your entries.

For NSF awareness, the output shows:

*** IP Routing is NSF aware *** EIGRP NSF enabled

Step 12

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

Configuring EIGRP Interfaces

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

Procedure

Command or Action Purpose

Step 1

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 2

interface interface-id

Example:


Switch(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 3

ip bandwidth-percent eigrp percent

Example:


Switch(config-if)# ip bandwidth-percent eigrp 60

(Optional) Configures the percentage of bandwidth that can be used by EIGRP on an interface. The default is 50 percent.

Step 4

ip summary-address eigrp autonomous-system-number address mask

Example:


Switch(config-if)# ip summary-address eigrp 109 192.161.0.0 255.255.0.0

(Optional) Configures a summary aggregate address for a specified interface (not usually necessary if auto-summary is enabled).

Step 5

ip hello-interval eigrp autonomous-system-number seconds

Example:


Switch(config-if)# ip hello-interval eigrp 109 10

(Optional) Change the hello time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 60 seconds for low-speed NBMA networks and 5 seconds for all other networks.

Step 6

ip hold-time eigrp autonomous-system-number seconds

Example:


Switch(config-if)# ip hold-time eigrp 109 40

(Optional) Change the hold time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 180 seconds for low-speed NBMA networks and 15 seconds for all other networks.

Caution

Do not adjust the hold time without consulting Cisco technical support.

Step 7

no ip split-horizon eigrp autonomous-system-number

Example:


Switch(config-if)# no ip split-horizon eigrp 109

(Optional) Disables split horizon to allow route information to be advertised by a router out any interface from which that information originated.

Step 8

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 9

show ip eigrp interface

Example:


Switch# show ip eigrp interface

Displays which interfaces EIGRP is active on and information about EIGRP relating to those interfaces.

Step 10

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

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.

Procedure

	Command or Action	Purpose
Step 1	configure terminal Example: `Switch# configure terminal`	Enters the global configuration mode.
Step 2	interface interface-id Example: `Switch(config)# interface gigabitethernet 1/0/1`	Enters interface configuration mode, and specifies the Layer 3 interface to configure.
Step 3	ip authentication mode eigrp `autonomous-system`md5 Example: `Switch(config-if)# ip authentication mode eigrp 104 md5`	Enables MD5 authentication in IP EIGRP packets.
Step 4	ip authentication key-chain eigrp `autonomous-system key-chain` Example: `Switch(config-if)# ip authentication key-chain eigrp 105 chain1`	Enables authentication of IP EIGRP packets.
Step 5	exit Example: `Switch(config-if)# exit`	Returns to global configuration mode.
Step 6	key chain `name-of-chain` Example: `Switch(config)# key chain chain1`	Identify a key chain and enter key-chain configuration mode. Match the name configured in Step 4.
Step 7	key `number` Example: `Switch(config-keychain)# key 1`	In key-chain configuration mode, identify the key number.
Step 8	key-string `text` Example: `Switch(config-keychain-key)# key-string key1`	In key-chain key configuration mode, identify the key string.
Step 9	accept-lifetime `start-time` {infinite \| `end-time` \| duration `seconds`} Example: `Switch(config-keychain-key)# accept-lifetime 13:30:00 Jan 25 2011 duration 7200`	(Optional) Specifies the time period during which the key can be received. The `start-time` and `end-time` syntax can be either `hh:mm:ss Month date year` or `hh:mm:ss date Month year` . The default is forever with the default `start-time` and the earliest acceptable date as January 1, 1993. The default `end-time` and duration is infinite .
Step 10	send-lifetime `start-time` {infinite \| `end-time` \| duration `seconds`} Example: `Switch(config-keychain-key)# send-lifetime 14:00:00 Jan 25 2011 duration 3600`	(Optional) Specifies the time period during which the key can be sent. The `start-time` and `end-time` syntax can be either `hh:mm:ss Month date year` or `hh:mm:ss date Month year` . The default is forever with the default `start-time` and the earliest acceptable date as January 1, 1993. The default `end-time` and duration is infinite .
Step 11	end Example: `Switch(config)# end`	Returns to privileged EXEC mode.
Step 12	show key chain Example: `Switch# show key chain`	Displays authentication key information.
Step 13	copy running-config startup-config Example: `Switch# copy running-config startup-config`	(Optional) Saves your entries in the configuration file.

EIGRP Stub Routing

The EIGRP stub routing feature reduces resource utilization by moving routed traffic closer to the end user.

Note

The feature set contains EIGRP stub routing capability, which 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.

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.

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 the figure given below, 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 4. EIGRP Stub Router Configuration

For more information about EIGRP stub routing, see “Configuring EIGRP Stub Routing” section of the Cisco IOS IP Configuration Guide, Volume 2 of 3: Routing Protocols.

Monitoring and Maintaining EIGRP

You can delete neighbors from the neighbor table. You can also display various EIGRP routing statistics. The table given below lists the privileged EXEC commands for deleting neighbors and displaying statistics. For explanations of fields in the resulting display, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.4.

Table 8. IP EIGRP Clear and Show Commands

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

Information About BGP

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

For details about BGP commands and keywords, see the “IP Routing Protocols” part of the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols .

BGP Network Topology

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). The figure given below shows a network that is running both EBGP and IBGP.

Figure 5. 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 the above figure, Rou

Bias-Free Language

Results

Chapter: Configuring IP Unicast Routing

Configuring IP Unicast Routing

Finding Feature Information

Information About Configuring IP Unicast Routing

Information About IP Routing

Types of Routing

IP Routing and Switch Stacks

How to Configure IP Routing

How to Configure IP Addressing

Default IP Addressing Configuration

Assigning IP Addresses to Network Interfaces

Procedure

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Using Subnet Zero

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Classless Routing

Disabling Classless Routing

Procedure

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Configuring Address Resolution Methods

Address Resolution

Defining a Static ARP Cache

Procedure

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Setting ARP Encapsulation

Procedure

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Enabling Proxy ARP

Procedure

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Routing Assistance When IP Routing is Disabled

Proxy ARP

Proxy ARP

Default Gateway