Cisco Systems 78-11741-02 manual Igrp Configuration Examples, IPC-219

Configuring IGRP

IGRP Configuration Examples

Enabling or Disabling Split Horizon

Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router out of any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks (such as Frame Relay and Switched Multimegabit Data Service [SMDS]), situations can arise for which this behavior is less than ideal. For these situations, you might want to disable split horizon.

If an interface is configured with secondary IP addresses and split horizon is enabled, updates might not be sourced by every secondary address. One routing update is sourced per network number unless split horizon is disabled.

To enable or disable split horizon, use the following commands in interface configuration mode as needed:

Command	Purpose
Router(config-if)# ip split-horizon	Enables split horizon.
Router(config-if)# no ip split-horizon	Disables split horizon.

Split horizon for Frame Relay and SMDS encapsulation is disabled by default. Split horizon is not disabled by default for interfaces using any of the X.25 encapsulations. For all other encapsulations, split horizon is enabled by default.

See the “Split Horizon Examples” section at the end of this chapter for examples of using split horizon.

Note In general, changing the state of the default is not recommended unless you are certain that your application requires making a change in order to advertise routes properly. Remember that if split horizon is disabled on a serial interface (and that interface is attached to a packet-switched network), you must disable split horizon for all routers in any relevant multicast groups on that network.

IGRP Configuration Examples

This section contains the following IGRP configuration examples:

•IGRP Feasible Successor Relationship Example

•Split Horizon Examples

Cisco IOS IP Configuration Guide

IPC-219