Cisco Systems 78-11741-02 manual Disabling Split Horizon, Process, IPC-267

Configuring EIGRP

EIGRP Configuration Task List

By default, hello packets are sent every 5 seconds. The exception is on low-speed, nonbroadcast multiaccess (NBMA) media, where the default hello interval is 60 seconds. Low speed is considered to be a rate of T1 or slower, as specified with the bandwidth interface configuration command. The default hello interval remains 5 seconds for high-speed NBMA networks. Note that for the purposes of EIGRP, Frame Relay and Switched Multimegabit Data Service (SMDS) networks may or may not be considered to be NBMA. These networks are considered NBMA if the interface has not been configured to use physical multicasting; otherwise they are not considered NBMA.

You can configure the hold time on a specified interface for a particular EIGRP routing process designated by the autonomous system number. The hold time is advertised in hello packets and indicates to neighbors the length of time they should consider the sender valid. The default hold time is three times the hello interval, or 15 seconds. For slow-speed NBMA networks, the default hold time is 180 seconds.

To change the interval between hello packets, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)#ip hello-interval eigrp	Configures the hello interval for an EIGRP routing
autonomous-system-number seconds	process.

On very congested and large networks, the default hold time might not be sufficient time for all routers to receive hello packets from their neighbors. In this case, you may want to increase the hold time.

To change the hold time, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip hold-time eigrp	Configures the hold time for an EIGRP routing process.
autonomous-system-number seconds

Note Do not adjust the hold time without advising your technical support personnel.

Disabling Split Horizon

Split horizon controls the sending of EIGRP update and query packets. When split horizon is enabled on an interface, update and query packets are not sent for destinations for which this interface is the next hop. Controlling update and query packets in this manner reduces the possibility of routing loops.

By default, split horizon is enabled on all interfaces.

Split horizon blocks route information from being advertised by a router out of any interface from which that information originated. This behavior usually optimizes communications among multiple routing devices, particularly when links are broken. However, with nonbroadcast networks (such as Frame Relay and SMDS), situations can arise for which this behavior is less than ideal. For these situations, including networks in which you have EIGRP configured, you may want to disable split horizon.

To disable split horizon, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)#no ip split-horizon eigrp	Disables split horizon.
autonomous-system-number

Cisco IOS IP Configuration Guide

IPC-267