EIGRP Split Horizon

EIGRP actually runs its own version of split horizon for all of the protocols that it supports: IP, Internetwork Packet Exchange (IPX), and Appletalk.

Example For Split Horizon Cisco

Frame Relay Technologies and split horizon are a routing technique used with classful routing protocols in which routing updates are prevented from being advertised out the same interface from which they were learned. Split horizon issues are most prevalent in NBMA hub and spoke networks. To remedy this, split horizon is disabled by default on physical interfaces and point-to-multipoint subinterfaces when they are configured for Frame Relay encapsulation.

This default setting alleviates reachability issues, but can cause other problems that you need to be aware of, such as routing loops. This default behavior applies to RIP and IGRP, however EIGRP runs its own version of split horizon that must be explicitly disabled for both IP and IPX in a NBMA hub and spoke environment. An alternative to disabling split horizon is the use of point-to-point subinterfaces.

As shown in the example, R2 receives routing updates from R4 and Rl, but because of split horizon, R2 does not advertise the 40.0.0.0/24 network to Rl and the 10.0.0.0/24 network to R4. Therefore, the remote sites do not have full reachability.

Gateway of last resort is not set

20.0.0.0/24 is subnetted, 1 subnets D 20.1.1.0 [90/2172416] via 192.168.1.2, 00:00:28,

Serial0/0

40.0.0.0/24 is subnetted, 1 subnets C 40.1.1.0 is directly connected, FastEthernet0/0

10.0.0.0/24 is subnetted, 1 subnets D 10.1.1.0 [90/2684416] via 192.168.1.2, 00:00:28,

Serial0/0

C 192.168.1.0/24 is directly connected, Serial0/0

Gateway of last resort is not set 20.0.0.0/24 is subnetted, 1 subnets

40.0.0.0/24 is subnetted, 1 subnets D 40.1.1.0 [90/2276352] via 192.168.1.2, 00:00:15,

10.0.0.0/24 is subnetted, 1 subnets C 10.1.1.0 is directly connected, FastEthernet0/0

C 192.168.1.0/24 is directly connected, Serial0/0

Serial0/0 Serial0/0

© 2002, Cisco Systems, Inc. All rights reservec

To allow full reachability between the remote sites, you must disable split horizon on R2. This is done with the no ip split-horizon eigrp autonomous system command. This is an interface configuration command that must be placed on the interface for which you wish to disable EIGRP split horizon.

Notice in the example that the spoke routers now have full routing tables and therefore can reach all remote sites.

+2 -1

Post a comment