Split Horizon

Recall from earlier that split horizon is a routing technique in which information about routes is prevented from exiting the router interface or subinterface through which that information was received. Split horizon is most prevalent in multipoint networks. Here, routing updates flow into one subinterface but also must be sent out that very same subinterface to reach the other routers on the multipoint network. Split horizon is enabled by default and prevents specific route updates for EIGRP, IGRP, and RIP from being propagated properly in a multipoint configuration. Disable this with the no ip split-horizon eigrp autonomous system command. This command has similar forms for IPX and AppleTalk.

In Figure 11-4, the grinch router receives updates from the whos and whoville routers, but because of split horizon, the grinch does not advertise 172.16.5.0 and 172.16.6.0 out its serial 0.1 multipoint interface. Because the grinch didn't learn about the 172.16.2.0 network from its 0.1 interface, it advertises that network to the whos and whoville routers.

Figure 11-4 EIGRP Split Horizons Route Suppression

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 4 subnets D 172.16.5.0 [90/23394560] via 172.16.1.5, 00:00:11, Serial0.1 D 172.16.6.0 [90/23394560] via 172.16.1.6, 00:00:11, Serial0.1 C 172.16.1.0 is directly connected, Serial0.1 C 172.16.2.0 is directly connected, Ethernet1 grinch# E1

grinch

S0.1 Multipoint IP=172.16.1.1/24

S0.1 Multipoint IP=172.16.1.1/24

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 3 subnets C 172.16.5.0 is directly connected, Ethernet0 C 172.16.1.0 is directly connected, Serial0.1 D 172.16.2.0 [90/46251776] via 172.16.1.1, 00:02:07, Serial0.1 whoville#

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 3 subnets C 172.16.6.0 is directly connected, Ethernet0 C 172.16.1.0 is directly connected, Serial0 D 172.16.2.0 [90/46251776] via 172.16.1.1, 00:03:52, Serial0 whos#

To make the whos and whoville happy again, we need to disable split horizon on the grinch by using the no ip split-horizon eigrp command, as demonstrated in Example 11-6.

Example 11-6 Disabling Split Horizon on the grinch Router grinch(config)#int s0.1

grinch(config-subif)#no ip split-horizon eigrp 2001

Figure 11-5 illustrates how the routing tables will look after disabling split horizon on the grinch router. Notice that all routes are being propagated.

Figure 11-5 Fully Functional EIGRP Network

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 4 subnets D 172.16.5.0 [90/23394560] via 172.16.1.5, 00:06:06, Serial0.1 D 172.16.6.0 [90/23394560] via 172.16.1.6, 00:06:06, Serial0.1 C 172.16.1.0 is directly connected, Serial0.1 C 172.16.2.0 is directly connected, Ethernet1 grinch# e^

I if grinch

S0.1 Multipoint IP=172.16.1.1/24

S0.1 Multipoint IP=172.16.1.1/24

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 4 subnets C 172.16.5.0 is directly connected, Ethernet0 D 172.16.6.0 [90/46763776] via 172.16.1.1, 00:06:41, Serial0.1 C 172.16.1.0 is directly connected, Serial0.1 D 172.16.2.0 [90/46251776] via 172.16.1.1, 00:06:41, Serial0.1 whoville#

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 4 subnets D 172.16.5.0 [90/46763776] via 172.16.1.1, 00:07:11, Serial0 C 172.16.6.0 is directly connected, Ethernet0 C 172.16.1.0 is directly connected, Serial0 D 172.16.2.0 [90/46251776] via 172.16.1.1, 00:07:11, Serial0 whos#

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