Command
IP Split-horizon
Use
Split Horizon protects against routing loops by not accepting routes on the same interface that a router sends out the updates out.
This normally has to be turned off in hub and spoke topologies to ensure full reachability.
Syntax
Router(config-if)#no ip split-horizon
Example
In this example, we will configure RIP on the frame-relay links.
R1(config)#router rip R1(config-router)#version 2 R1(config-router)#no auto R1(config-router)#network 10.0.0.0 R1(config-router)#network 1.1.1.1
R3(config)#router rip R3(config-router)#version 2 R3(config-router)#no auto R3(config-router)#network 3.3.3.3 R3(config-router)#network 10.0.0.0
R4(config)#router rip R4(config-router)#version 2 R4(config-router)#no auto R4(config-router)#network 4.0.0.0 R4(config-router)#network 10.0.0.0
Looking at the routing table on R1 and R3 reveals that they are only learning R4’s routes.
R1(config)#do sh ip route rip 4.0.0.0/24 is subnetted, 1 subnets R 4.4.4.0 [120/1] via 10.4.4.4, 00:00:08, Serial1/0
R3(config-if)#do show ip route rip 4.0.0.0/24 is subnetted, 1 subnets R 4.4.4.0 [120/1] via 10.4.4.4, 00:00:03, Serial1/0
R4 receives both R1 and R3 routes just fine.
R4(config-router)#do show ip route rip 1.0.0.0/32 is subnetted, 3 subnets R 1.1.1.1 [120/1] via 10.4.4.1, 00:00:11, Serial0/0 R 1.3.3.3 [120/1] via 10.4.4.1, 00:00:11, Serial0/0 R 1.2.2.2 [120/1] via 10.4.4.1, 00:00:11, Serial0/0 3.0.0.0/32 is subnetted, 1 subnets R 3.3.3.3 [120/1] via 10.4.4.3, 00:00:08, Serial0/0
Now we will disable split-horizon on R4’s S0/0.
R4(config-router)#int s0/0 R4(config-if)#no ip split-horizon
And now we receive all routes.
R3(config-if)#do show ip route rip 1.0.0.0/32 is subnetted, 3 subnets R 1.1.1.1 [120/2] via 10.4.4.1, 00:00:02, Serial1/0 R 1.3.3.3 [120/2] via 10.4.4.1, 00:00:02, Serial1/0 R 1.2.2.2 [120/2] via 10.4.4.1, 00:00:02, Serial1/0 4.0.0.0/24 is subnetted, 1 subnets R 4.4.4.0 [120/1] via 10.4.4.4, 00:00:02, Serial1/0