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 |
|
R3(config)#router rip |
|
R4(config)#router rip |
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 |
