Earning your Cisco CCNA and CCNP is a tough proposition, and part of that is the fact that you quickly learn that there’s usually more than one way to do things with Cisco routers – and while that’s generally a good thing, you better know the ins and outs of all options when it comes to test day and working on production networks. Working with Frame Relay subinterfaces and split horizon is just one such situation.

One reason for the use of subinterfaces is to circumvent the rule of split horizon. You recall from your CCNA studies that split horizon dictates that a route cannot be advertised out the same interface upon which it was learned in the first place. In the following example, R1 is the hub and R2 and R3 are the spokes. All three routers are using their physical interfaces for frame relay connectivity, and they are also running RIPv2 /24. Each router is also advertising a loopback interface, using the router number for each octet.

R1(config)#int s0

R1(config-if)#ip address

R1(config-if)#no frame inverse

R1(config-if)#frame map ip 122 broadcast

R1(config-if)#frame map ip 123 broadcast

R1(config-if)#no shut

R2(config)#int s0

R2(config-if)#encap frame

R2(config-if)#no frame inver

R2(config-if)#frame map ip 221 broadcast

R2(config-if)#frame map ip 221 broadcast

R2(config-if)#ip address

R3(config)#int s0

R3(config-if)#encap frame

R3(config-if)#no frame inver

R3(config-if)#frame map ip 321 broadcast

R3(config-if)#frame map ip 321 broadcast

R3(config-if)#ip address

R1#show ip route rip is subnetted, 1 subnets

R [120/1] via, 00:00:20, Serial0 is subnetted, 1 subnets

R [120/1] via, 00:00:22, Serial0

R2#show ip route rip is subnetted, 1 subnets

R [120/1] via, 00:00:06, Serial0

R3#show ip route rip is subnetted, 1 subnets

R [120/1] via, 00:00:04, Serial0

The hub router R1 has a route to both loopbacks, but neither spoke has a route to the other spoke’s loopback. That’s because split horizon prevents R1 from advertising a network via Serial0 if the route was learned on Serial0 to begin with.

We’ve got two options here, one of which is to disable spilt horizon on the interface. While doing so will have the desired effect in our little network, disabling split horizon is not a good idea and should be avoided whenever possible. We’re not going to do it in this lab, but here is the syntax to do so:

R1(config)#interface serial0

R1(config-if)#no ip split-horizon