To truly understand Frame Relay subinterfaces and split horizon, you've got to see both in action. Study for your exam and prepare for success with this fully illustrated lab from Chris Bryant, CCIE #12933.
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 172.12.123.0 /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 172.12.123.1 255.255.255.0
R1(config-if)#no frame inverse
R1(config-if)#frame map ip 172.12.123.2 122 broadcast
R1(config-if)#frame map ip 172.12.123.3 123 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 172.12.123.1 221 broadcast
R2(config-if)#frame map ip 172.12.123.3 221 broadcast
R2(config-if)#ip address 172.12.123.2 255.255.255.0
R3(config)#int s0
R3(config-if)#encap frame
R3(config-if)#no frame inver
R3(config-if)#frame map ip 172.12.123.1 321 broadcast
R3(config-if)#frame map ip 172.12.123.2 321 broadcast
R3(config-if)#ip address 172.12.123.3 255.255.255.0
R1#show ip route rip
2.0.0.0/32 is subnetted, 1 subnets
R 2.2.2.2 [120/1] via 172.12.123.2, 00:00:20, Serial0
3.0.0.0/32 is subnetted, 1 subnets
R 3.3.3.3 [120/1] via 172.12.123.3, 00:00:22, Serial0
R2#show ip route rip
1.0.0.0/32 is subnetted, 1 subnets
R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:06, Serial0
R3#show ip route rip
1.0.0.0/32 is subnetted, 1 subnets
R 1.1.1.1 [120/1] via 172.12.123.1, 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
A better solution is to configure subinterfaces on R1. The IP addressing will have to be revisited, but that's no problem here. R1 and R2 will use 172.12.123.0 /24 to communicate, while R1 and R3 will use 172.12.13.0 /24. R3's serial0 interface will need to be renumbered, so let's look at all three router configurations:
R1(config)#interface serial0
R1(config-if)#encap frame
R1(config-if)#no frame inverse-arp
R1(config-if)#no ip address
R1(config-if)#interface serial0.12 multipoint
R1(config-subif)#ip address 172.12.123.1 255.255.255.0
R1(config-subif)#frame map ip 172.12.123.2 122 broadcast
R1(config-subif)#interface serial0.31 point-to-point
R1(config-subif)#ip address 172.12.13.1 255.255.255.0
R1(config-subif)#frame interface-dlci 123
R2(config)#int s0
R2(config-if)#ip address 172.12.123.2 255.255.255.0
R2(config-if)#encap frame
R2(config-if)#frame map ip 172.12.13.3 221 broadcast
R2(config-if)#frame map ip 172.12.123.1 221 broadcast
R3(config)#int s0
R3(config-if)#ip address 172.12.13.3 255.255.255.0
R3(config-if)#encap frame
R3(config-if)#frame map ip 172.12.13.1 321 broadcast
R3(config-if)#frame map ip 172.12.123.2 321 broadcast
A frame map statement always names the REMOTE IP address and the LOCAL DLCI. Don't forget the broadcast option!
Show frame map shows us that all the static mappings on R1 are up and running. Note the "static" output, which indicates these mappings are a result of using the frame map command. Pings are not shown, but all three routers can ping each other at this point.
R1#show frame map
Serial0 (up): ip 172.12.123.2 dlci 122(0x7A,0x1CA0), static,
broadcast, CISCO, status defined, active
Serial0 (up): ip 172.12.13.3 dlci 123(0x7B,0x1CB0), static,
broadcast, CISCO, status defined, active
After the 172.12.13.0 /24 network is added to R1 and R3’s RIP configuration, R2 and R3 now have each other's loopback network in their RIP routing tables.
R2#show ip route rip
1.0.0.0/32 is subnetted, 1 subnets
R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:20, Serial0
3.0.0.0/32 is subnetted, 1 subnets
R 3.3.3.3 [120/1] via 172.12.123.1, 00:00:22, Serial0
R3#show ip route rip
1.0.0.0/32 is subnetted, 1 subnets
R 1.1.1.1 [120/1] via 172.12.13.1, 00:00:20, Serial0
2.0.0.0/32 is subnetted, 1 subnets
R 2.2.2.2 [120/1] via 172.12.13.1, 00:00:22, Serial0
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