Cisco CCNA / CCNP Certification Test Lab: Frame Relay Subinterfaces And Split Horizon
Earning the Cisco CCNA and CCNP is a tough proposition, and part of this is the fact that you quickly learn that there’s usually more than one way to try and do things with Cisco modems – and while that’s generally the good thing, you better know the ins and outs of almost all options when it comes to test evening and working on production sites. Working with Frame Relay subinterfaces and also split horizon is just one these kinds of situation.
One reason for the utilization of subinterfaces is to circumvent the principle of split horizon. You call to mind from your CCNA studies that break up horizon dictates that a way cannot be advertised out the identical interface upon which it was figured out in the first place. In the following example, R1 would be the hub and R2 and R3 will be the spokes. All three routers are using their own physical interfaces for framework relay connectivity, and they are also running RIPv2 172.12.123.0 /24. Each switch is also advertising a loopback user interface, using the router number for every single octet.
R1(config)#int s0
R1(config-if)#ip address 172.12.123.1 255.255.255.3
R1(config-if)#no frame inverse
R1(config-if)#frame map ip 172.12.123.2 122 broadcast
R1(config-if)#frame map ip address 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 internet protocol 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 internet protocol 172.12.123.1 321 broadcast
R3(config-if)#frame road ip 172.12.123.2 321 transmitted
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 internet protocol route rip
1.0.0.0/32 is subnetted, 1 subnets
3rd r 1.1.1.1 [120/1] via 172.12.123.1, 00:00:06, Serial0
R3#show ip option rip
1.0.0.0/32 is subnetted, 1 subnets
R 1.1.1.1 [120/1] by using 172.12.123.1, 00:00:04, Serial0
The hub hub R1 has a route to both loopbacks, although neither spoke has a route to the other spoke’s loopback. That’s because split skyline prevents R1 from advertising any network via Serial0 if the path was learned on Serial0 in the first place.
We’ve got two options in this article, one of which is to disable spilt horizon on the interface. While the process will have the desired effect in your little network, disabling split horizon is not a good idea and should be avoided whenever possible. We’re not likely to do it in this lab, yet here is the syntax to do so:
R1(config)#interface serial0
R1(config-if)#no internet protocol split-horizon
A better solution is to alter subinterfaces on R1. The IP addressing must be revisited, but that’s no problem here. R1 and also R2 will use 172.12.123.0 /24 to speak, while R1 and R3 will use 172.12.13.0 /24. R3′s serial0 interface will have to be renumbered, so let’s look at all three modem configurations:
R1(config)#interface serial0
R1(config-if)#encap frame
R1(config-if)#no frame inverse-arp
R1(config-if)#no ip
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 tackle 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 send out
R3(config-if)#frame map ip 172.12.123.2 321 broadcast
A frame chart statement always names your REMOTE IP address and the LOCAL DLCI. Don’t forget the broadcast option!
Show frame map shows people that all the static mappings with R1 are up and running. Note the “static” output, which indicates these mappings are a result of utilizing the frame map command. Pings will not be shown, but all three wireless routers can ping each other at this stage.
R1#show frame map
Serial0 (up): ip address 172.12.123.2 dlci 122(0x7A,0x1CA0), static,
transmitted, CISCO, status defined, energetic
Serial0 (up): ip 172.12.13.3 dlci 123(0x7B,0x1CB0), static,
broadcast, CISCO, status defined, active
Following 172.12.13.0 /24 community is added to R1 and R3’s Split configuration, R2 and R3 now have one another’s loopback network in their RIP course-plotting tables.
R2#show ip route tear
1.0.0.0/32 will be subnetted, 1 subnets
R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:20, Serial0
3.0.0.0/32 can be 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 will be subnetted, 1 subnets
R 2.2.2.2 [120/1] via 172.12.13.1, 00:00:22, Serial0
While turning divided horizon off is one way to attain total IP connectivity, the process can have other unintended outcomes. The use of subinterfaces is a more effective method of allowing the spokes to see the hub’s loopback network.
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