Gather your Cisco CCNA and CCNP is a statement hard, and one of them is that you quickly learn that it is usually more than one way, things with Cisco routers - and while this is generally a good thing, it's better to know the pros and cons of all options when it comes to days of testing and working on production networks. Working with Frame Relay subinterfaces and split horizon is just such a situation.
One reason for using subinterfaces is to circumvent the ruleSplit Horizon. Do you remember your CCNA studies requiring Split Horizon, has announced that a path can not be ruled out, the same interface that has been learned in the first place. In this example, the hub of R1 and R2 and R3 are the spokes. All three routers using their physical interfaces for frame relay connections, and run RIPv2 172.12.123.0 / 24, each router is advertising a loopback interface, the number of routers for eachOctet.
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) # ip 172.12.123.2 122 broadcast frame map
R1 (config-if) # ip 172.12.123.3 123 broadcast frame map
R1 (config-if) # no shutdown
R2 (config) # int s0
R2 (config-if) # encap frame
R2 (config-if) # no frame inversion
R2 (config-if) # frame map ip 172.12.123.1 221 broadcast
R2 (config-if) # frame map ip 172.12.123.3 221Mission
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 inversion
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 isSubnets, 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 path for both cycles, but did not speak a route to the other spoke of loopback. This is becauseSplit horizon prevents the advertisement of a network via Serial0 when the route was learned on Serial0, R1 to start.
We have two options, one of which paid off in the horizon on the interface. This is the desired effect in our small network, disable the split horizon is not a good idea and should be avoided whenever possible. We have no intention of doing so in the lab, but here is the syntax to do this:
R1 (config) # interface Serial0
R1 (config-if) # no ipSplit horizon
A better solution is to configure subinterfaces on R1. The IP address must be revised, but that is not a problem here. R1 and R2 use to communicate 172.12.123.0 / 24, while R1 and R3 to use 172.12.13.0 / 24. Interface Serial0 R3 must be numbered, and then 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 ipAddress
R1 (config-if) # interface serial0.12 multipoint
R1 (config-if) # ip address 172.12.123.1 255.255.255.0
R1 (config-if) # ip 172.12.123.2 122 broadcast frame map
R1 (config-if) # interface serial0.31 point-to-point
R1 (config-if) # ip address 172.12.13.1 255.255.255.0
R1 (config-if) # 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) # ip framework map172.12.13.3 221 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 statement setting accessible map the remote IP address and local DLCI. Do not forget the option of broadcasting!
Frame Show map shows us that all the static mappingR1 and running. Note: The "static" version, these mappings are a consequence of their use are part of the map command. Pings are not visible, 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 network 172.12.13.0 / 24R1 and R3 was added to the configuration of RIP, R2 and R3 are now mutually loopback network in its RIP routing table.
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
As you turn split horizon off is a way to reach a total of IP connectivity, may have other undesirable results. The use of subinterfaces is an effective way to see the rays so that the loopback network hub.
