If two neighboring routers are sending and receiving different versions of RIP, routes will not be properly exchanged. By default, RIP routers are sending version 1 updates and accepting version 1 and 2 updates. This can be changed on both a global level and an interface level, and in this CCNA exam tutorial, we'll do both.
Thanks to those defaults, R2 was originally able to successfully advertise its loopback to R3, as shown here:
R2(config)#int loopback2
R2(config-if)#ip address 2.2.2.2 255.255.255.255
R2(config)#router rip
R2(config-router)#network 172.12.0.0
R2(config-router)#network 2.0.0.0
R3#show ip route rip
R 2.0.0.0/8 [120/1] via 172.12.23.3, 00:00:08, Ethernet0
Now. R2 is hardcoded for RIPv2, but a mistyped command has R3 hardcoded for RIPv1.
R2(config)#router rip
R2(config-router)#version 2
R2(config-router)#no auto-summary
R3(config)#router rip
R3(config-router)#version 1
Viewing and clearing the RIP routing table on R3 shows that R3 did have a route for R2's loopback before the clear, but not after.
R3#show ip route rip
R 2.0.0.0/8 [120/1] via 172.12.23.2, 00:00:10, Ethernet0
R3#clear ip route *
R3#show ip route rip
<empty RIP table>
R3#
Debugging RIP and clearing the table again quickly shows us the issue.
R3#debug ip rip
RIP protocol debugging is on
R3#clear ip route *
08:01:54: RIP: ignored v2 packet from 172.12.23.2 (illegal version)
You may have a case where a router needs to send both versions! It's rare, but it does happen. The RIP version is usually changed globally, but it can be changed with the interface-level ip rip recieve version and ip rip send version commands. That's what we'll do here. IOS Help shows us the available options:
R3(config)#int e0
R3(config-if)#ip rip ?
authentication Authentication control
receive advertisement reception
send advertisement transmission
v2-broadcast send ip broadcast v2 update
R3(config-if)#ip rip receive ?
version version control
R3(config-if)#ip rip receive version ?
1 RIP version 1
2 RIP version 2
<cr>
R3(config-if)#ip rip receive version 1 2
R3(config-if)#ip rip send version 1 2
R3 is now set to send and accept RIP updates in both versions. After clearing the routing table, the route to R2's loopback appears again. Note the /32 mask appears in the R3 routing table - that's because auto-summarization was disabled on R2 when it was hardcoded for RIPv2.
R3#show ip route rip
2.0.0.0/32 is subnetted, 1 subnets
R 2.2.2.2 [120/1] via 172.12.23.3, 00:00:01, Ethernet0
To your success,
Chris Bryant
CCIE #12933