Connected vs Non-Connected Route with RIPv2

[nonamer15]

Does a router treat connected vs non-connected routes differently with RIPv2 from a summarization perspective? I would have said no, but testing says otherwise.

Let's assume I've got 3 routers and they are all configured with auto-summary enabled, which is the default. If any of those routers are directly connected to more than 1 classful network (all of my routers are), then that router will advertise summarized routes. I wouldn't see why there would be a difference whether the route was directly connected or not.

Below is the topology I have.

Here's what I see in the routing table of R1.

R1# sh ip route rip

10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
R 10.1.2.0/24 [120/1] via 10.1.1.2, 00:00:15, GigabitEthernet2
R 192.168.1.0/24 [120/1] via 10.1.1.2, 00:00:15, GigabitEthernet2

I don't understand the route for 10.1.2.0/24. I would expect to see a route for 10.0.0.0/8 learned via RIP.

• R2 has a connected route of 10.1.2.0/24
• R2 has auto-summary enabled, so it should advertise 10.0.0.0/8 to R1
• On R1, you would see a route to 10.0.0.0/8 learned via RIP

Can anyone explain this? I understand why there isn't a route for '172.16.' as R2 is receiving a 172.16.0.0/16 route from both R1 and R3. Due to split horizon, it won't send an advertisement for a route in which it has also learned that route via that interface. In this scenario, disabling auto-summary on either R1 or R3 would allow a route for '172.16.' to appear on both R1 and R3.

However, I still don't understand why '10.' is not being summarized by R2. To me, it should be.

Below are the RIP configurations if needed.

R1#sh run | s router
router rip
 version 2
 passive-interface default
 no passive-interface GigabitEthernet2
 network 10.0.0.0
 network 172.16.0.0

R2#sh run | s router
router rip
 version 2
 passive-interface GigabitEthernet4
 network 10.0.0.0
 network 192.168.1.0

router rip
 version 2
 passive-interface default
 no passive-interface GigabitEthernet2
 network 10.0.0.0
 network 172.16.0.0

Thanks.

[Peter Paluch · Accepted solution]

Hello,

As far as I can tell, R2 behaves as expected. The auto-summary is applied only when advertising a subnet of one major network out an interface that is in a different major network itself. In your case, with the 10.x.x.x subnets, this is not happening: R2 is advertising 10.1.2.0/24 (which belongs to the major network 10.0.0.0) through an interface that lies in 10.1.1.0/24 (which is in the same major network 10.0.0.0). That is why the auto-summary didn't take place. If your network between R1 and R2 was different, say, 11.1.1.0/24, then you would see the auto-summary in action just like you see it in action for the 172.16.0.0/16 subnets.

Feel welcome to ask further!

Best regards,
Peter

[nonamer15]

Thanks Peter. Appreciate it. I was unaware of that caveat ("The auto-summary is applied only when advertising a subnet of one major network out an interface that is in a different major network itself").