08-07-2018 06:13 PM
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.
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.
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08-07-2018 06:36 PM
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
08-07-2018 06:36 PM
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
08-07-2018 06:59 PM
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").
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