RIP v2 summarisation

Question

I am learning ccna, and doing some practice on packet tracer. I have a simple network with two routers, R0 and R1. They are connected to each other through a serial link (172.16.56.128/30). R0 is directly connected to a network (172.16.53.0/25). R1 is directly connected to two networks one on fa0/0 (172.16.54.0/23) and one on fa0/1 (172.16.56.0/25). I have configured rip v2 on both router using no auto-summary. I added both the networks on R1, while I did not add the network(172.16.56.0/25) on R0. However, the network 172.16.56.0/25 is in the routing table of r1.

 172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks
R       172.16.53.0/25 [120/1] via 172.16.56.129, 00:00:09, Serial0/0/1 ///WHY?
C       172.16.54.0/23 is directly connected, FastEthernet0/0
C       172.16.56.0/25 is directly connected, FastEthernet0/1
C       172.16.56.128/30 is directly connected, Serial0/0/1

Can anyone explain me why? shouldn't no auto-summary prevent this?

cuonglm · Accepted Answer · 2013-05-17T12:06:07.753

1

no auto-summary can not prevent that.

Because when you using dynamic routing protocol like RIP, it will send the subnet mask of the classful boundary, and not the actual subnet mask. The command no auto summary will turn this feature off in RIP, and send the actual subnet mask.

Update @lucaConfa

You see that, because you use no auto-summary command. so RIP willl use the real subnet mask and you have 172.16.53.0/25 in routing table.

If you don't use no auto-summary command, you will have 172.16.53.0/24

edited May 17 '13 at 12:06

answered May 17 '13 at 11:43

cuonglm

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Sorry I do not understand, you are saying that no auto-summary will force rip to use the subnet mask instead of the classful one. Am I correct? If that happens then why I have 172.16.53.0 in the routing table? using the actual subnet mask should keep this network out of rip. – lucaConfa May 17 '13 at 11:51
Ok, but why it is included in the routing table of R1? I did not add 172.16.53.0/25 to rip. It should not be there... – lucaConfa May 17 '13 at 12:19
"I did not add 172.16.53.0/25 to rip" ==> You did this on which router? – cuonglm May 17 '13 at 12:48
on R0 I did not issued the command `network 172.16.53.0` – lucaConfa May 17 '13 at 12:53
Ah, 172.16.53.0 directly connect to R0, by default, RIP will advertise it.Please show your RIP config. – cuonglm May 17 '13 at 13:00
router rip version 2 network 172.16.0.0 no auto-summary ! ip classless – lucaConfa May 17 '13 at 19:30
Why is the network 172.16.0.0 there in `my show run`? – lucaConfa May 17 '13 at 19:31
It's strange, try reload router and show config again. – cuonglm May 18 '13 at 02:10
I still have the same problem... maybe that is the way RIP works.. – lucaConfa May 20 '13 at 09:52

RIP v2 summarisation

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