RIPv2 Auto Summary Not Working

[asmith@tcwcomputers.com]

Hello all,

I'm trying to understand why auto summary is not working for some of my routers in my Packet Tracer lab. I basically have 4 routers all of them are setup with RIPv2, auto summary & have the network command set for each network. Auto summary is working for my class B and C networks but I have 3 contiguous class A networks (10.10.10.0/24, 10.10.11.0/24, 10.10.9.0/24) that appear to  not summarize their routes. I know that you really should auto summary networks anymore but I'm just testing things as I'm studying for my ICND-1. I read somewhere that current Cisco routers now disable auto summary by default but when do a show ip protocols every routers output does show that automatic network summarization and RIPv2 is in effect. What am I missing?

router5#show ip route

 ..................................................................

10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

R 10.10.9.0/24 [120/2] via 10.10.11.1, 00:00:08, Serial0/0/0

R 10.10.10.0/24 [120/1] via 10.10.11.1, 00:00:26, Serial0/0/0

C 10.10.11.0/24 is directly connected, Serial0/0/0

L 10.10.11.2/32 is directly connected, Serial0/0/0

R 172.16.0.0/16 [120/1] via 192.168.2.2, 00:00:22, Serial0/0/1

192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks

C 192.168.2.0/24 is directly connected, Serial0/0/1

L 192.168.2.1/32 is directly connected, Serial0/0/1

[Andrew Khalil]

asmith@tcwcomputers.com,

Greetings,

This is the normal behavior of RIPv2 as it's as same as RIPV1, Classfull!

The only difference that it's classless in the appearance inside the routing table, which means that you will see 1 summarized network (10.0.0.0/8) as well as the subnets that contributes this summarized network (10.10.9.0/24, 10.10.10.0/24, 10.10.11.0/24)

If you will enable now RIPV1, you will see instead of all the detailed networks only 10.0.0.0/8!

The solution for this issue can be one of the 3 options:

1- Continue using RIPV2 or even V1 but change the subnets.

2- Change the protocol to EIGRP or OSPF.

3- Use distribute list to deny some networks, as now any router can reach this 3 networks as they are summarized so they are treated as 1 network.

Please don't forget to rate all helpful responses and mark solutions!

Bst Rgds,

Andrew Khalil

[Jon Marshall]

The key to understanding auto summary is that it works across classful boundaries. 

So you have an entry for 172.16.0.0/16 on router 5 because that network is only reachable via either the 192.168.3.0/24 or via the 10.0.0.0/8 networks so it is summarised. 

But look at the 10.0.0.0/8 subnets and you will see that router 5 has an interface in one of those subnets and so there is no summary route.  

If you look at the routing table on router 6 however you should see summary routes for 10.0.0.0/8 because for router 6 to get to any of the 10.0.0.0/8 subnets it has to go via different classful networks. 

Jon

[asmith@tcwcomputers.com]

Interesting. That is starting to make more sense. My original assumption was that if a router was set to 'auto summary' then the only routes it would advertise via RIPv2 would be it's classfull route. I guess my big question is how was Router5 able to add a RIPv2 route for 10.10.9.x - Is Router3 advertising it's 10.10.9.x as classfull & classless and Router5 not being separated by another classfull boundary knows to choose the classless route or how is that classless route arriving at Router5?

Here's the show ip route from Router 6: 

router6>en

Password:

router6#show ip route

 --------------------------------------------------------------

10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

R 10.0.0.0/8 [120/1] via 192.168.2.1, 00:00:08, Serial0/0/0

[120/1] via 172.16.10.2, 00:00:02, Serial0/0/1

R 10.10.9.0/24 is possibly down, routing via 192.168.2.1, Serial0/0/0

R 10.10.10.0/24 is possibly down, routing via 192.168.2.1, Serial0/0/0

R 10.10.11.0/24 is possibly down, routing via 172.16.10.2, Serial0/0/1

172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks

R 172.16.0.0/16 is possibly down, routing via 192.168.2.1, Serial0/0/0

C 172.16.10.0/24 is directly connected, Serial0/0/1

L 172.16.10.1/32 is directly connected, Serial0/0/1

192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks

C 192.168.2.0/24 is directly connected, Serial0/0/0

L 192.168.2.2/32 is directly connected, Serial0/0/0

Looking at that it's starting to make more sense as i can see all the possibly down routes and piece together why it's bad to have auto summary running in this scenario which is why i set things up like this in the first place - to see it break. Now if i can just understand my above question that would be super. Thank you for your help

[Jon Marshall · Accepted solution]

It will advertise a summary if the interface IP it is advertising it out of it is not part of the classful network. 

So if you look back to your diagram you can see R3 is advertising 10.10.9.0/24 to R4 across a subnet of the 10.0.0.0/8 network so there will be no summary route and the same logic for R4 to R5. 

Whereas R3 advertising to R6 is going out of an interface in a different classful network and so it will advertise a summary route. 

Jon

[asmith@tcwcomputers.com]

Ahhh eureka!

That's what I was missing. I envisioned any router set with 'auto summary' to ONLY have the capability to send out classfull RIPv2 advertisements. Like a toggle switch either summary or not. Now I get it and understand that the route 'type' that gets sent out (classfull or classless) is dependent on the interfaces network the route is coming out of (is it contiguous w/ auto summary enabled? Yes, then send a classless route. No, then send a summary). Thank you so much!

"It will advertise a summary if the interface IP it is advertising it out of it is not part of the classful network."