RIPv2 / VLSM

I'm practicing VLSM with RIPv2 and have a question about the output from the show ip route command on R2.

With reference to this topology...

... you should be able to see the output from the two Routers, from the show ip route command.

I'm a little confused as to why R2 is showing the 170.0.0.0 network with a /16 mask??

The network works fine; the PC can communicate with the server and all is well. I'm just confused about the routing table from R2, as I expected it to be 170.0.0.0 /28, not /16.

B.t.w, it's not that I don't know what the natural mask for a Class B Network is; I know it's /16. It's just that I can't understand why R2 sees it that way, given that the RIPv2 updates include the subnet mask.

I'm sure there must be a reason, it's just that I don't know, and I want to know.

Cheers.

RIPv2.jpg

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Comments

hurricane1091

Turn off auto-summary. I believe RIPv2 has auto-summary enabled by default. Your IP address is a class B, so it defaults to advertising a /16 when you use auto-summary. It's probably "no auto-summary" under RIP config mode.

rob42

Thanks; I'll try that. Does that need to be done on both Routers, please?

hurricane1091

rob42 wrote: »

Thanks; I'll try that. Does that need to be done on both Routers, please?

No, you do it on the router advertising the subnet. If the router learning the network were to advertise it to another neighbor, you would need to turn off auto-summary on that router too though. Let me know if it works! Not really familiar with RIP except when it comes time to pass a test, but I think this will fix your problem.

rob42

For 3 minutes, the R2 Routing Table looks like this...

10.0.0.0/30 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial2/0
170.0.0.0/16 is variably subnetted, 2 subnets, 2 masks
R 170.0.0.0/16 [120/1] via 10.0.0.1, 00:02:51, Serial2/0
R 170.0.0.0/28 [120/1] via 10.0.0.1, 00:00:03, Serial2/0
C 192.168.2.0/24 is directly connected, FastEthernet0/0

... then changes to...

10.0.0.0/30 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial2/0
170.0.0.0/16 is variably subnetted, 2 subnets, 2 masks
R 170.0.0.0/16 is possibly down, routing via 10.0.0.1, Serial2/0
R 170.0.0.0/28 [120/1] via 10.0.0.1, 00:00:05, Serial2/0
C 192.168.2.0/24 is directly connected, FastEthernet0/0

After another 3 minutes (or 4; timing is not exact), it changes to...

10.0.0.0/30 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial2/0
170.0.0.0/28 is subnetted, 1 subnets
R 170.0.0.0 [120/1] via 10.0.0.1, 00:00:07, Serial2/0
C 192.168.2.0/24 is directly connected, FastEthernet0/0

Thank you for your help: I really like learning new stuff and this kind of thing is only learned by 'doing it'; I've not read about this in any of my books.

All the best to you.

hurricane1091

rob42 wrote: »

For 3 minutes, the R2 Routing Table looks like this...

10.0.0.0/30 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial2/0
170.0.0.0/16 is variably subnetted, 2 subnets, 2 masks
R 170.0.0.0/16 [120/1] via 10.0.0.1, 00:02:51, Serial2/0
R 170.0.0.0/28 [120/1] via 10.0.0.1, 00:00:03, Serial2/0
C 192.168.2.0/24 is directly connected, FastEthernet0/0

... then changes to...

10.0.0.0/30 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial2/0
170.0.0.0/16 is variably subnetted, 2 subnets, 2 masks
R 170.0.0.0/16 is possibly down, routing via 10.0.0.1, Serial2/0
R 170.0.0.0/28 [120/1] via 10.0.0.1, 00:00:05, Serial2/0
C 192.168.2.0/24 is directly connected, FastEthernet0/0

After another 3 minutes (or 4; timing is not exact), it changes to...

10.0.0.0/30 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial2/0
170.0.0.0/28 is subnetted, 1 subnets
R 170.0.0.0 [120/1] via 10.0.0.1, 00:00:07, Serial2/0
C 192.168.2.0/24 is directly connected, FastEthernet0/0

Thank you for your help: I really like learning new stuff and this kind of thing is only learned by 'doing it'; I've not read about this in any of my books.

All the best to you.

Great work mate!