RIPv1 Classful Behavior with Multiple Subnet Masks

There was some confusion this week on how RIPv1 would handle a subnet with multiple masks. As a result of the discussion, I decided to test it out. I configured three routers, RouterA, RouterB, and RouterC, as shown.

RouterA, RouterB, and RouterC could all communicate using a mask of 255.255.255.252 (/30). RouterA’s routing table included both the serial links with the 255.255.255.252 mask (networks 172.16.9.0, 172.16.9.4, and 172.16.9.8) and the Fast Ethernet link (172.16.1.0)configured with a mask of 255.255.255.0 (/24). RouterB and RouterC's routing table did not include the FastEthernet route to 172.16.1.0/24.

I am including the routing tables and debug ip rip output for your review. Notice that RouterA did not include network 172.16.1.0 (which had been configured on the FastEthernet0/0 interface with a mask of 255.255.255.0) in the update send out its egress interface to RouterB and RouterC:

02:38:21: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (172.16.1.1)
02:38:21: RIP: build update entries - suppressing null update

RouterA#show ip route
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
R 172.16.9.4/30 [120/1] via 172.16.9.2, 00:00:12, Serial0/0
[120/1] via 172.16.9.10, 00:00:16, Serial0/1
C 172.16.9.0/30 is directly connected, Serial0/0
C 172.16.1.0/24 is directly connected, FastEthernet0/0
C 172.16.9.8/30 is directly connected, Serial0/1

RouterB#show ip route
172.16.0.0/30 is subnetted, 3 subnets
C 172.16.9.4 is directly connected, Serial0/2/1
C 172.16.9.0 is directly connected, Serial0/2/0
R 172.16.9.8 [120/1] via 172.16.9.6, 00:00:22, Serial0/2/1
[120/1] via 172.16.9.1, 00:00:00, Serial0/2/0

RouterC#show ip route
172.16.0.0/30 is subnetted, 3 subnets
C 172.16.9.4 is directly connected, Serial0/2/1
R 172.16.9.0 [120/1] via 172.16.9.9, 00:00:09, Serial0/2/0
[120/1] via 172.16.9.5, 00:00:16, Serial0/2/1
C 172.16.9.8 is directly connected, Serial0/2/0

RouterA#debug ip rip
03:31:04: RIP: received v1 update from 172.16.9.2 on Serial0/0
03:31:04: 172.16.9.4 in 1 hops
03:31:05: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (172.16.
1.1)
03:31:05: RIP: build update entries - suppressing null update
03:31:05: RIP: sending v1 update to 255.255.255.255 via Serial0/0 (172.16.9.1)
03:31:05: RIP: build update entries
03:31:05: subnet 172.16.9.8 metric 1
03:31:05: RIP: sending v1 update to 255.255.255.255 via Serial0/1 (172.16.9.9)
03:31:05: RIP: build update entries
03:31:05: subnet 172.16.9.0 metric 1
03:31:16: RIP: received v1 update from 172.16.9.10 on Serial0/1
03:31:16: 172.16.9.4 in 1 hops

RouterB#debug ip rip
RIP protocol debugging is on
*Apr 20 04:15:35.767: RIP: sending v1 update to 255.255.255.255 via Serial0/2/0
(172.16.9.2)
*Apr 20 04:15:35.767: RIP: build update entries
*Apr 20 04:15:35.767: subnet 172.16.9.4 metric 1
*Apr 20 04:15:38.755: RIP: received v1 update from 172.16.9.6 on Serial0/2/1
*Apr 20 04:15:38.755: 172.16.9.8 in 1 hops
*Apr 20 04:15:39.899: RIP: sending v1 update to 255.255.255.255 via Serial0/2/1
(172.16.9.5)
*Apr 20 04:15:39.899: RIP: build update entries
*Apr 20 04:15:39.899: subnet 172.16.9.0 metric 1
*Apr 20 04:15:40.067: RIP: received v1 update from 172.16.9.1 on Serial0/2/0
*Apr 20 04:15:40.067: 172.16.9.8 in 1 hops

RouterC#debug ip rip
*Apr 20 04:22:26.219: RIP: sending v1 update to 255.255.255.255 via Serial0/2/1
(172.16.9.6)
*Apr 20 04:22:26.219: RIP: build update entries
*Apr 20 04:22:26.219: subnet 172.16.9.8 metric 1
*Apr 20 04:22:28.267: RIP: received v1 update from 172.16.9.9 on Serial0/2/0
*Apr 20 04:22:28.267: 172.16.9.0 in 1 hops
*Apr 20 04:22:31.123: RIP: sending v1 update to 255.255.255.255 via Serial0/2/0
(172.16.9.10)
*Apr 20 04:22:31.123: RIP: build update entries
*Apr 20 04:22:31.123: subnet 172.16.9.4 metric 1
*Apr 20 04:22:32.891: RIP: received v1 update from 172.16.9.5 on Serial0/2/1
*Apr 20 04:22:32.895: 172.16.9.0 in 1 hops