Nowadays, however, we are using OSPF in the multi-vendor environment, and EIGRP in Cisco-only environments, the importance of RIP is not lowered. The RIP protocol has two versions, as we all know – version 1, the classful protocol; and version 2, the classless protocol. RIPv1 being a classful protocol, while implementing it, we must be careful in case of implementing subnets of a single classful IP network. We are discussing three of the cases below:

Case 1:

Each router R1 and R2 has a LAN with subnet 172.16.X.0/24, where X is router number. R1 and R2 are connected using serial interface and the subnet used is 172.16.0.0/30. Router R3 is connected to R2 using 192.168.1.0/30 on serial interface. R3 has a LAN 192.168.3.0/24.

Show ip route output:

R1:

Gateway of last resort is not set

172.16.0.0/16 is variably subnetted,2 subnets, 2 masks
C 172.16.0.0/30 is directly connected, Serial1/0
C 172.16.1.0/24 is directly connected, Ethernet0/0
R 192.168.1.0/24 [120/1] via 172.16.0.2, 00:00:04, Serial1/0
R 192.168.3.0/24 [120/1] via 172.16.0.2, 00:00:04, Serial1/0

Router#

Here, the R1-R2 serial link has /30 subnet-bits. The same network 172.16.2.0/24 is not propagated here.

R2:

Gateway of last resort is not set

172.16.0.0/16 is variably subnetted,2 subnets, 2 masks
C 172.16.0.0/30 is directly connected, Serial1/0
C 172.16.2.0/24 is directly connected, Ethernet0/0
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial1/1
R 192.168.3.0/24 [120/1] via 192.168.1.1, 00:00:06, Serial1/1

Router#

Same issues here. The network 172.16.1.0/24 is not in the routing table.

R3:

Gateway of last resort is not set

R 172.16.0.0/16 [120/1] via 192.168.1.2, 00:00:05, Serial1/1
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial1/1
C 192.168.3.0/24 is directly connected, Ethernet0/1

Router#

Here RIP learns all the 172.x.x.x networks as that is separate network than 192.168.1.0 network implemented on the P2P link.

This way of design is not recommended.

Case 2:

Each router R1 and R2 has a LAN with subnet 172.16.X.0/24, where X is router number. R1 and R2 are connected using serial interface and the subnet used is 172.16.0.0/24. Router R3 is connected to R2 using 192.168.1.0/24 on serial interface. R3 has a LAN 192.168.3.0/24.

Show ip route output:

R1:

Gateway of last resort is not set

172.16.0.0/24 is subnetted,3 subnets
C 172.16.0.0 is directly connected, Serial1/0
C 172.16.1.0 is directly connected, Ethernet0/0
R 172.16.2.0 [120/1] via 172.16.0.2, 00:00:04, Serial1/0
R 192.168.1.0/24 [120/1] via 172.16.0.2, 00:00:04, Serial1/0
R 192.168.3.0/24 [120/1] via 172.16.0.2, 00:00:04, Serial1/0

Router#

R2:

Gateway of last resort is not set

172.16.0.0/24 is subnetted,3 subnets
C 172.16.0.0 is directly connected, Serial1/0
R 172.16.1.0 [120/1] via 172.16.0.1, 00:00:03, Serial1/0
C 172.16.2.0 is directly connected, Ethernet0/0
C 192.168.1.0/24 is directly connected, Serial1/1
R 192.168.3.0/24 [120/1] via 192.168.1.1, 00:00:03, Serial1/1

Router#

R3:

Gateway of last resort is not set

R 172.16.0.0/16 [120/1] via 192.168.1.2, 00:00:06, Serial1/1
C 192.168.1.0/24 is directly connected, Serial1/1
C 192.168.3.0/24 is directly connected, Ethernet0/1

Router#

In this case, everything is okay, as the link between R1 and R2 is also /24 network.

Case 3:

Each router R1, R2 and R3 has a LAN with subnet 172.16.X.0/24, where X is router number. R1 and R2 are connected using serial interface and the subnet used is 172.16.0.0/24. Router R3 is connected to R2 using 192.168.1.0/24 on serial interface. R3 has additional LAN 192.168.3.0/24.

Show ip route output:

R1:

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 3 subnets
C 172.16.0.0 is directly connected, Serial1/0
C 172.16.1.0 is directly connected, Ethernet0/0
R 172.16.2.0 [120/1] via 172.16.0.2, 00:00:03, Serial1/0
R 192.168.1.0/24 [120/1] via 172.16.0.2, 00:00:03, Serial1/0
R 192.168.3.0/24 [120/1] via 172.16.0.2, 00:00:03, Serial1/0

Router#

Here you can see that the routing table is not containing the 172.16.3.0/24 network in it, but it contains information of 192.168.3.0/24.

R2:

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 3 subnets
C 172.16.0.0 is directly connected, Serial1/0
R 172.16.1.0 [120/1] via 172.16.0.1, 00:00:04, Serial1/0
C 172.16.2.0 is directly connected, Ethernet0/0
C 192.168.1.0/24 is directly connected, Serial1/1
R 192.168.3.0/24 [120/1] via 192.168.1.1, 00:00:04, Serial1/1

Router#

Here again, you can see that the routing table is not containing the 172.16.3.0/24 network in it, but it is receiving routing updates from R3 (that’s why 192.168.3.0/24 is in the routing table).

R3:

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 1 subnets
C 172.16.3.0 is directly connected, Ethernet0/0
C 192.168.1.0/24 is directly connected, Serial1/1
C 192.168.3.0/24 is directly connected, Ethernet0/1

Router#

In this case, since 172.16.0.0/16 is directly connected network for each router, the 172.16.3.0/24 network from R3 will not be propagated to other routers, and it will not learn the same classful network from other routers as well.

Hence, Case 3 type of designs are strictly not recommended!

Conclusion

When using RIP v1 for networks, we must be careful about such scenario, where subnetworks of same classful network are directly connected to separate routers.

In the scenarios presented above, do remember that, even if RIP v1 is configured, a router sends the entire subnet address when an update packet involves a subnet of the same classful network as the IP address of the transmitting interface.

If the routing update contains anything other than the interface subnet bits, the update is discarded, check Case 1.

Thus, when using a classful routing protocol, it is important to use the same subnet mask on all subnets belonging to the same classful network. But avoid such cases as Case 3.

I would like to thank Mr. Maheswar Nepal for working/assisting on this.