RIP Route Summarization

Let's understand with the topology below, how we can do route summarization in RIP. Router R1 has 3 loopbacks(host prefix) configured which are advertised to R2. Without summarization, R2 will advertised these 3 prefixes to R3. While we configure route summarization on R2's f1/1 interface (connected to R3) it will generate a single summary route towards R3 and 3 host prefixes (/32) will be suppressed and not advertised anymore to R3.

Without Route summarization on R2, we can see R3 receives all 3 host prefixes from R2.

R2#sh ip route rip | i 172.
      172.9.0.0/32 is subnetted, 3 subnets
R        172.9.0.1 [120/1] via 10.10.12.1, 00:00:13, FastEthernet1/0
R        172.9.0.2 [120/1] via 10.10.12.1, 00:00:13, FastEthernet1/0
R        172.9.0.3 [120/1] via 10.10.12.1, 00:00:13, FastEthernet1/0


R3#sh ip route | i 172.
      172.9.0.0/16 is variably subnetted, 4 subnets, 2 masks
R        172.9.0.0/16 [120/2] via 10.10.23.2, 00:00:35, FastEthernet1/0
R        172.9.0.1/32 [120/2] via 10.10.23.2, 00:00:07, FastEthernet1/0
R        172.9.0.2/32 [120/2] via 10.10.23.2, 00:00:07, FastEthernet1/0
R        172.9.0.3/32 [120/2] via 10.10.23.2, 00:00:07, FastEthernet1/0


Now let's configure summarization on R2's f1/1 interface and we'll see that now R2 will only advertise summary prefix 172.9.0.0/16 to R3 and R3 will have only single summary route and not the 3 host prefixes.

R2(config)#int f1/1
R2(config-if)#ip summary-address rip 172.9.0.0 255.255.0.0


R3#sh ip route | i 172.
R     172.9.0.0/16 [120/2] via 10.10.23.2, 00:00:24, FastEthernet1/0


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