RIP Route Summarization

Route Summarization - Overview

Route summarization or route aggregation is a technique using which we create one summary route that is a superset of  multiple smaller network subnets. Route summarization help us in multiple ways, for example, it helps in saving memory by reducing the size of  routing tables reducing memory requirements. It also helps in saving bandwidth as after summarization we will have to advertise lesser number of routes. Furthermore, it also helps in saving device resources for example, using lesser memory, processor utilization. Finally, it helps a lot in providing network stability when there are frequent route-flaps.

However, there are few disadvantages of summarization. It could lead to sub-optimal routing by preferring more specific route if available via other path in the routing table. Additionally, a summary route can forward the traffic for the networks that are note in use.

Route Summarization in RIP

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.

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

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

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

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

Imp Point: By default, RIPv2 and EIGRP perform automatic network summarization at classful boundaries, just like a classful protocol does. They will limit the network to its default classful boundary mask before advertising. Automatic summarization lets RIPv2 and EIGRP be backward compatible with their predecessors, RIPv1 and Interior Gateway Routing Protocol (IGRP). The difference between these protocols and their predecessors is that you can manually turn off automatic summarization, using the "no auto-summary" router configuration command. You do not need this command when you are using OSPF or IS-IS, because neither protocol performs automatic network summarization by default.

The autosummarization behavior can cause problems in a network that has discontiguous subnets or if some of the summarized subnets cannot be reached via the advertising router. If a summarized route indicates that certain subnets can be reached via a router, when in fact those subnets are discontiguous or unreachable via that router, the network might have problems similar to those caused by a classful protocol.

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