Route Summarization is a method to reduce the number of routes in a routing table.

So why do this you ask?

    • Reduce the CPU workload of the Router.
    • Reduce Memory and Bandwidth usage
    • Can reduce flapping of routes

In the Below diagram A: we can see five different routers I have assigned them with Static IP routes for an easier explanation.

Each Route has a Statically assigned route to each network

R1 is able to Ping to R5 and visa versa

If we run the Ip Show Route command on each router you will note that there are directly connected routes as well as statically assigned routes as well.(If IOS Ver 15 was running on the routers you will also see locally connected routes)

In the Below diagram B: We will apply the route summarization to R1 , the reason we can do this is the networks are contigous and can easily be summarized into one ip address .

We can see we have the following Networks:

To apply summarization we need to work out what the new subnet will be.

We need to work out where to seperate our common and uncommon bits.

As we can see from the below we have a common bit of 21 till the red line.

This leaves us with the subnet of

Our address range that will cover the above 4 networks run from: –

Within our diagram we can see our static route summarised to the following in R1