Route Summarisation

Route summarisation can be explained with a simple analogy. When someone asks you for directions most of the time you can say.

“Take the x motorway/road and follow the directions for x”.

In other words you are pointing them in the right direction, and while on their journey they will learn the route to their destination by following street signs. This process is much simpler to follow than explaining the whole route, you may also not know the whole route, but only the direction in which to head.

Route summarisation within a network is very similar to this process. In many cases routers need not know about specific routes, they may only need to know how to reach a router that connects to those routes. Once the traffic gets to the router in question, it will know how to direct the traffic to specific networks that only it may know about.

Router summarisation can significantly reduce a routers routing table and also reduce the number of updates needed when sections of a network change.

However if you have discontigous networks you should avoid using route summarisation.Route summarisation also requires classless routing protocols. E.g OSPF, RIP version 2 or EIGRP, and is most effective when the addresses are arranged in a hierarchical fashion, much like the telephone area code analogy described earlier to explain IP addresses and their hierarchy.

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