Route Summarization or Route aggregation is a method of reducing the size of routing table in and IP (Internet Protocol) network, in this method, multiple selected routes are consolidated to form a single advertisement, in contrast to flat routing where there is a unique entry for each route in the routing table.
A summarized address is the longest match for all the entries in the routing table which it is representing, pointing to the same interface.
Route summarization is not possible if the routes are pointing towards different interfaces.
Example of Route Summarization
Flat routing is more specific, as there's an individual entry in the routing table for each destination whereas in Route Summarization, there's one entry for several destination.
Flat Routing
If you want to go to New Jersey follow this route
If you want to go to New York follow this route
If you want to go to New Mexico Follow this route
Summarized Route
If you want to go to any state whose name start with "N" go to this route
Why do we Summarize Routes
In large networks, there comes a time when there are too many routes and size of routing table starts increasing, that is the right time we should start route summarization.
Bigger the size of routing table, more resources we need to process the information leading to latency in the network
Benefits of Route Summarization
There are several advantages of route summarization over flat routing
- Route summarization can reduce latency in complex large networks, especially when there are too many routers are involved.
- Reducing the number of entries in the routing table decreases the size of routing table hence reducing the overhead for the routing protocol.
- Network stability is improved by reducing or eliminating the unnecessary network updates as a part of the network undergoes a change.
Disadvantage of Route Summarization
As flat routing is more specific, there's an entry for each destination, but in Route summarization, there's one entry for several destinations in this way our users can reach many destinations (can communicate with many destinations) where you don't want your users to be.
Example
When you say
If you want to go to New Jersey follow this route
If you want to go to New York follow this route
If you want to go to New Mexico Follow this route
Users will be able to reach only New Jersey, New York and New Mexico, but when you say
If you want to go to a state whose name starts with N go in this direction, now users will be able to reach many other destinations which you don't want to declare for security reasons as routers cannot route for unknown destinations.
But now when you say for any state whose name starts with N send traffic in this direction for sure users will be able to reach New Jersey, New York and New Mexico, but because of this statement route will also start sending traffic for New Hampshire, Nevada, Nebraska, North Carolina, and North Dakota in the same direction which is a security loophole
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