Route summarization involves creating a single route which represents more than one subnet, and is a contiguous range of subnets implied by the route. Routes whose subnet falls within the range are referred to as component routes.
Certain routing protocols handle route summarization in different ways, here are a few points in which they are common:
- Advertised summary is assigned the metric of the lowest component subnet
- Router does not advertise component subnets
- Router does not advertise the summary when its routing table does not have any component subnets
- the summarizing router creates a local route summary with destination of null0 to prevent loops
- summary routes reduce the size of the routing table and improves convergence
- summary routes decrease the amount of specific information in routing tables causing suboptimal routing
Route Summarization is the act of advertising a larger supernet to the network that encompasses multiple contiguous subnets. Doing this allows for more efficient and leaner routing tables, and benefits to Query propagation.
Should a Query be sent to a router advertising a summary route, if that router did not have the more specific route in its routing table it will immediately Reply back with an infinite metric.
Whenever summary routes are advertised the router performing the summarization also installs a ‘discard route’, this discard route is identical to the network and netmask of the advertised route, and its interface is set to Null0.
The purpose for this route is for any traffic traveling to a subnet encompassed by the summary route, where the destination network is not reachable or does not exist, it will cause that traffic to be dropped to Null0 instead of following the default route…so it’s an effective tool to prevent routing loops from occurring.
EIGRP also takes into account all component routes of the summary and chooses the best metric to advertise the summary. This also means that if those component route metrics change, the summary route metric will also change. This can be CPU intensive for networks who have thousands of component routes for a single summary. To get around this you can set the summary metric statically.