Route Summarization

Overview

Route Summarization or supernetting advertises a single route prefix instead of sending a bunch of contiguous route prefixes. However, with Route Summarization, there is a need to design the networks with summarization in mind,or there is a possibility for introducing suboptimal routing and forwarding traffic for unused networks. Similarly, the router drops the traffic if the routing table lacks a matching destination route prefix for the advertised summary prefix.

The Route Summarization feature requires a thorough network design analysis before specifying the summarized prefix for advertisement to neighbors.

Route Summarization Use Case

In the example topology, a router learns route prefixes from multiple sources. When the router advertises route prefixes to its neighbor via BGP, it advertises the unique route prefixes.The router advertises eight unique route prefixes. Instead, with Route Summarization, the router can advertise a single summary prefix to the neighbor through BGP.

Black Hole Routing

A black hole route, or a null route, is a network route that discards incoming traffic. In Route Summarization, the system utilizes black hole routing to drop traffic for destinations that belong to a summary route but lack a locally learned prefix. Starting with release 5.2, the system requires this process because advertising a summary route directs all traffic for that supernet to the Edge or Gateway—including traffic for non-existent destination prefixes. Configuring a summary route automatically installs a black hole route for that prefix in the routing table until the system removes the configuration.