Route summarization allows routes to the same natural network segment but different subnets to be summarized into a single route belonging to the same network segment before it is transmitted to other network segments. RIP-1 packets do not carry mask information, and therefore RIP-1 can advertise only routes with natural masks. RIP-2 supports route summarization because RIP-2 packets carry mask information. Therefore, RIP-2 supports subnetting.
In RIP-2, route summarization can reduce the size of the routing table and improve the extensibility and efficiency of a large-scale network.
Route summarization has two modes:
Process-based classful summarization
Summary routes are advertised with natural masks. If split horizon or poison reverse is configured, classful summarization becomes invalid because split horizon or poison reverse suppresses some routes from being advertised. In addition, when classful summarization is configured, routes learned from different interfaces may be summarized into a single route. As a result, a conflict occurs in the advertisement of the summary route.
For example, a RIP process summarizes the route 10.1.1.0 /24 with metric 2 and route 10.2.2.0/24 with metric 3 into the route 10.0.0.0/8 with metric 2.
Interface-based summarization
Users can specify a summary address.
For example, users can configure a RIP-enabled interface to summarize the route 10.1.1.0/24 with metric 2 and route 10.1.2.0/24 with metric 3 into the route 10.1.0.0/16 with metric 2.