Route Summarization and Redistribution

By Dan DiNicolo, March 19th, 2007 Posted in CCDA Study Guide. Subscribe to our RSS Feed

If you recall from our look at Classless Interdomain Routing (CIDR), it is possible to represent many individual subnets or networks in a single routing table entry by allocating a custom subnet mask. Sometimes referred to as supernetting, route summarization simply involves collapsing a number of contiguous routing table entries into a single entry instead. This not only saves routing table space, but also makes routing more efficient.

In the world of routing protocols, route summarization refers to the process by which a protocol (like OSPF) will summarize multiple entries into a single routing table entry at a boundary on the network. For example, consider a network where networks 10.0.8.0/24 through 10.0.15.0/24 all exist behind Router A. Instead of Router A advertising each of these eight networks to a neighboring router, it would make much more sense to summarize the routes into a single routing table entry, and forward information about one network only.

Networks 10.0.8.0/24 through 10.0.15.0/24 can be summarized into a single routing table entry or advertisement – 10.0.8.0/21. If you are having trouble remembering how I came up with the new network prefix of /21, I would strongly suggest reviewing the CIDR section of Chapter 5. For those looking for a quick refresher, eight networks need to be summarized in this example. By stealing 3 bits from the network portion of the existing subnet mask, I am capable of summarizing 8 addresses, since 23 equals 8.

Written by Dan DiNicolo - Visit Website

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