Route Summarization and Redistribution

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 through 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 through can be summarized into a single routing table entry or advertisement – 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.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.