Summarization

Summarization is the process in which the administrator collapses many routes with a long mask to form another route with a shorter mask. Route summarization reduces the size of routing tables and makes routing function more efficiently. Route summarization also helps make networks more stable by reducing the number of updates that are sent when subnets change state. Route summarization makes classless interdomain routing (CIDR) possible. Variable-length subnet masking (VLSM) promotes the use of route summarization. Some dynamic routing protocols engage in route summarization automatically for changes in a major classful network, whereas others do not.

For any routing protocol within the scope of the CCIE written exam, an administrator can disable any automatic summarization that might be occurring and configure “manual” summarization. To engage in route summarization, find all the leftmost bits that are in common and create a mask that encompasses them. An example follows:-

The following routes exist in the routing table—all routes use a 24-bit mask:

10.108.48.0 = 00001010 01101100 00110000 00000000
10.108.49.0 = 00001010 01101100 00110001 00000000
10.108.50.0 = 00001010 01101100 00110010 00000000
10.108.51.0 = 00001010 01101100 00110011 00000000
10.108.52.0 = 00001010 01101100 00110100 00000000
10.108.53.0 = 00001010 01101100 00110101 00000000
10.108.54.0 = 00001010 01101100 00110110 00000000
10.108.55.0 = 00001010 01101100 00110111 00000000

Notice that the first 21 bits of the subnetwork IDs are all common. These can be masked off. You can use the single route entry for all these subnetworks as follows:

10.108.48.0/21