2.5 Route Summarization Overview
2.5.1 What is route summarization?
This section discusses what route summarization is and how VLSMs maximize the use of route summarization to address the issue of routing table growth and manageability. The concept of summarization is covered here so that its configuration can be the focus in the protocol-specific chapters that follow.

In large internetworks, hundreds or even thousands or tens of thousands, or hundreds of thousands of IP addresses can exist. In these environments, some routers may become overwhelmed. Route summarization, also called route aggregation or supernetting, reduces the number of routes that a router must maintain because it represents a series of network numbers as a single summary address. In the main figure, for example, you can either send three routing update entries or summarize the addresses into a single network number (172.16.0.0/16).

Another advantage to using route summarization in a large, complex network is that it can isolate topology changes from other routers. That is, if a specific link in the 172.16.27.0/24 domain was intermittently failing, the summary route would not change, so no router external to the domain would need to keep modifying its routing table due to this problematic activity.

Route summarization is most effective within a subnetted environment when the network addresses are in contiguous (sequential) blocks in powers of two. For example, consider these two addresses:

  • 130.129.0.0 
  • 130.192.0.0

Both addresses have nine matching bits in the beginning. If you were going to add more network addresses after you've used all numbers possible with these nine bits matching, you can dip into the next bit, the tenth bit, to define another group of addresses.

Routing protocols summarize or aggregate routes based on shared network numbers within the network. RIP2, OSPF, and Enhanced IGRP support route summarization based on subnet addresses, including VLSM addressing.

Summarization is described in RFC 1518, An Architecture for IP Address Allocation with CIDR.