Routing protocols summarize or aggregate routes based on shared network numbers within the network. Classless routing protocols (such as RIPv2, OSPF, IS-IS, and EIGRP) support route summarization based on subnet addresses, including VLSM addressing. Classful routing protocols (RIPv1 and IGRP) automatically summarize routes on the classful network boundary and do not support summarization on any other bit boundaries. Classless routing protocols (RIPv2, OSPF, ISIS, EIGRP) support summarization on any bit boundary.
Route summarization reduces memory use on routers and routing protocol network traffic, because it results in fewer entries in the routing table (on the routers that receive the summarized routes). For summarization to work correctly, the following requirements must be met:
● Multiple IP addresses must share the same highest-order bits.
● Routing protocols must base their routing decisions on a 32-bit IP address and a prefix length that can be up to 32 bits.
● Routing updates must carry the prefix length (the subnet mask) along with the 32-bit IP address.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2780-2781). . Kindle Edition.