Autosummarization Support and Configuration

Autosummarization is a feature in some classful routing protocols that automatically summarizes network routes at the classful network boundary. This can simplify routing table entries and reduce the size of routing updates, but it can also cause routing problems if the summarized route is not accurate or if it causes subnets to be dropped.

Autosummarization is enabled by default in some classful routing protocols, such as RIPv1 and IGRP. However, some newer protocols, such as OSPF and EIGRP, do not support autosummarization by default.

To disable autosummarization in a classful routing protocol, you can use the “no auto-summary” command. For example, to disable autosummarization in RIPv1, you would use the following command:

Router(config)# router rip

Router(config-router)# no auto-summary

In EIGRP, autosummarization is disabled by default, but you can enable it using the “auto-summary” command. For example:

Router(config)# router eigrp 1

Router(config-router)# auto-summary

Note that it is generally recommended to disable autosummarization in classful routing protocols if you are using variable-length subnet masking (VLSM) or if you have discontiguous classful networks. These scenarios can cause problems with autosummarization and can lead to routing issues.