Autosummarization and Discontiguous Classful Networks

Autosummarization is a feature that automatically summarizes routes at the classful network boundary. In classful routing protocols such as RIPv1 and IGRP, the routers do not send subnet masks in their routing updates, so they have no knowledge of the subnetted networks. Instead, they assume that all networks within a classful network have the same subnet mask, and they automatically summarize the routes at the classful network boundary.

For example, suppose we have two networks:

Network A: 192.168.1.0/24

Network B: 192.168.2.0/24

If these two networks are part of the same classful network, which is 192.168.0.0/16, then RIPv1 or IGRP will automatically summarize them as a single route, 192.168.0.0/16.

However, if the two networks are not part of the same classful network, for example, if Network A is 192.168.1.0/24 and Network B is 172.16.2.0/24, then the routers will not be able to summarize them automatically, because they have different classful network addresses.

Discontiguous classful networks are networks that have different classful network addresses, such as 192.168.1.0/24 and 172.16.2.0/24. In these networks, autosummarization can cause routing problems, because the routers may incorrectly summarize routes or drop packets that belong to subnets that are not part of the summarized route. To avoid these problems, it is recommended to use classless routing protocols such as OSPF and EIGRP, which support variable-length subnet masks (VLSM) and do not perform autosummarization. With these protocols, the routers can advertise subnets with their correct subnet masks, and the routing tables can contain more specific routes for each subnet.

Manual route summarization can improve routing efficiency, reduce memory consumption, and improve convergence by reducing the length of routing tables. The final sections of this chapter examine the automatic summarization of routes at the boundaries of classful networks using a feature called autosummarization.

Because classful routing protocols do not advertise subnet mask information, the routing updates simply list subnet numbers but no accompanying mask. A router receiving a routing update with a classful routing protocol looks at the subnet number in the update, but the router must make some assumptions about what mask is associated with the subnet.