Classful and classless routing protocols

Classful routing protocols are considered legacy and do not include subnet mask information with routing updates. Examples of classful routing protocols are RIPv1 and IGRP. Because subnet mask information is not included in updates, consistency of the mask is assumed throughout the network. Classful routing protocols also feature automatic summarization of routing updates when sent across a major classful network boundary. For example, the 10.16.0.0/16 network would be advertised as 10.0.0.0/8 when sent into a 172.16.0.0 domain.

BGP and EIGRP are not classful routing protocols, both engage in automatic summarization behavior by default, and in that sense they act classful. The no auto-summary command is used to disable this behavior. Classful routing protocols feature a fixed-length subnet mask (FLSM) as a result of their inherent limitations. The FLSM leads to inefficient use of addresses and limits the network’s overall routing efficiency. By default, classful routing protocols discard traffic bound for any unknown subnet of the major classful network. For example, if your classful routing protocol receives traffic destined for 10.16.0.0 and it knows of only the 10.8.0.0 and 10.4.0.0 subnets in its routing table, it discards the traffic—even if a default route is present! The ip classless command was introduced to change this behavior. The ip classless command allows the protocol to use the default route in this case. This command is on by default with Cisco IOS Release 12.0 and later routers.

As a classic example of a classless routing protocol, OSPF carries subnet mask information in updates. Wireless LAN Services Module (WLSM) is possible with such protocols.