Classless and Classful Capabilities
A routing protocol can be classless or classful. You might be wondering why this has any bearing on how a router builds its routing table. If a routing protocol is classful, such as RIP and IGRP, it does not carry subnet masks in its routing updates. However, classless routing protocols, such as EIGRP and OSPF, do carry subnet masks. When a classful routing protocol receives a routing update in the same major network as the interface it is received on, the interface must have the same subnet mask as the one in the update. If the update is in a different class, it is automatically summarized at that class's major bit boundary. This means that if you were to redistribute a classless routing protocol into a classful ...
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