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OSPF and IS-IS: Choosing an IGP for Large-Scale Networks by Jeff Doyle

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3.2. Message Encapsulation

OSPF operates directly over IP, with a protocol number of 89. The source address of all OSPF messages is always the local end of an adjacency, and all messages are either multicast to one of two reserved multicast addresses, 224.0.0.5 and 224.0.0.6, or unicast to the distant end of an adjacency. At no time does OSPF broadcast its messages. The rules for when to multicast and when to unicast, and what the two reserved multicast addresses signify, are discussed in context in later chapters. For now, it is enough to know how the messages are sent.

IS-IS operates not over the network layer like OSPF, but over the data link layer. But like OSPF, IS-IS messages are either unicast or multicast—never broadcast. The source ...

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