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

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4.2. The Hello Protocol

When the OSPF or IS-IS protocol process starts on a router, neighbors must be discovered and adjacencies established. Both protocols send and listen for Hello messages to discover neighbors. Functionally, the Hello protocol is the same for both OSPF and IS-IS; they differ only in the details. As you learned in Chapter 3, the Hello protocol performs several functions:

  • It discovers neighboring OSPF or IS-IS routers.

  • It performs three-way handshaking to ensure bidirectional communication between the neighbors.

  • It communicates information necessary for establishing whether an adjacency can be formed with a neighboring router.

  • After an adjacency is formed, it serves as a keepalive mechanism to detect failed neighbors or adjacencies. ...

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