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. ...