4.3. Adjacencies
As you progress through this book, you will learn that IS-IS and OSPF have quite a few optional capabilities—particularly OSPF. Before two neighbors that have discovered each other can begin exchanging routing information, they must ensure that they understand each other’s capabilities. Otherwise, some of the information exchanged might be misunderstood, or interpreted differently between the two neighbors, resulting in inaccurate or broken routing. Additionally, the neighbors must ensure that they agree on some basic parameters such as that they are on the same IP subnet, that their interface MTUs are the same, how often each neighbor should expect a Hello message from the other, and so on.
To ensure that two IS-IS or OSPF ...
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