Building Adjacencies
When an IS-IS router is connected to a broadcast (or multi-access) network, it immediately begins sending out IS-IS hellos. When connected to a point-to-point link, a router waits until it builds an ES-IS adjacency with the device on the other end before it determines to transmit IS-IS hellos.
These hellos are always padded to the maximum transmission unit (MTU) size of the link. This way, two routers will not build an adjacency over a link with different MTUs configured on either end.
When two IS-IS neighbors first begin bringing up an adjacency, they exchange Complete Sequence Number Packets (CNSPs) to synchronize their databases. Once a pair of routers are adjacent, Partial Sequence Number Packets (PSNPs) are used to ...
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