An adjacency is an acknowledged relationship between two IS-IS routers. An adjacency must be established before two IS-IS routers can exchange routing information.
IS-IS routers establish adjacencies by exchanging hello PDUs, which are also called Intermediate System to Intermediate System Hellos (IIHs). Hello PDUs contain some interface configuration and capability information. Once a pair of neighbors exchanges hello PDUs with acceptably matching configuration and capabilities, an adjacency is formed. Hello PDUs are sent periodically by each party to maintain the adjacency.
After an adjacency is formed, information about the adjacency is stored in a link state PDU (LSP), which is stored in the router link state database (LSDB). Routers periodically flood all of their LSPs to all other network nodes. When a router receives LSPs, it adds the LSPs to its LSDB, and uses the LSDB to calculate routes to other routers. These database maintenance operations are performed a little differently for the two adjacency types, point-to-point, and broadcast.