adjacency

An adjacency is a close link-state relationship between compatible neighboring routers that allows them to share routing information and forward network traffic. In OSPF, routers become fully adjacent when their compatibility is confirmed and they synchronize their link-state databases. In IS-IS, adjacencies proceed in stages from Down to Up; they are Up when their compatibility is confirmed. IS-IS adjacencies are level 1 or level 2, depending on the level capability of the routers.