IS-IS uses its neighbor‘s MAC address to form an adjacency and stores the neighbors MAC address to recognize the adjacency in the future. This causes no problems with directly adjacent devices but problems can occur when adjacency is required between devices that are more than one hop away. IS-IS Point-to-Point over Ethernet accommodates an IS-IS network with this type of configuration.
When IS-IS Point-to-Point over Ethernet is used, devices that are several hops away or available through an IP GRE tunnel can form an IS-IS adjacency. IS-IS Point-to-Point over Ethernet can also be used when only two IS‘s are part of the broadcast network. IS-IS Point-to-Point over Ethernet is configured at the interface level of the devices that are forming an adjacency.The figure below shows two devices several hops away from each other that are configured for IS-IS adjacency.
