With which type of IS-IS router can an IS-IS Level 2-only router form an adjacency?

An IS-IS Level 2-only router can form an adjacency with any other IS-IS Level 2 router. This capability is fundamental to the design and function of the IS-IS routing protocol, which delineates router types based on their operational levels. Level 2 routers are responsible for routing between different areas in the network and facilitate inter-area communication.

The reason this answer is correct lies in the nature of IS-IS itself. Level 2 routers operate at a broader level than Level 1 routers, which is confined to intra-area routing. The adjacency between Level 2 routers allows for the sharing of link-state information that is vital for building a complete and efficient topology map of routes across multiple areas. This allows IS-IS Level 2-only routers to effectively communicate and maintain routing tables that reflect the current state of the network.

Recognizing the importance of this adjacency permits those routers to properly exchange routing updates and act as a backbone within the network, thus ensuring that data can flow efficiently across different areas. Such design principles highlight the significance of adhering to the specified levels within IS-IS to maintain correct operational behavior and optimal routing paths.