With which type of IS-IS router can an IS-IS Level 1-only router form an adjacency?

An IS-IS Level 1-only router can form an adjacency specifically with another IS-IS Level 1 router that resides within the same area. This is due to the design and functionality of the IS-IS protocol, which separates networks into Level 1 and Level 2 areas. Level 1 routers are meant to handle intra-area communication and can only establish neighbor relationships (or adjacencies) with other Level 1 routers that are part of the same area. This ensures that routing information is properly managed and contained within the confines of that designated area, resulting in a more efficient and organized routing environment.

In contrast, Level 2 routers operate at a broader level and can connect with other Level 2 routers across different areas, which is why a Level 1-only router would not be able to form an adjacency with them. Proper configuration of an IS-IS router can enable various forms of adjacencies, but the fundamental relationship between Level 1 routers is limited to their specific area. Thus, the correct relationship is strictly between Level 1 routers within the same area, reinforcing the IS-IS protocol's hierarchical structure for optimized routing.