Can two Level-1 routers in different areas establish a neighbor relationship?

Two Level-1 routers situated in different areas cannot establish a neighbor relationship because Level-1 routing is specific to a single area in an IS-IS (Intermediate System to Intermediate System) networking environment. Each Level-1 router is designed to communicate within its own area to maintain a localized routing structure, which helps in managing and optimizing routing updates.

IS-IS uses a hierarchical structure, where Level-1 routers deal with intra-area routing, while Level-2 routers handle inter-area routing. A Level-1 router in one area is not able to exchange routing information or establish direct connections with a Level-1 router in another area since their routing information is scoped to their respective areas. This segmentation helps maintain the efficiency and scalability of the routing protocol by minimizing the amount of routing information that needs to be processed and propagated across the network.

In summary, the nature of Level-1 routing limits neighbor relationships to routers within the same area, preventing two Level-1 routers from forming a neighbor relationship if they are situated in different areas.