In the context ofJuniper Networks Junos OSand theIS-IS (Intermediate System to Intermediate System)protocol, understanding the hierarchical relationship between router levels is critical for effective troubleshooting and design. IS-IS uses a two-level hierarchy to manage scalability:Level 1 (L1), which represents intra-area routing, andLevel 2 (L2), which represents inter-area backbone routing.
When a router is configured as aLevel 1/2 (L1/L2)device, it acts as a bridge between the two levels. According to Juniper technical documentation, an L1/L2 router maintains two completelyseparate Link-State Databases (LSDB)—one for Level 1 and one for Level 2. It does not merge these into a single topology. This separation ensures that local area topology changes (L1) do not necessarily flood into the backbone (L2) unless specific redistribution is configured, and vice versa. Therefore, statementCis correct because the L1/L2 router maintains distinct SPF (Shortest Path First) computations for each level.
Regarding the visibility of the Level 1 router, IS-IS is designed to keep L1 areas "stubby" by default. ALevel 1 routeronly possesses the topology information for its own area (the Level 1 LSDB). It does not receive specific L2 routes or the L2 topology. Instead, the L1/L2 router sets theAttached (ATT) bitin its L1 Link-State PDUs (LSPs) to signal to L1-only routers that it has a connection to the backbone. The L1 router then generates a default route pointing to the L1/L2 router to reach inter-area destinations. This confirms that statementDis correct: the L1 router's knowledge is limited to its local L1 topology.
Conversely, statements A and B are incorrect because merging topologies would violate the hierarchical scaling principles of IS-IS, and L1 routers never learn the full L2 topology without explicit, non-standard route leaking.
