Comprehensive and Detailed In-Depth Explanation:
Understanding the IS-IS Levels and Default Route Advertisement:
* R1 is a Level-1 router
* R2 is a Level-1-2 router (meaning it connects both Level-1 and Level-2 areas)
* R3 is a Level-2-only router
Step 1: What Does "default-route-advertise always level-1" Do?
* This command forces R1 to advertise a default route (0.0.0.0/0) to all its Level-1 IS-IS neighbors, regardless of whether R1 has a default route in its routing table.
* Since R1 is a Level-1 router, it does not participate in Level-2 routing, meaning it relies on R2 (a Level-1-2 router) for external connectivity.
Step 2: Checking the Routing Tables
* R1: Since it is only a Level-1 router and does not receive a default route from any higher-level router, it does not have a default route in its own routing table. # (D is correct)
* R2: Since R1 advertises the default route at Level-1, R2 (a Level-1-2 router) receives this default route in its Level-1 database. # (B is correct)
* R3: R3 is a Level-2-only router, and default routes are not automatically leaked between Level-1 and Level-2 unless explicitly configured.
* Since no explicit route leaking configuration is mentioned, R3 will not have the default route
# (C is incorrect).
* A is incorrect because the command successfully advertises a default route from R1 to Level-1 neighbors like R2.
Final Answer: # B, D
References:
* HCIP-Datacom-Advanced Routing & Switching Technology V1.0 - IS-IS Default Route Advertisement
* IS-IS Routing Level-1 and Level-2 Behavior
* IS-IS Route Leaking and Default Route Propagation