Comprehensive and Detailed In-Depth Explanation:
Understanding the IS-IS Network Structure in the Question:
* Router Roles & IS-IS Levels:
* R1 and R3: Level-1 routers.
* R2: Level-1-2 router (interconnects Level-1 and Level-2 areas).
* R4: Level-2 router.
* Key Command Used: default-route-advertise always level-1
* This command forces R1 to advertise a default route (0.0.0.0/0) to all Level-1 routers.
* In an IS-IS network:
* Level-1 routers do not see Level-2 routes unless a default route is provided.
* The default route is propagated only within the Level-1 area and does not affect Level-
2 routers unless explicitly redistributed.
Route Distribution Analysis:
* R1 (Level-1) imports the default route and advertises it to all Level-1 routers.# R1 has the default route.
* R3 (Level-1) is in the same Level-1 area as R1, so it receives the default route from R1.# R3 has the default route.
* R2 (Level-1-2) connects Level-1 and Level-2. Since it participates in Level-1, it also learns the default route from R1.# R2 has the default route.
* R4 (Level-2) is not in Level-1 and does not receive the default route unless it is explicitly redistributed from Level-1 to Level-2, which is not stated in the question.# R4 does NOT have the default route.
Final Conclusion:
* R1, R2, and R3 have the default route.
* R4 does not have the default route.
Thus, the correct answer is:# R1, R2, and R3 have the default route in their routing tables.