352-001 · Question #170
352-001 Question #170: Real Exam Question with Answer & Explanation
The correct answer is B: when MPLS L3VPN PE devices are configured in the Level 1 areas. IS-IS Level 1 routers rely on a default route to reach destinations outside their area, but MPLS L3VPN PE devices require specific prefix reachability to build LSPs and forward VPN traffic. Route leaking from Level 2 into Level 1 is necessary to provide those specific routes.
Question
Options
- Awhen a multicast RP is configured in the nonbackbone area
- Bwhen MPLS L3VPN PE devices are configured in the Level 1 areas
- Cwhen equal cost load balancing is required between the backbone and nonbackbone areas
- Dwhen unequal cost load balancing is required between the backbone and nonbackbone areas
Explanation
IS-IS Level 1 routers rely on a default route to reach destinations outside their area, but MPLS L3VPN PE devices require specific prefix reachability to build LSPs and forward VPN traffic. Route leaking from Level 2 into Level 1 is necessary to provide those specific routes.
Common mistakes.
- A. Multicast RP reachability does not require route leaking because PIM can resolve the RP address using the default route that Level 1 routers already receive from the nearest L1/L2 boundary router.
- C. Equal cost load balancing between backbone and nonbackbone areas is handled natively by the default route mechanism in Level 1 and does not require specific route leaking from Level 2.
- D. Unequal cost load balancing is not a recognized technical justification for IS-IS route leaking; Level 1 routers forward traffic toward the closest L1/L2 router regardless of metric variation to the destination.
Concept tested. IS-IS Level 2 to Level 1 route leaking for MPLS VPN
Community Discussion
No community discussion yet for this question.