Cisco
352-001 · Question #252
352-001 Question #252: Real Exam Question with Answer & Explanation
The correct answer is D: Manually turn off the backdoor link when the Layer 3 service is up.. When retaining backup WAN links during an MPLS L3 VPN migration, manually disabling those links while the VPN is active is the definitive way to prevent them from carrying transit traffic.
Question
You have been asked to establish a design that will allow your company to migrate from a WAN service to a Layer 3 VPN service. In your design, you are keeping some WAN links between critical sites as a backup for this service. You plan to use OSPF as your IGP and BGP for the PE- CE protocol. When the Layer 3 VPN service is available, how will you prevent transit traffic over the backup links?
Options
- ARedistribute routes at the CE as external type 1.
- BRedistribute routes at the CE as external type 2.
- CUse conditional advertisement only when the Layer 3 service is down.
- DManually turn off the backdoor link when the Layer 3 service is up.
Explanation
When retaining backup WAN links during an MPLS L3 VPN migration, manually disabling those links while the VPN is active is the definitive way to prevent them from carrying transit traffic.
Common mistakes.
- A. Redistributing routes as OSPF external type 1 changes how the external metric accumulates across the domain but does not prevent the backdoor link from forming adjacencies or attracting transit traffic when both paths are simultaneously active.
- B. Redistributing as OSPF external type 2 applies a flat external metric that does not grow with internal path cost, but this metric adjustment still does not block the physical backdoor link from being used for transit.
- C. BGP conditional advertisement controls whether prefixes are sent to peers based on the presence of other routes - it affects route advertisement policy, not which physical path transit traffic uses when OSPF and BGP paths are both present.
Concept tested. Preventing backup link transit traffic during L3 VPN migration
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