350-501 Services Practice Question
A service provider is deploying MPLS Layer 3 VPNs and wants to ensure that customer traffic is not dropped when a PE-CE link fails. The CE router is using static routing. Which design should be implemented to provide fast convergence?
⚠ Common exam trap
Candidates often choose IP SLA (Option B) thinking it provides fast failure detection, but they overlook that IP SLA is poll-based and slower than BFD, which is the Cisco-recommended mechanism for sub-second convergence with static routes.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable BFD on the PE-CE interface and associate it with the static route.
BFD (Bidirectional Forwarding Detection) provides sub-second failure detection on the PE-CE link, and when associated with a static route, it allows the PE to quickly remove the failed route from the routing table. This triggers fast convergence without waiting for routing protocol timers, which is critical for MPLS Layer 3 VPNs using static routing on the CE side.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure OSPF on the PE-CE link and set hello timers to 1 second.
Why it's wrong here
OSPF hello timers improve convergence but OSPF is not used for static routes; also, BFD is faster.
- ✗
Use IP SLA tracking to monitor the CE and adjust the static route.
Why it's wrong here
IP SLA is slower than BFD and adds configuration complexity.
- ✗
Increase the administrative distance of the static route to 255.
Why it's wrong here
Changing AD does not affect failure detection speed.
- ✓
Enable BFD on the PE-CE interface and associate it with the static route.
Why this is correct
BFD provides sub-second failure detection, triggering fast route withdrawal.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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