350-501 Networking Practice Question
An MPLS-TE tunnel is configured with Fast Reroute using link protection. The primary path traverses links A-B and B-C. If link B-C fails, which action does the head-end router take?
⚠ Common exam trap
Cisco often tests the misconception that the head-end router handles all rerouting decisions in MPLS-TE, but FRR delegates local repair to the PLR, so candidates must remember that link protection is handled at the point of failure, not the head-end.
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
✓
The router at node B switches traffic to a pre-computed backup tunnel around link B-C.
MPLS-TE Fast Reroute (FRR) with link protection pre-computes a backup tunnel that bypasses the protected link. When link B-C fails, the router at node B (the Point of Local Repair, or PLR) immediately switches traffic to this pre-established backup tunnel, ensuring sub-50ms failover without involving the head-end router.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The router at node B switches traffic to a pre-computed backup tunnel around link B-C.
Why this is correct
Link protection works by having the PLR (point of local repair) at the upstream node.
- ✗
The head-end router immediately switches to a secondary explicit path.
Why it's wrong here
Fast Reroute is local repair; the head-end may not be aware immediately.
- ✗
RSVP-TE signals a new LSP from the head-end after detecting the failure.
Why it's wrong here
That would be much slower than FRR; FRR uses pre-signaled backup tunnels.
- ✗
Traffic is dropped until the IGP converges on the new topology.
Why it's wrong here
FRR is designed to provide sub-50ms convergence; no packet loss.
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