350-501 Networking Practice Question
An engineer configures MPLS TE tunnels. After configuration, the tunnel remains down. The 'show mpls traffic-eng tunnels' output shows 'Tunnel is down - path computation failed'. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between control-plane failures (path computation, TED) and signaling failures (RSVP, label distribution); the trap here is that candidates assume RSVP is the root cause because MPLS TE relies on RSVP, but the specific error message points to a missing TED, not a signaling issue.
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
✓
IGP TE extensions are not enabled on the head-end router.
The 'path computation failed' error indicates that the head-end router cannot find a valid path for the MPLS TE tunnel. IGP TE extensions (OSPF TE or IS-IS TE) must be enabled on the head-end router to flood link attributes (bandwidth, TE metrics) and build the TED (Traffic Engineering Database). Without the TED, CSPF (Constrained Shortest Path First) cannot compute a path, leaving the tunnel down.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
IGP TE extensions are not enabled on the head-end router.
Why this is correct
Correct. Without TE extensions, the router cannot compute a path.
- ✗
RSVP is not enabled on the head-end router.
Why it's wrong here
Incorrect. RSVP is for signaling, but path computation occurs before signaling.
- ✗
MPLS LDP is not enabled on the head-end router.
Why it's wrong here
Incorrect. LDP is used for label distribution, not for TE path computation.
- ✗
There is an MTU mismatch along the path.
Why it's wrong here
Incorrect. MTU mismatch would cause signaling errors, not path computation failure.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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