The answer is that MPLS TE Fast Reroute (FRR) is not configured on the tunnels. MPLS TE tunnels, by default, provide only a primary path for traffic engineering; they do not automatically reroute traffic around a link failure unless explicit backup protection is enabled. FRR works by pre-computing and signaling a backup path (often using a bypass or detour tunnel) that is activated immediately upon a link failure, typically within 50 milliseconds. On the ENCOR 350-401 exam, this scenario tests your understanding that MPLS TE and MPLS TE FRR are separate features—a common trap is assuming that simply enabling TE tunnels and RSVP provides redundancy. Remember that RSVP signals the primary path, but FRR is the mechanism that installs the backup. A useful memory tip is: "TE gives you the road, FRR gives you the detour." Without FRR, a link failure will cause traffic to drop until the IGP reconverges, which is not the fast protection that MPLS TE is designed to provide.
350-401 MPLS Practice Question
This 350-401 practice question tests your understanding of mpls. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer is configuring MPLS TE (Traffic Engineering) in an MPLS core to optimize bandwidth utilization. After enabling MPLS TE on all core routers and configuring tunnels, the engineer notices that traffic is not being rerouted when a link fails. The 'show mpls traffic-eng tunnels' shows the tunnels are up but not using the backup path. What is the most likely missing configuration?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MPLS TE FRR (Fast Reroute) is not configured on the tunnels.
MPLS TE requires explicit backup path configuration, such as FRR (Fast Reroute) or backup tunnels, to provide protection. Without it, traffic will not be rerouted automatically. Option A is correct. Option B is wrong because LDP is not required for TE; Option C is wrong because RSVP is the signaling protocol, not the issue; Option D is wrong because link-state routing is fine.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
MPLS TE FRR (Fast Reroute) is not configured on the tunnels.
Why this is correct
Correct because FRR provides backup paths for link failures in MPLS TE.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Incorrect because LDP is not used for TE tunnel protection.
✗
RSVP is not configured on the core routers.
Why it's wrong here
Incorrect because RSVP is needed for TE signaling, but its presence alone does not provide backup paths.
✗
OSPF is not configured with MPLS TE extensions.
Why it's wrong here
Incorrect because TE extensions are needed for path computation, not for backup path activation.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
MPLS — This question tests MPLS — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: MPLS TE FRR (Fast Reroute) is not configured on the tunnels. — MPLS TE requires explicit backup path configuration, such as FRR (Fast Reroute) or backup tunnels, to provide protection. Without it, traffic will not be rerouted automatically. Option A is correct. Option B is wrong because LDP is not required for TE; Option C is wrong because RSVP is the signaling protocol, not the issue; Option D is wrong because link-state routing is fine.
What should I do if I get this 350-401 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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