The answer is to configure the backup path to use adjacency SIDs instead of node SIDs. This is correct because TI-LFA calculates a post-convergence backup path, and if that path includes a node SID belonging to a router that is only reachable via the failed link, the backup path itself becomes unreachable, causing traffic loss far exceeding the expected sub-50ms failover. The core issue is that a node SID forces traffic to be forwarded to that specific router, but if that router is dependent on the failed link, the backup path collapses. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of how TI-LFA interacts with SR-MPLS and IGP metrics, often appearing as a trap where candidates overlook that node SIDs imply strict forwarding to a specific router, while adjacency SIDs provide a more flexible, link-specific path that avoids the failed dependency. A helpful memory tip is: node SIDs name the router, adjacency SIDs name the link—when the link is down, don’t name the router that needs it.
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. 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 service provider has deployed Segment Routing (SR-MPLS) with OSPF as the IGP. They enabled TI-LFA for link protection. During a maintenance window, they shut down a core link between two P routers. Expected behavior is that TI-LFA should provide sub-50ms failover. However, after the shutdown, traffic loss exceeds 10 seconds. Analysis shows that the backup path uses a segment list that includes a node SID from a router that is currently unreachable due to the same failure. The TI-LFA backup path calculation appears to have included a node that is dependent on the failed link. What design issue is most likely causing this?
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
✓
Configure the backup path to use adjacency SIDs instead of node SIDs
TI-LFA calculates backup paths using the post-convergence topology. If the backup path includes a node that is only reachable through the failed link, it means the TI-LFA calculation did not properly exclude nodes that are dependent on the failure. This can happen if the IGP metric on the backup path is not properly set, or if the router does not consider remote node failures correctly. The correct action is to ensure that the IGP metric on the backup path is lower than the direct path, or to implement MICRO-LOOP avoidance. Alternatively, the TI-LFA may need to use adjacency SIDs instead of node SIDs for the backup path.
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.
✗
Increase the prefix-SID index on all routers to avoid conflicts
Why it's wrong here
Prefix-SID index conflict is not the issue.
✓
Configure the backup path to use adjacency SIDs instead of node SIDs
Why this is correct
Adjacency SIDs are link-specific and do not depend on the reachability of a node that may be affected by the failure.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Reduce the IGP metric on the backup links to ensure they are perceived as shorter paths
Why it's wrong here
Metrics are already set; the backup path includes a node that becomes unreachable, so metric change alone may not fix it.
✗
Disable TI-LFA and rely on LFA
Why it's wrong here
LFA would also fail if the topology does not provide a loop-free alternate; TI-LFA is more robust.
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.
What to study next
Got this wrong? Here's your next step.
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 350-501 OSPF questions on adjacency and route selection.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Configure the backup path to use adjacency SIDs instead of node SIDs — TI-LFA calculates backup paths using the post-convergence topology. If the backup path includes a node that is only reachable through the failed link, it means the TI-LFA calculation did not properly exclude nodes that are dependent on the failure. This can happen if the IGP metric on the backup path is not properly set, or if the router does not consider remote node failures correctly. The correct action is to ensure that the IGP metric on the backup path is lower than the direct path, or to implement MICRO-LOOP avoidance. Alternatively, the TI-LFA may need to use adjacency SIDs instead of node SIDs for the backup path.
What should I do if I get this 350-501 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 350-501 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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