The answer is to check and adjust the SRGB configuration on PE-5 to match the range used by other routers. This is correct because when a prefix-SID is learned in the IS-IS database but not installed in the MPLS forwarding table, it typically indicates a label allocation failure caused by an SRGB mismatch or an out-of-range label index. Since the IGP advertises the prefix-SID, the issue lies in the label mapping: if PE-5’s SRGB does not overlap with PE-3’s, the resulting label falls outside the allocated block and is rejected by the LFIB. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of Segment Routing label propagation and the critical role of consistent SRGB configuration across all routers in an SR-MPLS domain. A common trap is to assume the problem is with BGP or IS-IS adjacency, but the forwarding table absence despite a learned prefix-SID points directly to a label range conflict. Memory tip: “SID in the database, missing in the table? Check the SRGB label—if it’s out of range, it’s a mismatch, not a flap.”
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 large service provider runs a dual-stack MPLS core with Segment Routing (SR-MPLS) and IS-IS as the IGP. The network has been operating stably for months. Recently, a new PE router (PE-5) was added and configured with IS-IS and SR. After the addition, some remote prefixes in the VRF on other PEs become unreachable. Troubleshooting reveals that the BGP next hop for those prefixes is the loopback of another PE (PE-3), but the MPLS forwarding table on PE-3 shows no label for the prefix. The IS-IS database on PE-3 shows the prefix-SID for PE-5's loopback, but the forwarding table does not have a label for that prefix. Commands like 'show mpls forwarding-table' show no entry for PE-5's loopback. What is the most likely cause and correct action?
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
✓
Check and adjust the SRGB configuration on PE-5 to match the range used by other routers
The issue is that PE-3 does not have a label for PE-5's loopback because the prefix-SID for PE-5 is not programmed in the LFIB. This commonly happens when the SRGB (Segment Routing Global Block) on PE-5 does not overlap with PE-3's SRGB, or when the label index is not configured correctly. Since the IS-IS database shows the prefix-SID, the IGP is advertising it, but the label may be out of range. The correct action is to check and align the SRGB configuration on all routers. Option B (clear IS-IS adjacency) is a generic reset that might not fix the underlying mismatch. Option C (enable MPLS on the interface) is already done if LDP was used, but SR uses IGP, not LDP. Option D (redistribute into BGP) is not relevant.
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.
✗
Clear the IS-IS adjacency between PE-3 and PE-5
Why it's wrong here
Clearing the adjacency will reset the relationship but not fix a fundamental configuration mismatch.
✗
Enable MPLS on the interface connecting to PE-5
Why it's wrong here
MPLS is already enabled implicitly with SR; the issue is label allocation, not interface configuration.
✓
Check and adjust the SRGB configuration on PE-5 to match the range used by other routers
Why this is correct
If SRGB ranges are mismatched, the label allocated may be invalid, causing the forwarding entry not to be installed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Configure BGP to redistribute IS-IS routes into BGP
Why it's wrong here
BGP redistribution is not related to the missing label in the MPLS forwarding table.
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: Check and adjust the SRGB configuration on PE-5 to match the range used by other routers — The issue is that PE-3 does not have a label for PE-5's loopback because the prefix-SID for PE-5 is not programmed in the LFIB. This commonly happens when the SRGB (Segment Routing Global Block) on PE-5 does not overlap with PE-3's SRGB, or when the label index is not configured correctly. Since the IS-IS database shows the prefix-SID, the IGP is advertising it, but the label may be out of range. The correct action is to check and align the SRGB configuration on all routers. Option B (clear IS-IS adjacency) is a generic reset that might not fix the underlying mismatch. Option C (enable MPLS on the interface) is already done if LDP was used, but SR uses IGP, not LDP. Option D (redistribute into BGP) is not relevant.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A service provider is migrating their MPLS core network to Segment Routing (SR-MPLS). All existing core routers run IOS-XR and are configured with an SRGB of 16000-23999 and OSPF as the IGP. A new router (R5) is added as a PE. The engineer configures R5 with the same SRGB and enables segment-routing under OSPF. However, when configuring an SR-TE policy from R5 to the remote loopback 10.0.0.1 on R1, the policy remains down. Show commands reveal that R5 is not learning the prefix-SID for 10.0.0.1. On R1, the prefix-SID is configured with index 100. The engineer verifies that OSPF adjacencies are up and that R5 can ping 10.0.0.1. What is the most likely cause of the issue?
medium
✓ A.The OSPF process on R5 is not configured with segment-routing mpls.
B.The SR-TE policy on R5 is missing the color attribute.
C.R1 is not configured with the same SRGB as R5.
D.The prefix-SID index 100 is not within the SRGB range of R5 (16000-23999).
Why A: Option B is correct because without 'segment-routing mpls' under the OSPF process, R5 cannot participate in Segment Routing, meaning it does not advertise its own prefix-SIDs nor learn the prefix-SIDs of other routers. This prevents the SR-TE policy from obtaining the necessary label bindings. Option A is incorrect because index 100 maps to label 16100 (16000+100), which is within the SRGB range. Option C is incorrect because while color is a key attribute in SR-TE policies, the policy can still be defined with a segment-list; however, the primary issue here is the lack of prefix-SID learning. Option D is incorrect because the SRGB is consistent across all routers.
Last reviewed: Jun 24, 2026
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