The answer is that LDP is not enabled on the interface between PE1 and P1. This is the most likely cause because when the MPLS forwarding table on PE1 shows the label for the BGP next-hop (PE2’s loopback) as ‘Untagged’, it means LDP has failed to assign a label for that IGP prefix on the incoming interface. In an L3VPN, LDP must distribute labels for all IGP loopback routes so that PE routers can push the correct transport label for the BGP next-hop; without it, PE1 cannot encapsulate VPNv4 traffic, and the core routers (P1, P2) drop the packets since they lack the VPN route. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the MPLS label stack and the dependency of LDP on directly connected interfaces—a common trap is assuming that reachability via ICMP guarantees MPLS label assignment. Remember the memory tip: “ICMP reachable does not mean LDP label stable”—always verify the MPLS forwarding table for the ‘Untagged’ status to pinpoint missing LDP adjacency.
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 a network with PE1, P1, P2, and PE2 connected in a chain: PE1-P1-P2-PE2. The IGP is IS-IS with wide metrics, and MPLS LDP is enabled on all interfaces. The goal is to provide L3VPN services between PE1 and PE2. The engineer has configured MP-iBGP between PE1 and PE2, and the VPNv4 routes are exchanged. However, when a CE router behind PE1 tries to reach the CE behind PE2, the traffic fails. The engineer checks the MPLS forwarding table on PE1 and sees that the label for the BGP next-hop (PE2's loopback) is 'Untagged'. The BGP next-hop is reachable via ICMP. What is the most likely cause of this issue?
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
✓
LDP is not enabled on the interface between PE1 and P1.
The MPLS forwarding table on PE1 shows the label for PE2's loopback as 'Untagged', which indicates that LDP has not assigned a label for that prefix on the incoming interface. Since LDP is responsible for distributing labels for IGP routes (like loopbacks) in an MPLS LDP-enabled network, the missing label means LDP is not operational on the link between PE1 and P1. Without a label, PE1 cannot push an MPLS label for the BGP next-hop, causing the VPNv4 traffic to be dropped or forwarded as IP, which fails because the core routers (P1, P2) do not have the VPN route.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 IS-IS metric between PE1 and P1 is too high, causing suboptimal routing.
Why it's wrong here
High metric might affect path, but not label assignment.
✗
The MPLS TTL propagation is disabled, causing packets to be dropped.
Why it's wrong here
TTL propagation does not affect label binding.
✓
LDP is not enabled on the interface between PE1 and P1.
Why this is correct
Without LDP, no label is received from P1 for the BGP next-hop.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The MTU on the link between PE1 and P1 is set to 1500, causing fragmentation.
Why it's wrong here
Fragmentation does not cause 'Untagged'.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between BGP route exchange (which works) and MPLS label assignment (which fails), leading candidates to incorrectly focus on routing protocol issues or MTU/TTL parameters instead of verifying LDP adjacency and label bindings.
Detailed technical explanation
How to think about this question
In MPLS LDP networks, each LSR must establish an LDP session and exchange label bindings for all IGP prefixes (including loopbacks) on every interface where LDP is enabled. The 'show mpls forwarding-table' command displays the outgoing label for a given prefix; 'Untagged' means the router has no label binding for that prefix on the outgoing interface, often because LDP is not enabled or the session is down. A common real-world scenario is forgetting to enable LDP on a core-facing interface, which breaks MPLS forwarding for all VPN traffic traversing that link.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: LDP is not enabled on the interface between PE1 and P1. — The MPLS forwarding table on PE1 shows the label for PE2's loopback as 'Untagged', which indicates that LDP has not assigned a label for that prefix on the incoming interface. Since LDP is responsible for distributing labels for IGP routes (like loopbacks) in an MPLS LDP-enabled network, the missing label means LDP is not operational on the link between PE1 and P1. Without a label, PE1 cannot push an MPLS label for the BGP next-hop, causing the VPNv4 traffic to be dropped or forwarded as IP, which fails because the core routers (P1, P2) do not have the VPN route.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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