The answer is a route target mismatch between PE1 and PE2. This is correct because the route to CE2’s loopback appears as valid in the VRF BGP table on PE1, yet no MPLS label is assigned in the forwarding table, indicating that PE1 learned the VPNv4 route via MP-BGP but cannot install it into the CUST VRF due to mismatched route target import and export policies. Without a matching RT, PE1 does not consider the route as belonging to the VRF, so it cannot resolve the next hop or allocate a label for MPLS forwarding. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of how MP-BGP uses route targets to control VRF route distribution, and it is a common trap where candidates confuse a valid BGP route with a usable forwarding entry. A helpful memory tip: “Valid BGP, no label? Check the RT—if it doesn’t match, the route won’t catch.”
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.
An engineer is troubleshooting an MPLS VPN issue where CE1 cannot ping CE2 across the provider network. The PE routers are configured with MP-BGP and LDP. On PE1, 'show ip bgp vpnv4 vrf CUST' shows the route to CE2's loopback as valid, but 'show mpls forwarding-table' does not list a label for that prefix. What is the most likely cause?
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
✓
Route target mismatch between PE1 and PE2
The route is present in the VRF BGP table on PE1 (valid), but no MPLS label is assigned for that prefix in the forwarding table. This indicates that PE1 has learned the route via MP-BGP but cannot install it into the MPLS forwarding table because the route target (RT) import policy on PE1 does not match the RT export policy on PE2. Without a matching RT, PE1 does not consider the VPNv4 route as belonging to the CUST VRF, so it cannot resolve the next hop or assign a label for forwarding.
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.
✗
MTU mismatch on the MPLS core interfaces
Why it's wrong here
MTU issues cause packet drops, not missing label entries.
✗
LDP is not enabled on the core link between PE1 and P
Why it's wrong here
LDP is needed for transport labels, but the issue is about VPN label in forwarding table.
✓
Route target mismatch between PE1 and PE2
Why this is correct
Incorrect RT prevents the VPN label from being installed in the forwarding table despite BGP advertisement.
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.
✗
BGP session between PE1 and PE2 is not established
Why it's wrong here
If BGP session is down, the route would not appear in 'show ip bgp vpnv4'.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between BGP route validity (learned via MP-BGP) and VRF route installation (controlled by RT import), leading candidates to mistakenly suspect BGP session or LDP issues when the real problem is a route target mismatch.
Trap categories for this question
Command / output trap
If BGP session is down, the route would not appear in 'show ip bgp vpnv4'.
Detailed technical explanation
How to think about this question
In MPLS VPNs, the route target (RT) extended community controls VPN route import/export between VRFs. Even if MP-BGP advertises a VPNv4 route, the receiving PE will only install it into a VRF if the route's RT matches the VRF's import RT. The missing label in the forwarding table occurs because the route is not imported into the VRF, so no MPLS label is allocated for that prefix in the forwarding table. This is a common misconfiguration when RT values are not consistently defined across PEs.
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: Route target mismatch between PE1 and PE2 — The route is present in the VRF BGP table on PE1 (valid), but no MPLS label is assigned for that prefix in the forwarding table. This indicates that PE1 has learned the route via MP-BGP but cannot install it into the MPLS forwarding table because the route target (RT) import policy on PE1 does not match the RT export policy on PE2. Without a matching RT, PE1 does not consider the VPNv4 route as belonging to the CUST VRF, so it cannot resolve the next hop or assign a label for forwarding.
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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Question Discussion
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