- A
The command 'show ip bgp vpnv4 vrf CUSTOMER_A' shows no prefixes on the remote PE.
Indicates that VPNv4 routes are not being received, a control plane issue.
- B
The command 'show mpls forwarding-table vrf CUSTOMER_A' shows no labels for remote prefixes.
Missing labels mean the LFIB is not populated, often due to BGP label allocation failure.
- C
The MP-BGP session between PEs is in the 'Idle' or 'Active' state.
A down MP-BGP session prevents VPN route exchange.
- D
Ping from CE1 to CE2 fails, but ping from CE1 to the local PE succeeds.
Why wrong: This could be due to many issues (e.g., routing, MTU, label switching) and is not exclusively a control plane symptom.
- E
The IGP adjacency between PE and P routers is down.
Why wrong: This is an underlay issue, not a direct VPN control plane problem.
Quick Answer
The answer is that an MP-BGP session between PEs stuck in the 'Idle' or 'Active' state is a direct symptom of an MPLS L3VPN control plane misconfiguration. This occurs because the control plane relies on Multiprotocol BGP to exchange VPNv4 prefixes and their corresponding MPLS labels between provider edge routers; if the session fails to establish, no VPN routes can be advertised, effectively breaking the Layer 3 VPN. On the Cisco CCNP ENARSI 300-410 exam, this question tests your ability to isolate control plane faults from data plane or underlay issues—a common trap is confusing a ping failure from CE to CE, which could stem from many causes, with the specific control plane symptom of a missing MP-BGP session. Remember that while a missing IGP adjacency affects the underlay, it is not a direct VPN control plane symptom; the key is to focus on BGP session states and the presence of VPNv4 prefixes. Memory tip: "No session, no VPN" — if the MP-BGP session is down, the control plane cannot exchange routes.
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. 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.
Which THREE symptoms indicate a misconfiguration in the MPLS L3VPN control plane between two PEs? (Choose THREE.)
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The command 'show ip bgp vpnv4 vrf CUSTOMER_A' shows no prefixes on the remote PE.
Common control plane issues include missing VPNv4 prefixes in BGP, lack of MPLS labels for VPN routes, and failure to establish the MP-BGP session. Correct routing table entries on the PE but no labels suggests a label allocation problem. Ping failure from CE to CE could be due to many issues, not specifically control plane. IGP adjacency down affects the underlay but is not a direct VPN control plane symptom.
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.
- ✓
The command 'show ip bgp vpnv4 vrf CUSTOMER_A' shows no prefixes on the remote PE.
Why this is correct
Indicates that VPNv4 routes are not being received, a control plane issue.
Related concept
OSPF neighbours must agree on key parameters.
- ✓
The command 'show mpls forwarding-table vrf CUSTOMER_A' shows no labels for remote prefixes.
- ✓
The MP-BGP session between PEs is in the 'Idle' or 'Active' state.
- ✗
Ping from CE1 to CE2 fails, but ping from CE1 to the local PE succeeds.
Why it's wrong here
This could be due to many issues (e.g., routing, MTU, label switching) and is not exclusively a control plane symptom.
- ✗
The IGP adjacency between PE and P routers is down.
Why it's wrong here
This is an underlay issue, not a direct VPN control plane problem.
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 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
MPLS L3VPN — This question tests MPLS L3VPN — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The command 'show ip bgp vpnv4 vrf CUSTOMER_A' shows no prefixes on the remote PE. — Common control plane issues include missing VPNv4 prefixes in BGP, lack of MPLS labels for VPN routes, and failure to establish the MP-BGP session. Correct routing table entries on the PE but no labels suggests a label allocation problem. Ping failure from CE to CE could be due to many issues, not specifically control plane. IGP adjacency down affects the underlay but is not a direct VPN control plane symptom.
What should I do if I get this 300-410 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 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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