The correct answer is to verify that LDP has allocated a label for the BGP next-hop address on PE2. In MPLS L3VPN troubleshooting, even when BGP successfully advertises a VPNv4 route with a valid next-hop, actual packet forwarding depends on LDP binding a label to that next-hop address; without this label, the router cannot push the necessary MPLS header to transport traffic across the service provider core. This scenario tests your understanding of the separation between control plane reachability (BGP) and data plane forwarding (LDP label switching), a classic trap on the ENCOR 350-401 exam where candidates assume a route in BGP means end-to-end connectivity. A common memory tip is “BGP knows the road, but LDP builds the tunnel”—if the tunnel isn’t built, traffic can’t travel.
350-401 MPLS Practice Question
This 350-401 practice question tests your understanding of mpls. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 uses MPLS L3VPN with OSPF as the PE-CE routing protocol. A customer reports that a new subnet added on CE1 is not reachable from CE2, even though the PE1 router has the route in its VRF and BGP table. The 'show ip bgp vpnv4 vrf CUSTOMER' on PE2 shows the prefix with a valid next-hop. What should the engineer check next?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Verify that LDP has allocated a label for the BGP next-hop address on PE2.
In MPLS L3VPN, even if BGP has the route, the packet forwarding requires a valid MPLS label for the next-hop. If LDP is not resolving the BGP next-hop, the route is not usable. Option A is correct. Option B is wrong because OSPF is fine; Option C is wrong because the route is in BGP; Option D is wrong because redistribution is already working.
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.
✓
Verify that LDP has allocated a label for the BGP next-hop address on PE2.
Why this is correct
Correct because without a label for the next-hop, the VPN route cannot be forwarded.
Incorrect because the route is already in BGP, so redistribution is working.
✗
Ensure the route is present in the global BGP table on PE2.
Why it's wrong here
Incorrect because the route is in the VPNv4 BGP table, which is sufficient.
✗
Confirm that the VRF on PE2 has the correct route-target import.
Why it's wrong here
Incorrect because the route is already in the VRF BGP table, so import is working.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
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 350-401 OSPF questions on adjacency and route selection.
MPLS — This question tests MPLS — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Verify that LDP has allocated a label for the BGP next-hop address on PE2. — In MPLS L3VPN, even if BGP has the route, the packet forwarding requires a valid MPLS label for the next-hop. If LDP is not resolving the BGP next-hop, the route is not usable. Option A is correct. Option B is wrong because OSPF is fine; Option C is wrong because the route is in BGP; Option D is wrong because redistribution is already working.
What should I do if I get this 350-401 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 350-401 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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