The answer is that redistribution from OSPF into BGP under the VRF is not configured on PE1. In an MPLS L3VPN, the PE router must redistribute CE-learned OSPF routes into the VPNv4 address family of BGP so that the prefix can be advertised across the provider backbone to the remote PE. Without this redistribution, the OSPF route for 10.2.2.0/24 remains confined to the VRF routing table and never enters the global BGP table, breaking end-to-end connectivity between the CEs. On the ENCOR 350-401 exam, this scenario tests your understanding of the route redistribution boundary between the CE-PE IGP and the MP-BGP control plane—a common trap is assuming OSPF routes automatically propagate into BGP. Remember that in MPLS VPNs, OSPF and BGP are separate routing domains; the VRF acts as a bridge, but explicit redistribution is required. A helpful memory tip: “OSPF in the VRF, BGP in the VPN—redistribute to connect the domain.”
350-401 MPLS Practice Question
This 350-401 practice question tests your understanding of mpls. 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 where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are running OSPF with the CE routers. On PE1, the 'show ip route vrf CUSTOMER' output shows 10.2.2.0/24 as an OSPF route, but the prefix is not present in the global BGP table. 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
✓
Redistribution from OSPF into BGP under the VRF is not configured on PE1.
In MPLS L3VPN, CE routes must be redistributed into BGP (VPNv4) on the PE. If OSPF routes are present in the VRF but not in BGP, redistribution is missing. Option A correctly identifies this. Option B is wrong because OSPF is running; Option C is irrelevant; Option D would affect label allocation, not route advertisement.
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.
✓
Redistribution from OSPF into BGP under the VRF is not configured on PE1.
Why this is correct
Correct because VRF routes must be redistributed into BGP to be advertised as VPNv4 prefixes.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Incorrect because the route is present in the VRF, indicating the adjacency is up.
✗
The VRF forwarding table on PE1 is full.
Why it's wrong here
Incorrect because a full VRF table would not prevent a specific route from being in BGP.
✗
MPLS LDP is not enabled on the PE1-CE1 link.
Why it's wrong here
Incorrect because LDP is used for label distribution in the core, not for VPN route exchange.
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: Redistribution from OSPF into BGP under the VRF is not configured on PE1. — In MPLS L3VPN, CE routes must be redistributed into BGP (VPNv4) on the PE. If OSPF routes are present in the VRF but not in BGP, redistribution is missing. Option A correctly identifies this. Option B is wrong because OSPF is running; Option C is irrelevant; Option D would affect label allocation, not route advertisement.
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.
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.
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