The answer is that the route-target export on PE1 does not match the route-target import on PE2. In MPLS L3VPN, route targets are the fundamental mechanism that controls the import and export of VPN routes between VRFs; even when overlapping IP addresses exist between sites, correctly configured route distinguishers make each prefix globally unique, but without matching RTs, the BGP VPNv4 routes will never be installed in the remote VRF. This scenario tests your understanding of how MPLS L3VPN separates control-plane route distribution from address uniqueness—a key distinction on the ENCOR 350-401 exam, where a common trap is confusing route distinguishers with route targets. Remember, RDs solve overlapping addresses, but RTs solve route exchange; a helpful memory tip is “RTs are the gatekeepers—they decide who gets in and who stays out.”
350-401 MPLS Practice Question
This 350-401 practice question tests your understanding of mpls. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 designing an MPLS L3VPN service for a customer that requires overlapping IP addresses between two sites. The customer uses OSPF as the PE-CE protocol. The engineer configures VRFs on the PE routers and assigns unique route distinguishers (RDs) and route targets (RTs). However, the customer reports that routes from one site are not being installed in the other site's VRF. 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
✓
The route-target export on PE1 does not match the route-target import on PE2.
In MPLS L3VPN, route targets control the import/export of routes between VRFs. If the RTs are not configured correctly, routes will not be exchanged. Option A is correct. Option B is wrong because overlapping addresses are handled by VRFs; Option C is wrong because OSPF can handle overlapping addresses with proper configuration; Option D is wrong because RDs only make prefixes unique, they do not control route exchange.
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 route-target export on PE1 does not match the route-target import on PE2.
Why this is correct
Correct because route targets must match for routes to be imported into the remote VRF.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The overlapping IP addresses cause a routing loop in OSPF.
Why it's wrong here
Incorrect because VRFs keep the routing tables separate, preventing loops.
✗
OSPF cannot carry overlapping prefixes in different VRFs.
Why it's wrong here
Incorrect because OSPF can run in multiple VRFs without issue.
✗
The route distinguisher is not unique between the two sites.
Why it's wrong here
Incorrect because RDs only need to be unique per VRF, not across sites.
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: The route-target export on PE1 does not match the route-target import on PE2. — In MPLS L3VPN, route targets control the import/export of routes between VRFs. If the RTs are not configured correctly, routes will not be exchanged. Option A is correct. Option B is wrong because overlapping addresses are handled by VRFs; Option C is wrong because OSPF can handle overlapping addresses with proper configuration; Option D is wrong because RDs only make prefixes unique, they do not control route exchange.
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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