The correct answer is to configure the domain-id under the OSPF process within the VRF on the PE router. This OSPF attribute, known as the domain-id, is used in MPLS L3VPN to tag routes originating from the customer’s OSPF domain, preventing them from being redistributed back into that same domain when they return from the MPLS backbone. Without this tag, the PE router would treat the VPNv4 routes as external and could re-inject them, causing routing loops. On the ENCOR 350-401 exam, this concept tests your understanding of OSPF route redistribution control in a service provider context, often appearing as a distractor where options like sham-link or VRF-lite seem plausible. A common trap is confusing domain-id with sham-link, which is used for backdoor links, not loop prevention. Remember: domain-id keeps OSPF routes from coming back home—think of it as a “do not re-enter” stamp for your customer’s routing domain.
CCNP 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 configuring MPLS L3VPN with OSPF as the PE-CE protocol. The customer wants to ensure that routes from the MPLS backbone are not redistributed back into the customer's OSPF domain. What configuration should the engineer apply on the PE router?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the 'domain-id' under the OSPF process in the VRF.
In MPLS L3VPN, the 'domain-id' OSPF attribute prevents redistribution of routes from the backbone into the customer's OSPF domain. Option A is correct. Option B is wrong because 'sham-link' is for backdoor links; Option C is wrong because 'capability vrf-lite' is for VRF-lite; Option D is wrong because 'redistribute connected' is not related.
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.
✓
Configure the 'domain-id' under the OSPF process in the VRF.
Why this is correct
Correct because the domain-id prevents routes from being redistributed back into the customer's OSPF domain.
Incorrect because sham-links are used for backdoor links, not to prevent redistribution.
✗
Use the 'capability vrf-lite' command under OSPF.
Why it's wrong here
Incorrect because vrf-lite is used for non-MPLS environments.
✗
Apply a route-map to filter routes redistributed into OSPF.
Why it's wrong here
Incorrect because route-maps can filter, but the domain-id is the standard method.
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 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: Configure the 'domain-id' under the OSPF process in the VRF. — In MPLS L3VPN, the 'domain-id' OSPF attribute prevents redistribution of routes from the backbone into the customer's OSPF domain. Option A is correct. Option B is wrong because 'sham-link' is for backdoor links; Option C is wrong because 'capability vrf-lite' is for VRF-lite; Option D is wrong because 'redistribute connected' is not related.
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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