The answer is that PE2 lacks a label for the CE1 prefix in its LFIB, causing the return traffic to fail. While PE1 correctly installs the VPNv4 route for 10.2.2.0/24 with a valid MPLS label and forwarding path to the next-hop 192.168.1.2, the problem lies in the reverse direction: when CE2 sends traffic back to CE1 (10.1.1.0/24), PE2 must impose a label for that prefix, and if that label is missing from PE2’s label forwarding information base (LFIB), the packet is dropped. This scenario tests your understanding of MPLS L3VPN unidirectional label allocation and the need for symmetric label bindings on both PEs. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap—engineers often check only the forward path and overlook that the remote PE must also have a label for the local CE prefix. A reliable memory tip is “check both directions: if the label is missing on the far side, the return ride is denied.”
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.
An engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using OSPF with the CEs. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route. However, traffic from CE1 to CE2 fails. The show ip cef vrf CUSTOMER 10.2.2.0 command on PE1 shows the next-hop as 192.168.1.2 and the output interface as GigabitEthernet0/0. The show mpls forwarding-table 192.168.1.2 detail command on PE1 shows a label with outgoing interface GigabitEthernet0/0. The show ip route 192.168.1.2 command on PE1 shows the route with a next-hop of 10.0.0.2 and output interface GigabitEthernet0/0. The show ip cef 192.168.1.2 command on PE1 shows the next-hop as 10.0.0.2 and output interface GigabitEthernet0/0. 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 PE2 router does not have a label for the CE1 prefix in its LFIB.
All forwarding components on PE1 are correct. The issue is likely on the remote side, such as PE2 not having a label for the return traffic or CE2 not having a route back. The engineer should check PE2's forwarding table for the CE1 prefix.
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 VRF route-target import on PE2 is misconfigured.
Why it's wrong here
Incorrect because the route is installed on PE1, and import is for receiving routes.
✓
The PE2 router does not have a label for the CE1 prefix in its LFIB.
Why this is correct
Correct: If PE2 cannot forward return traffic due to missing label, traffic will be dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The OSPF process on PE1 is not redistributing BGP routes into OSPF.
Why it's wrong here
Incorrect because the route is in the VRF, and OSPF redistribution is for CE-PE, not for forwarding.
✗
The MP-BGP session is using an incorrect update-source.
Why it's wrong here
Incorrect because the route is received and installed.
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 300-410 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 300-410 OSPF questions on adjacency and route selection.
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 PE2 router does not have a label for the CE1 prefix in its LFIB. — All forwarding components on PE1 are correct. The issue is likely on the remote side, such as PE2 not having a label for the return traffic or CE2 not having a route back. The engineer should check PE2's forwarding table for the CE1 prefix.
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.
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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