The answer is that the PE2 loopback is not advertised into the service provider core IGP, causing an MPLS L3VPN next-hop unreachable recursive lookup failure. This occurs because CEF performs a recursive lookup on the BGP next-hop 192.168.1.2 to find an output interface, but when the IGP (OSPF or IS-IS) lacks a route to that loopback, the lookup fails and CEF marks the interface as 'no route'. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the BGP next-hop resolution process in MPLS L3VPNs, where the provider core must have an IGP route to the remote PE’s loopback for successful label-switched path establishment. A common trap is assuming the BGP route is valid simply because it appears in the VRF table; the key is verifying the IGP reachability to the next-hop. Memory tip: “No route in CEF means no IGP to the next-hop—check the core, not the VRF.”
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.
A network 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 and MP-BGP between them. 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 as well. 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 but the output interface is 'no route'. 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 loopback is not advertised into the IGP (OSPF/IS-IS) of the service provider core.
CEF has a next-hop but no output interface because the recursive routing table lookup for the BGP next-hop (192.168.1.2) fails. The IGP (OSPF or IS-IS) does not have a route to the PE2 loopback, so CEF cannot resolve the adjacency.
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 OSPF process on PE1 is not redistributing connected routes.
Why it's wrong here
Incorrect because the IGP route to the PE2 loopback is missing, not the CE routes.
✓
The PE2 loopback is not advertised into the IGP (OSPF/IS-IS) of the service provider core.
Why this is correct
Correct: The BGP next-hop must be reachable via IGP for CEF to resolve the output interface.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Incorrect because the route is present in the VRF routing table.
✗
MPLS is not enabled on the core-facing interfaces.
Why it's wrong here
Incorrect because the issue is IGP reachability, not MPLS label switching.
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 loopback is not advertised into the IGP (OSPF/IS-IS) of the service provider core. — CEF has a next-hop but no output interface because the recursive routing table lookup for the BGP next-hop (192.168.1.2) fails. The IGP (OSPF or IS-IS) does not have a route to the PE2 loopback, so CEF cannot resolve the adjacency.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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 have MP-BGP peering and the VRF is configured with route-target import 100:100. 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, but the show ip route vrf CUSTOMER command does not have this route. The show ip bgp vpnv4 all 10.2.2.0/24 command on PE1 shows the route is received but not best. What is the most likely cause?
hard
A.The route-target import on PE1 is missing.
✓ B.The BGP next-hop (PE2 loopback) is not reachable in the global routing table.
C.The VRF on PE1 has a different route-target export.
D.The MP-BGP session is using an incorrect address family.
Why B: The route is received but not marked as best, so it is not installed in the routing table. Common reasons include the route being suppressed due to a higher AD from another source or the next-hop being unreachable. In this scenario, the most likely cause is that the BGP next-hop is not reachable in the global routing table.
Last reviewed: Jun 18, 2026
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.