- A
OSPF route redistribution from PE-B into BGP is set to external type 2, while PE-D expects type 1.
Why wrong: Incorrect: OSPF route type does not affect BGP import or next-hop reachability; all OSPF routes are treated similarly.
- B
The route target for Site B is misconfigured on PE-D, not matching the export RT from PE-B.
Why wrong: Incorrect: The VPNv4 table shows the route with the correct RT, so import is not the issue.
- C
PE-D is not assigning an MPLS label for the route to Site B, causing packets to be dropped.
Why wrong: Incorrect: The route in the VPNv4 table shows mpls labels in/out, so label allocation is intact.
- D
The BGP next-hop (10.0.2.2) for Site B's route is not reachable in PE-D's global routing table.
Correct: If the next-hop is unreachable, the VPNv4 route cannot be installed in the VRF, causing loss of connectivity to that specific site.
Quick Answer
The answer is that the BGP next-hop 10.0.2.2 for Site B’s route is not reachable in PE-D’s global routing table. This is the most common cause of an MPLS L3VPN BGP next-hop unreachable scenario: even though the VPNv4 route is imported with the correct Route Target, the route will not be installed into the VRF unless the next-hop address is resolvable via the global IGP. In this case, the route for 10.10.20.0/24 appears in the BGP VPNv4 table on PE-D with RT:100:200, confirming import works, but a failed ping from PE-D to that prefix indicates the VRF lacks the route—pointing directly to a global routing table reachability failure. On the Cisco SPCOR 350-501 exam, this tests your understanding of the BGP next-hop resolution requirement for VPNv4 prefixes, a classic trap where candidates assume a visible VPNv4 route means it is active in the VRF. A quick memory tip: “If the next-hop is missing, the VRF route is dismissing.”
350-501 Services Practice Question
This 350-501 practice question tests your understanding of services. 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 ISP operates an MPLS Layer 3 VPN backbone. A customer, Corporation X, has four sites (A, B, C, D) each connected to a different PE router (PE-A, PE-B, PE-C, PE-D). They use OSPF as the CE-PE routing protocol. Sites A, B, and C can exchange routes without issue. However, Site D suddenly loses connectivity to Site B, though it can still reach Sites A and C. Show commands on PE-D reveal that the VRF for Corporation X imports RT:100:200, and the route for Site B (prefix 10.10.20.0/24) is present in the BGP VPNv4 table with RT:100:200 and next-hop 10.0.2.2. The OSPF neighbor between PE-D and the CE at Site D is up, and no route filters are configured. ‘ping 10.10.20.0’ from PE-D fails. What is the most likely cause of the issue?
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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The BGP next-hop (10.0.2.2) for Site B's route is not reachable in PE-D's global routing table.
The route is in the VPNv4 table with the correct RT, so import is working. The next-hop is 10.0.2.2. If that next-hop is not reachable in the global routing table of PE-D, the VPNv4 route will not be installed in the VRF. Checking ‘show ip route 10.0.2.2’ on PE-D would confirm. Option A is plausible but incorrect because an RT mismatch would prevent the route from even appearing in the VPNv4 table. Option C is incorrect because OSPF is redistributed, and the route type does not affect reachability. Option D is incorrect because label allocation is not the issue; the prefix is present with labels.
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.
- ✗
OSPF route redistribution from PE-B into BGP is set to external type 2, while PE-D expects type 1.
- ✗
The route target for Site B is misconfigured on PE-D, not matching the export RT from PE-B.
Why it's wrong here
Incorrect: The VPNv4 table shows the route with the correct RT, so import is not the issue.
- ✗
PE-D is not assigning an MPLS label for the route to Site B, causing packets to be dropped.
Why it's wrong here
Incorrect: The route in the VPNv4 table shows mpls labels in/out, so label allocation is intact.
- ✓
The BGP next-hop (10.0.2.2) for Site B's route is not reachable in PE-D's global routing table.
Why this is correct
Correct: If the next-hop is unreachable, the VPNv4 route cannot be installed in the VRF, causing loss of connectivity to that specific site.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
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.
Trap categories for this question
Similar concept trap
Incorrect: OSPF route type does not affect BGP import or next-hop reachability; all OSPF routes are treated similarly.
Command / output trap
Incorrect: The VPNv4 table shows the route with the correct RT, so import is not the issue.
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 350-501 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-501 question test?
Services — This question tests Services — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The BGP next-hop (10.0.2.2) for Site B's route is not reachable in PE-D's global routing table. — The route is in the VPNv4 table with the correct RT, so import is working. The next-hop is 10.0.2.2. If that next-hop is not reachable in the global routing table of PE-D, the VPNv4 route will not be installed in the VRF. Checking ‘show ip route 10.0.2.2’ on PE-D would confirm. Option A is plausible but incorrect because an RT mismatch would prevent the route from even appearing in the VPNv4 table. Option C is incorrect because OSPF is redistributed, and the route type does not affect reachability. Option D is incorrect because label allocation is not the issue; the prefix is present with labels.
What should I do if I get this 350-501 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 350-501 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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Last reviewed: Jun 24, 2026
This 350-501 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 350-501 exam.
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