- A
The BGP session between the PEs is flapping due to a hold timer mismatch or connectivity issue.
If the BGP session between PEs goes down, VPN routes are withdrawn, causing connectivity loss. When the session comes back, routes are re-advertised, restoring connectivity. The pattern matches.
- B
The LDP graceful restart timer is causing periodic re-establishment of LDP sessions.
Why wrong: LDP graceful restart helps during session loss, but if sessions are stable, the timer does not cause drops.
- C
The OSPF network type on the core links is set to point-to-point, causing SPF recalculation every 5 minutes.
Why wrong: OSPF point-to-point links do not cause periodic SPF; SPF is triggered by topology changes.
- D
One of the core interfaces is flapping due to a hardware issue, triggering IGP and LDP convergence.
Why wrong: Interface flapping would cause repeated route updates, but the engineer reports stable OSPF and LDP.
Quick Answer
The answer is a BGP session flapping between the PEs due to a hold timer mismatch or connectivity issue. In an MPLS VPN, the BGP session carries VPNv4 routes between PE routers; if that session drops, the routes are withdrawn from the RIB, causing the customer’s connectivity to vanish until the session re-establishes and the routes are re-advertised. This explains the periodic pattern of a few minutes of connectivity followed by a drop, which aligns with the BGP hold timer expiring. On the Cisco SPCOR 350-501 exam, this scenario tests your ability to distinguish BGP session stability from LDP or IGP issues—a common trap is to blame LDP graceful restart, but here all LDP sessions are up and OSPF is stable. Remember: in MPLS VPN, the BGP session is the glue for customer routes; if it flaps, the VPN service flaps. Memory tip: “BGP holds the VPN routes—if the hold timer fails, the VPN fails.”
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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 small service provider is deploying MPLS in its core network for the first time. They have a simple topology: three P routers in the core and two PE routers connecting customers. They have configured LDP on all interfaces and OSPF as the IGP. They also configured a basic L3VPN for a customer with a single CE attached to each PE. The customer reports that they can ping between CEs for a few minutes, but then the connectivity drops. After a few seconds, it returns. This pattern repeats every few minutes. The engineer checks the LDP neighbors and sees that all LDP sessions are up. The OSPF adjacencies are stable. The engineer also notices that the pings time out exactly when the LDP graceful restart timer is expiring on one of the P routers. 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:
"first"Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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 session between the PEs is flapping due to a hold timer mismatch or connectivity issue.
Option D is correct because if the BGP session between PEs is going down (e.g., due to a hold timer issue or misconfiguration), the VPN routes would be withdrawn and re-advertised, causing intermittent connectivity. Option A is wrong because LDP graceful restart timer is for LDP session recovery, but LDP sessions are up. Option B is wrong because OSPF network type does not cause periodic drops. Option C is wrong because interface flaps would cause more permanent outages.
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 BGP session between the PEs is flapping due to a hold timer mismatch or connectivity issue.
Why this is correct
If the BGP session between PEs goes down, VPN routes are withdrawn, causing connectivity loss. When the session comes back, routes are re-advertised, restoring connectivity. The pattern matches.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The LDP graceful restart timer is causing periodic re-establishment of LDP sessions.
Why it's wrong here
LDP graceful restart helps during session loss, but if sessions are stable, the timer does not cause drops.
- ✗
The OSPF network type on the core links is set to point-to-point, causing SPF recalculation every 5 minutes.
Why it's wrong here
OSPF point-to-point links do not cause periodic SPF; SPF is triggered by topology changes.
- ✗
One of the core interfaces is flapping due to a hardware issue, triggering IGP and LDP convergence.
Why it's wrong here
Interface flapping would cause repeated route updates, but the engineer reports stable OSPF and LDP.
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 350-501 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-501 question test?
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The BGP session between the PEs is flapping due to a hold timer mismatch or connectivity issue. — Option D is correct because if the BGP session between PEs is going down (e.g., due to a hold timer issue or misconfiguration), the VPN routes would be withdrawn and re-advertised, causing intermittent connectivity. Option A is wrong because LDP graceful restart timer is for LDP session recovery, but LDP sessions are up. Option B is wrong because OSPF network type does not cause periodic drops. Option C is wrong because interface flaps would cause more permanent outages.
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: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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