350-501 Architecture Practice Question
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?
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.
The symptom of periodic connectivity drops that recover after a few seconds, coinciding with the LDP graceful restart timer, indicates that the BGP session between the PEs is flapping. When the BGP session goes down, the VPN routes are withdrawn, causing the CE-to-CE connectivity to drop. Once the BGP session re-establishes, routes are re-advertised and connectivity returns. LDP graceful restart timer expiration is not the cause (LDP sessions are up), but may coincide with BGP hold timer failures. Options B, C, and D are incorrect because LDP graceful restart does not cause the described pattern, OSPF network type does not cause periodic drops, and interface flapping would cause more permanent outages.
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If the BGP session between PEs goes down, VPN routes are withdrawn, causing connectivity loss.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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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