The answer is EBGP with BFD, as this combination delivers the fastest convergence for PE-CE link failures in a Layer 3 MPLS VPN design. BFD provides sub-second, hardware-based failure detection that is significantly faster than any routing protocol’s hello timers, including OSPF fast hello or EIGRP’s proprietary mechanisms, because BFD operates independently of the routing protocol and can detect link loss in milliseconds. On the Cisco SPCOR 350-501 exam, this question tests your understanding of convergence mechanics in service provider MPLS VPNs, often appearing as a direct multiple-choice trap where OSPF with fast hello seems correct but lacks BFD’s reliability and speed advantage. A common memory tip is to remember that BFD is the “bullet train” for failure detection—no routing protocol hello can outrun it.
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 service provider is designing a Layer 3 MPLS VPN for a customer with two sites. The customer requires fast convergence in case of a PE-CE link failure. Which routing protocol should be used between PE and CE to achieve the fastest convergence?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
EBGP with BFD
EBGP with BFD provides sub-second failure detection due to hardware-based BFD, which is faster than any routing protocol's hello timers. OSPF with fast hello can achieve sub-second detection but is not as reliable as BFD. EIGRP has fast convergence but is proprietary. Static routing with object tracking requires additional configuration and is slower. IS-IS with SPF tuning does not address failure detection speed directly.
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.
✗
Static routing with object tracking
Why it's wrong here
Static routing with tracking is slow and requires manual intervention.
✗
IS-IS with SPF tuning
Why it's wrong here
SPF tuning does not affect link failure detection; it optimizes path computation after detection.
✗
EIGRP
Why it's wrong here
EIGRP is proprietary and not typically used in multi-vendor environments.
✓
EBGP with BFD
Why this is correct
BFD provides sub-second failure detection independent of routing protocol, enabling fast convergence.
Fast hello can achieve detection in milliseconds but BFD is more deterministic and widely used in SP cores.
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.
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: EBGP with BFD — EBGP with BFD provides sub-second failure detection due to hardware-based BFD, which is faster than any routing protocol's hello timers. OSPF with fast hello can achieve sub-second detection but is not as reliable as BFD. EIGRP has fast convergence but is proprietary. Static routing with object tracking requires additional configuration and is slower. IS-IS with SPF tuning does not address failure detection speed directly.
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Question Discussion
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