350-501 Architecture Practice Question
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?
⚠ Common exam trap
Cisco often tests the misconception that OSPF with fast hello timers or EIGRP provides the fastest convergence, but the trap here is that BFD is the only mechanism that offers hardware-assisted, sub-second detection independent of routing protocol timers, making it the fastest option for PE-CE link failure convergence in MPLS VPNs.
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
✓
EBGP with BFD
EBGP with BFD provides the fastest convergence for PE-CE link failures because BFD can detect link failures in sub-second intervals (as low as 50 ms) and trigger BGP to withdraw routes immediately, without waiting for routing protocol timers. This is critical for Layer 3 MPLS VPNs where fast failover is required, and EBGP is commonly used as the PE-CE routing protocol in service provider environments.
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.
- ✗
OSPF with fast hello timers
Why it's wrong here
Fast hello can achieve detection in milliseconds but BFD is more deterministic and widely used in SP cores.
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
Go deeper
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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