350-501 Inter-AS Option B Practice Question
A large enterprise uses MPLS L3VPN to connect multiple sites. They want to implement inter-AS option B (ASBR-to-ASBR MP-eBGP) for scalability. Which statement about this model is correct?
⚠ Common exam trap
Cisco often tests the misconception that ASBRs need VRFs or per-VRF sessions in Option B, when in fact they operate at the VPNv4 address family level without any customer-specific configuration.
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
✓
ASBRs exchange VPNv4 routes using MP-eBGP
In Inter-AS Option B, ASBRs use MP-eBGP to exchange VPNv4 routes. They do not maintain VRFs (Option A incorrect), and sessions are per address family, not per VRF (Option C incorrect). Option B is incorrect because a full mesh of MP-iBGP between PEs is not required. Option D correctly describes the VPNv4 route exchange using MP-eBGP.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Each ASB router maintains VRFs for all attached customers
Why it's wrong here
In Inter-AS Option B, ASBRs do not maintain VRFs for attached customers; they operate at the VPNv4 address family level.
- ✗
Inter-AS option B requires a full mesh of MP-iBGP between all PEs
Why it's wrong here
Inter-AS Option B does not require a full mesh of MP-iBGP between all PEs; it uses MP-eBGP between ASBRs.
- ✗
MP-eBGP sessions are established per VRF between ASBRs
Why it's wrong here
MP-eBGP sessions in Option B are established per address family (VPNv4), not per VRF.
- ✓
ASBRs exchange VPNv4 routes using MP-eBGP
Why this is correct
In Option B, ASBRs exchange VPNv4 routes, not IPv4 labeled unicast routes. IPv4 labeled unicast is characteristic of Option C.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.