350-501 Architecture Practice Question
In Inter-AS MPLS Option B, which routers exchange labeled VPNv4 prefixes directly?
⚠ Common exam trap
Cisco often tests the misconception that PE routers directly exchange VPNv4 prefixes in Option B, but the correct answer is ASBR routers because the inter-AS VPNv4 exchange happens at the ASBR level using MP-eBGP, not between PEs.
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
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ASBR routers
In Inter-AS MPLS Option B, ASBR routers (Autonomous System Boundary Routers) exchange labeled VPNv4 prefixes directly across the AS boundary. This is achieved by having each ASBR perform two label operations: it pops the outer transport label from the VPNv4 prefix received from its own AS, then swaps the inner VPN label and forwards the labeled VPNv4 prefix to the neighboring ASBR in the adjacent AS. This direct exchange eliminates the need for end-to-end LSPs or MP-eBGP sessions between PE routers across different ASes.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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ASBR routers
Why this is correct
ASBRs exchange labeled VPNv4 prefixes via MP-eBGP in Option B.
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PE routers
Why it's wrong here
PE routers in different AS do not exchange directly; they go through ASBRs.
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P routers
Why it's wrong here
P routers do not run BGP.
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CE routers
Why it's wrong here
CE routers are customer routers, not part of MPLS VPN signaling.
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Route reflectors
Why it's wrong here
Route reflectors are used within an AS, not between ASes.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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