350-501 Networking Practice Question
A service provider is deploying L3VPN with inter-AS option B (ASBR-to-ASBR). Which TWO statements are true about this design?
⚠ Common exam trap
Cisco often tests the misconception that Option B requires route reflectors or that the VPN label is removed at the ASBR, when in fact the ASBR performs a label swap and directly exchanges VPNv4 prefixes via eBGP without needing route reflectors.
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 peer using eBGP and exchange labeled VPN-IPv4 prefixes.
In inter-AS Option B, ASBRs peer using eBGP and exchange labeled VPN-IPv4 prefixes (AFI 1, SAFI 128). This allows the VPNv4 routes to be carried across AS boundaries without requiring a full mesh of MP-IBGP between PEs, as the ASBRs re-advertise the routes with a new next-hop and perform label allocation for the VPN labels.
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 VPN label is removed by the ASBR before forwarding to the neighbor AS.
Why it's wrong here
The VPN label is swapped, not removed, to maintain MPLS encapsulation.
- ✓
ASBRs peer using eBGP and exchange labeled VPN-IPv4 prefixes.
Why this is correct
ASBRs eBGP peer and exchange VPNv4 prefixes with labels, allowing end-to-end MPLS.
- ✗
Route reflectors are required to propagate VPN routes between ASes.
Why it's wrong here
Route reflectors are not required for inter-AS option B; ASBRs directly exchange routes.
- ✓
ASBRs perform label swap for VPN labels when forwarding traffic.
Why this is correct
ASBRs swap the VPN label (inner label) when forwarding cross-AS traffic.
- ✗
ASBRs exchange unlabeled IPv4 routes and use MP-BGP to carry VPNv4 routes.
Why it's wrong here
In option B, ASBRs exchange labeled VPNv4 routes, not unlabeled IPv4.
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