- A
The VPN label is removed by the ASBR before forwarding to the neighbor AS.
Why wrong: The VPN label is swapped, not removed, to maintain MPLS encapsulation.
- B
ASBRs peer using eBGP and exchange labeled VPN-IPv4 prefixes.
ASBRs eBGP peer and exchange VPNv4 prefixes with labels, allowing end-to-end MPLS.
- C
Route reflectors are required to propagate VPN routes between ASes.
Why wrong: Route reflectors are not required for inter-AS option B; ASBRs directly exchange routes.
- D
ASBRs perform label swap for VPN labels when forwarding traffic.
ASBRs swap the VPN label (inner label) when forwarding cross-AS traffic.
- E
ASBRs exchange unlabeled IPv4 routes and use MP-BGP to carry VPNv4 routes.
Why wrong: In option B, ASBRs exchange labeled VPNv4 routes, not unlabeled IPv4.
Quick Answer
The answer is that ASBRs perform label swap for VPN labels when forwarding traffic in inter-AS Option B. This is correct because in this design, ASBRs from different autonomous systems peer using eBGP and exchange labeled VPN-IPv4 prefixes (AFI 1, SAFI 128), which allows the VPNv4 routes to cross AS boundaries without requiring a full mesh of MP-IBGP between PEs. The ASBR re-advertises these routes with a new next-hop and allocates a fresh VPN label, effectively swapping the label as traffic traverses the inter-AS link. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of how MPLS L3VPN inter-AS option B scales by pushing label operations to the ASBR, avoiding per-VRF state on intermediate routers. A common trap is confusing this with Option A, where the ASBR maintains separate VRF instances; in Option B, the ASBR simply swaps labels without VRF context. Memory tip: think “B for Border swap” — the ASBR at the border swaps the VPN label.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 deploying L3VPN with inter-AS option B (ASBR-to-ASBR). Which TWO statements are true about this design?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
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.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
In inter-AS Option B, the ASBR receives a VPNv4 route from the PE with a VPN label (e.g., MPLS label 100). When re-advertising to the neighbor ASBR via eBGP, the ASBR allocates a new VPN label (e.g., 200) and updates the BGP next-hop to its own interface address. This label swap ensures that the MPLS label stack is correctly maintained across the AS boundary, and the receiving ASBR can then forward traffic to the correct egress PE using the new label.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
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FAQ
Questions learners often ask
What does this 350-501 question test?
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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