The correct answer is that ASBRs exchange IPv4 labeled unicast routes using MP-eBGP. This is correct because in Inter-AS MPLS VPN Option B, the ASBRs establish a multihop MP-eBGP session between them specifically for the IPv4 labeled unicast address family, allowing them to pass VPNv4 routes across autonomous systems without needing per-VRF sessions or VRFs on the ASBRs. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding of scalable inter-AS connectivity, often appearing as a distractor against Option A (which uses VRF-to-VRF at the ASBR) or Option C (which uses multihop MP-eBGP between route reflectors). A common trap is confusing the address family—remember, Option B uses the IPv4 labeled unicast AFI/SAFI, not VPNv4 directly between ASBRs. Memory tip: “B for Between ASBRs, using labeled unicast to bridge VPNv4.”
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. 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 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?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
ASBRs exchange IPv4 labeled unicast routes using MP-eBGP
In Inter-AS Option B, ASBRs exchange IPv4 labeled unicast routes using MP-eBGP (Multiprotocol BGP) with the IPv4 labeled unicast address family. This allows the ASBRs to pass VPNv4 routes between autonomous systems without requiring per-VRF sessions or VRFs on the ASBRs, enabling scalable inter-AS connectivity.
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.
✗
Each ASB router maintains VRFs for all attached customers
Why it's wrong here
In option B, ASBRs do not maintain VRFs; they simply forward labeled VPN routes.
✗
Inter-AS option B requires a full mesh of MP-iBGP between all PEs
Why it's wrong here
Inter-AS option B avoids full mesh of iBGP by using eBGP between ASBRs.
✗
MP-eBGP sessions are established per VRF between ASBRs
Why it's wrong here
Only one MP-eBGP session is needed between ASBRs to exchange all VPNv4 routes.
✓
ASBRs exchange IPv4 labeled unicast routes using MP-eBGP
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
Under the hood, Option B uses ASBRs to re-advertise VPNv4 routes from one AS to another via MP-eBGP, preserving the MPLS labels and VPN route distinguishers. A subtle behavior is that the ASBR must be configured with the 'no bgpdefault route-target filter' command to accept all VPNv4 routes, as it does not have VRFs to filter by route-target. In real-world deployments, this model scales well for large service providers interconnecting multiple customer VPNs across AS boundaries.
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 practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: ASBRs exchange IPv4 labeled unicast routes using MP-eBGP — In Inter-AS Option B, ASBRs exchange IPv4 labeled unicast routes using MP-eBGP (Multiprotocol BGP) with the IPv4 labeled unicast address family. This allows the ASBRs to pass VPNv4 routes between autonomous systems without requiring per-VRF sessions or VRFs on the ASBRs, enabling scalable inter-AS connectivity.
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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Question Discussion
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