The picture below shows MPLS VPN cross-domain option Method C: The data packet sent from PE2 with the destination of 192.168.1.1 carries a three-layer label. Which of the following descriptions of…

Question

Exhibit

Options

• AB2 is assigned by ASBR-PE2
• BB2 is assigned by ASBR-PE1
• CT4 is allocated by ASBR-PE2
• DV1 is allocated by PE1

• A
  83% (20)
• B
  8% (2)
• C
  8% (2)

Explanation

In MPLS VPN Inter-AS Option C, PE2 pushes a three-label stack where each label originates from a specific router. V1 (VPN label, innermost) is assigned by PE1 - the egress PE - to identify the specific VPN forwarding entry for 192.168.1.1, which is why D is correct. B2 (BGP label, middle) is assigned by ASBR-PE2 when it re-advertises labeled BGP routes from AS1 into AS2 toward PE2, allowing ASBR-PE2 to perform a label swap when forwarding traffic back toward ASBR-PE1 - which is why A is correct.

B is wrong because B2 is locally generated by ASBR-PE2 for its own re-advertisement; ASBR-PE1 assigns a separate label for its own re-advertisement within AS1, which ASBR-PE2 receives and maps to B2. C is wrong because T4 is the outer LDP transport label assigned by an intermediate P router (or next-hop LSR) within AS2, not by ASBR-PE2 - ASBR-PE2's label contribution is B2, the BGP label layer.

Memory tip: Work inward through the label stack - VPN label comes from the VPN egress PE (PE1 → V1), the BGP label comes from the local Border router advertising into your AS (ASBR-PE2 → B2), and the Transport label comes from the next-hop Transit router in the local AS core, not from the ASBR itself.

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