Refer to the exhibit. A customer carrier running MPLS VPN wants to utilize a backbone carrier to forward traffic and exchange VPNv4 prefixes between the two customer carrier networks depicted. Which…

Question

Exhibit

Options

• ABB-PE-A and CC-PE-B
• BCC-PE-A and CC-PE-B
• CBB-PE-A and BB-PE-B
• DCC-PE-A and BB-PE-A
• EBB-PE-A and BB-P-A
• FCC-PE-A and CC-P-A

• A
  13% (7)
• B
  77% (41)
• D
  2% (1)
• E
  2% (1)
• F
  6% (3)

Explanation

In Carrier Supporting Carrier (CSC) architecture, two separate iBGP planes must exist - one within each autonomous system. The customer carrier's PE routers (CC-PE-A and CC-PE-B) must establish MP-iBGP to exchange VPNv4 prefixes for their own VPN customers across the backbone, while the backbone carrier's PE routers (BB-PE-A and BB-PE-B) must establish MP-iBGP to exchange labeled IPv4 routes used to stitch the LSP across the backbone provider network.

Why the distractors are wrong:

• A (BB-PE-A ↔ CC-PE-B): These routers are in different ASes on opposite ends of the network - no direct session is needed or practical between them.
• D (CC-PE-A ↔ BB-PE-A): This session does exist, but it is eBGP (inter-AS labeled unicast), not iBGP - the question specifically asks for MP-iBGP.
• E (BB-PE-A ↔ BB-P-A): P (core) routers in the backbone don't participate in BGP at all; they simply forward labeled packets using the existing LSP.
• F (CC-PE-A ↔ CC-P-A): Same reasoning - customer carrier P routers forward traffic via MPLS but don't need MP-BGP sessions.

Memory tip: "iBGP stays in-house" - each carrier only runs iBGP within its own AS (CC-PE↔CC-PE for VPNv4, BB-PE↔BB-PE for labeled routes). Anything crossing AS boundaries is eBGP, and P routers never touch BGP in this design.

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