How does Inter-AS Option-A function when two PE routers in different autonomous systems are directly connected?

Question

Options

• AThe two routers share all Inter-AS VPNv4 routes and redistribute routes within an IBGP session
• BThe two routers establish an MP-EBGP session to share their customers' respective VPNv4
• CThe two routers treat one another as CE routers and advertise unlabeled IPv4 routes through an
• DThe two routers share VPNv4 routes over a multihop EBGP session and set up an Inter-AS

• A
  3% (1)
• B
  6% (2)
• C
  89% (32)
• D
  3% (1)

Explanation

Inter-AS Option-A (also called "Back-to-Back VRF") works by having each ASBR (AS Border Router) maintain a separate VRF per customer and treat the directly connected peer ASBR as if it were a CE router. Because each ASBR sees the other as a CE device, they exchange plain, unlabeled IPv4 routes using a standard per-VRF routing protocol (eBGP, static, OSPF, etc.) - no MPLS labels or VPNv4 prefixes cross the AS boundary. Each ASBR then independently re-originates those routes as VPNv4 within its own AS.

Why the distractors are wrong:

• A is wrong because Option-A never uses an IBGP session between ASBRs; that would be within the same AS.
• B describes Option-B, where ASBRs exchange VPNv4 routes via MP-EBGP - the key difference is labels do cross the boundary.
• D describes Option-C (multihop MP-EBGP between RRs/PEs across ASes), which requires LSP stitching end-to-end.

Memory tip: Think of Option-A as the "dumb pipe" method - the ASBRs strip everything down to plain IP, acting like CEs to each other. It's the simplest option but scales poorly because each ASBR needs a VRF per customer per neighbor. "A = Acts like a CE" keeps the concept anchored.

No community discussion yet for this question.