In an Inter-AS VPRN Model B architecture, the Autonomous System Boundary Routers (ASBRs) exchange VPN-IPv4 routes with other ASBRs using Multiprotocol eBGP (MP-eBGP). Within each Autonomous System (AS), the Provider Edge (PE) routers establish Multiprotocol iBGP (MP-iBGP) sessions with their local ASBR. As PE1 and ASBR1 are both within AS1, they use an internal BGP session. This session must support the VPN-IPv4 address family to carry VPRN routes, which necessitates the use of MP-iBGP. The ASBR effectively acts as a route reflector for the PE within its own AS for VPRN prefixes.

A. iBGP: This is incorrect because standard iBGP does not support the advertisement of VPN-IPv4 address family routes, which is a requirement for VPRN services.

B. eBGP: This is incorrect as eBGP is used for routing between different Autonomous Systems. The connection between PE1 and ASBR1 is within a single AS (AS1).

D. MP-eBGP: This is incorrect because MP-eBGP is used between ASBRs in different Autonomous Systems (e.g., between ASBR1 and ASBR2), not between a PE and an ASBR within the same AS.

1. Nokia 7750 SR and 7950 XRS Layer 3 Services Guide: VPRN, Release 22.10.R1, Document ID: DG-2210-0111.

Section: Inter-AS VPRNs > Inter-AS VPRN Model B

Quote/Paraphrase: This section explicitly states, "The PEs in each AS have MP-iBGP sessions to their local ASBR. The ASBRs in each AS are interconnected via MP-eBGP sessions." This confirms that the session between a PE (like PE1) and its local ASBR (ASBR1) is MP-iBGP.

2. Nokia Service Routing Architect (SRA) Self-Study Guide: Preparing for the BGP, VPRN and Multicast Exams, Document ID: SRC-SSG-BVM-R19.

Section: Chapter 4: Virtual Private Routed Network (VPRN) Services > Inter-AS VPRNs > Option B

Quote/Paraphrase: The guide describes Option B by detailing that VPN-IPv4 routes are distributed from the PE to the ASBR using MP-iBGP. The ASBR then redistributes these routes to the remote ASBR using MP-eBGP. This directly supports the use of MP-iBGP between PE1 and ASBR1.