The three standard models for implementing inter-AS VPRNs, as defined in RFC 4364, are Option A, Option B, and Option C. Options A, C, and D in the question correspond to these valid models. Option B describes the exchange of labeled IPv4 routes (BGP labeled-unicast address family) between route reflectors. This mechanism is used to establish transport MPLS Label Switched Paths (LSPs) across autonomous systems, not to exchange VPRN service routes. VPRN service connectivity requires the exchange of VPN-IPv4 routes (BGP vpn-ipv4 address family), which carry the necessary Route Distinguisher (RD) and Route Target (RT) information to maintain VPN separation and routing policies.

A. This describes Inter-AS VPRN Option B, a standard and widely used method where AS Border Routers (ASBRs) exchange VPN-IPv4 routes using MP-eBGP.

C. This describes Inter-AS VPRN Option A, also known as back-to-back VRF. It is a valid, albeit less scalable, implementation method.

D. This describes Inter-AS VPRN Option C, where Provider Edge (PE) routers in different autonomous systems establish a direct, multi-hop MP-eBGP session to exchange VPN-IPv4 routes.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPRN Services" chapter, Section: "Inter-AS VPRNs". This document explicitly details the three valid inter-AS VPRN models: Option A (Back-to-Back VRF at the ASBR), Option B (MP-eBGP between ASBRs for VPN-IPv4), and Option C (MP-eBGP between PEs for VPN-IPv4). This confirms that A, C, and D are viable options.

2. RFC 4364, "BGP/MPLS IP Virtual Private Networks (VPNs)", Section 10, "Multi-AS Backbones". This foundational RFC defines the three inter-AS VPN architectures.

Section 10a describes the VRF-to-VRF model (Option A/C).

Section 10b describes the MP-eBGP exchange of VPN-IPv4 routes between ASBRs (Option A/A).

Section 10c describes the multi-hop MP-eBGP between PEs in different ASes (Option D).

3. RFC 8277, "Using BGP to Bind MPLS Labels to Address Prefixes", Section 1, "Introduction". This RFC, which obsoletes RFC 3107, defines the BGP Labeled Unicast address family (AFI=1, SAFI=4). It clarifies that its purpose is to advertise an MPLS label for an IP prefix, which is a transport plane function for building LSPs, and is distinct from the VPN-IPv4 address family (AFI=1, SAFI=128) used for VPRN services. This supports why option B is not a method for implementing the VPRN service itself.