In an Inter-AS Model C VPRN, the ingress PE (PE1) pushes a three-label stack onto the packet destined for the remote PE (PE2). This is required to traverse the multiple autonomous systems.

1. VPN Label (Innermost): This label is advertised by the egress PE (PE2) via the direct MP-eBGP session with PE1. It identifies the specific VPRN service instance on PE2.

2. BGP Label (Middle): This label is used for transport between the Autonomous System Boundary Routers (ASBRs). ASBR2 advertises a labeled route for PE2 to ASBR1, which then advertises it to PE1. This BGP label ensures the packet is correctly forwarded from ASBR1 to PE2.

3. Transport/LDP Label (Outermost): This label is learned via LDP within the source AS (AS1). It is used to forward the packet from PE1 to its BGP next-hop, which is ASBR1.

A. One label is insufficient for an Inter-AS scenario; it cannot provide transport across both the intra-AS and inter-AS links while also identifying the VPRN service.

B. A two-label stack is characteristic of Inter-AS Model B, where ASBRs exchange VPN-IPv4 routes and perform a label swap, not Model C.

D. Four labels are not used in a standard Inter-AS Model C deployment. This might be seen in more complex hierarchical models like Carrier-of-Carriers, but not this scenario.

1. Nokia 7750 SR OS Services Guide, Release 20.10.R1, VPRN Services, Section: Inter-AS VPRNs. This guide details the operation of different Inter-AS VPRN models. For Model C (Option C), it describes the data plane forwarding, which involves a three-label stack pushed by the ingress PE: an LDP label to reach the local ASBR, a BGP label to cross the inter-AS link and reach the remote PE, and the VPN label for service demultiplexing at the remote PE.

2. Rosen, E., & Rekhter, Y. (2006). BGP/MPLS IP Virtual Private Networks (VPNs). RFC 4364, Section 10. This foundational RFC describes the architecture for BGP/MPLS VPNs. Section 10, "Multi-AS Backbones," explicitly states for the Option C scenario: "This requires a label stack of depth 3. The top label takes the packet to the egress ASBR of the first AS. The middle label is the one assigned by the egress ASBR of the second AS for the egress PE. The bottom label is the VPN label."

3. Nokia 4A0-106 Courseware, Nokia Virtual Private Routed Networks, Module 4: VPRN Service Provisioning, Section: Inter-AS VPRNs. The official course material explains that in Model C, the data plane requires three labels to be imposed by the ingress PE to ensure end-to-end transport and service identification across the separate autonomous systems.