In a Carrier Supporting Carrier (CSC) VPRN model where the customer carrier is a BGP/MPLS service provider, the backbone carrier's PE (CSC-PE) routers do not learn or advertise the end-customer's routes. The backbone network's responsibility is to provide transport connectivity between the customer carrier's PE routers (CSC-CEs). The CSC-PEs use MP-iBGP to exchange labeled routes for the customer carrier's infrastructure (its PEs and P-routers), not the individual VPN routes of the end-customers. The customer carrier runs its own, separate MP-BGP sessions between its own PEs over the backbone. Therefore, the statement that MP-iBGP advertises end-customer routes is not applicable to the BGP/MPLS service provider customer type, and thus is not applicable to both types of customer carriers.

A. This is the standard and recommended method. The CSC-PE (backbone carrier) and CSC-CE (customer carrier) are in different autonomous systems, making eBGP the appropriate protocol for exchanging labeled routes.

B. While eBGP is standard, using iBGP is technically possible if the CSC-PE and CSC-CE are configured in the same AS. This configuration would be applicable to either customer carrier type.

C. This is the fundamental mechanism for the backbone VPRN. CSC-PE routers use MP-iBGP to distribute the customer carrier's routes (as VPN-IPv4 prefixes) across the backbone network to other CSC-PEs.

1. Nokia 3HE15095AAABTQZZA 01 - 7450 ESS, 7750 SR, 7950 XRS, and VSR Triple Play Service Delivery Architecture Guide, Release 20.10.R1, Section: "Carrier Supporting Carrier".

Page 598: "In the CSC model, the backbone carrier does not need to know the end-customer’s routes. The backbone carrier only needs to carry the customer carrier’s PE-to-PE routing information." This directly supports why option D is incorrect for the BGP/MPLS SP customer type.

Page 599: "The CSC-PE router uses an eBGP session to the CSC-CE router to send and receive labeled IPv4 routes." This confirms option A is correct.

Page 599: "The CSC-PE router uses an MP-iBGP session to other CSC-PE routers to send and receive VPN-IPv4 routes." This confirms option C is correct.

2. RFC 4364 - BGP/MPLS IP Virtual Private Networks (VPNs), Section 10: "Carrier's Carrier".

Page 29, Paragraph 2: "The backbone carrier's PEs (the 'CSC-PEs') do not need to know the routes to the end-customers' CEs... The CSC-PEs only need to know the routes to the customer carrier's PEs (the 'CSC-CEs')." This standard reinforces that the backbone carrier is agnostic to the end-customer routes, making statement D inapplicable to the BGP/MPLS SP scenario.