In cross-domain VPN-OptionB packet forwarding, two ASBRs exchange the same LSP tags once.

The statement is false. In cross-domain VPN-OptionB, two Autonomous System Boundary Routers (ASBRs) establish an MP-EBGP session to exchange labeled VPNv4 routes. Each ASBR allocates a VPN label for the routes it advertises. This label is locally significant to the advertising ASBR. The receiving ASBR uses this label when forwarding traffic back to the advertising ASBR. The labels exchanged for a given prefix in opposite directions are generated independently and are not the same. Furthermore, this exchange is a continuous process as part of the BGP protocol, not a one-time event.

A. If the statement were taken as “true”, it would ignore the label swap that occurs at the first ASBR; hence it is incorrect.

1. Huawei HedEx Documentation, NE40E V800R012C10 Configuration Guide - BGP/MPLS IP VPN, "Inter-AS VPN Option B". The documentation explains that ASBRs exchange labeled VPN-IPv4 routes, with each ASBR advertising a next-hop label for prefixes in its own AS.

2. RFC 4364, "BGP/MPLS IP Virtual Private Networks (VPNs)", Section 10, "Multi-AS Backbones". This RFC details the procedure for Option B, where ASBRs exchange VPN-IPv4 NLRI with a new next hop (the advertising ASBR) and a new label. The labels are allocated by each ASBR independently.