In the upstream direction, when the BR receives an IPv6 packet destined for the BR prefix, a source-based IPv6 address lookup (anti-spoofing) is performed to verify that the packet is sent by the credible CE.
In the downstream direction, a destination-based IPv4 lookup is performed. This leads to the MAP-T rule entry, which provides the information necessary to derive the IPv6 address of the destination CE.
The MAP-T forwarding function in the VSR is also responsible for:
address conversion between IPv4 and IPv6 based on the BMR rule
header translation between IPv4 and IPv6, as described in RFC 6145
In address-sharing scenarios, address translation is performed for TCP/UDP and a subset of ICMP traffic; everything else is dropped. In contrast, 1:1 and prefix-sharing scenarios are protocol agnostic.