What is the primary role of a BR router in a 6rd environment?

The primary role of a BR (Border Relay) router in a 6rd (IPv6 Rapid Deployment) environment is to connect the CE (Customer Equipment) routers with the IPv6 network. The BR router de-encapsulates IPv6 packets that are encapsulated in IPv4 packets from the CE routers and forwards them to the IPv6 internet. This process is stateless and allows for high availability and load balancing by potentially having more than one BR router configured with the same IPv4 address (anycast), so CE routers are routed to the closest BR router12.

References:

NetworkLessons.com, “IPv6 6RD (Rapid Deployment)”

RFC 5969, “IPv6 Rapid Deployment on IPv4 Infrastructures (6rd) – Protocol Specification”