Initially, a PPPoE RG follows the same procedure as a PPPoE host: the BNG receives a prefix from RADIUS (in this case through a Delegated-IPv6-Prefix attribute), which is used as a trigger to suggest the IPv6CP protocol to the client. The prefix that is suggested to the client should have the same prefix length as configured under the subscriber>if>ipv6 node (delegated-prefix-length). This length should be between 48 and 64 bits, inclusive.
After the IPv6CP protocol has completed, however, the client should run the DHCPv6 protocol over its PPPoE tunnel to receive a Delegated Prefix (IA_PD) and optionally IPv6 DNS server information. This Delegated Prefix can then be subdivided by the client and distributed over its downstream interfaces. During DHCPv6, no extra RADIUS requests are made; the information is stored during the initial (PPPoE or PPP) authentication until the client starts DHCPv6.
Only after DHCPv6 has completed, the IPv6 subscriber host is instantiated and the BNG starts sending Router Advertisements (if configured.) The router advertisements do not contain any prefix information, which has already been provided by DHCPv6, but it is used as an indication to the client that its default gateway should be the BNG.