What is DHCPv6-PD?

DHCPv6-PD (DHCPv6 Prefix Delegation) hands a whole IPv6 prefix, such as a /56, to a downstream router so it can number its own LANs.

Ordinary DHCP gives a host one address. Prefix delegation gives a requesting router a block of address space instead, using the IA_PD option. It was first defined in RFC 3633 and is now part of the main DHCPv6 specification, RFC 8415. The exchange is the usual DHCPv6 one, sent over UDP: the requesting router sends a Solicit asking for a prefix (optionally hinting at a size), the delegating router offers one in an Advertise, and a Request and Reply confirm the lease. The requesting router then splits the prefix into /64s, puts one on each LAN, and announces them with Router Advertisements so hosts can use SLAAC. The delegating router normally installs a route for the delegated prefix pointing at the requesting router, so return traffic finds its way back.

This is how most ISPs give IPv6 to customer routers: the home or office router asks on its WAN interface and gets a /56 or /48 (sometimes smaller) to spread across its LANs. Leases are tied to the client's DUID, so servers usually hand the same prefix back, but not always, which is one reason to avoid hard-coding a delegated prefix into firewall rules. A frequent mistake is to delegate a prefix but leave the downstream router without a blackhole route for the unused part of it, so traffic to unused addresses loops between the two routers until its hop limit runs out. The IPv6 on VyOS lesson sets up prefix delegation between two routers and shows that loop and its fix.