# How Does DHCPv6 Prefix Delegation Work?

## wswartzendruber

I'm using ISC stuff here.  Evidently, dhclient has an option to accept whole prefixes along with addresses.  You then assign your machines addresses within this prefix.

The question is, how do you actually serve these prefixes out to your LAN?  I can't find anything in dhcpd that deals with it.  radvd makes a mention of an accessory setting relating to expiration time, but doesn't say how it acquires the prefix to broadcast in the first place.  Is there some universal socket for getting prefix information from the DHCPv6 client to radvd or dhcpd?

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## Bones McCracker

The subordinate DHCPv6 server is configured to request configuration information from the parent delegating DHCPv6 server.  I assume these messages are handling using normal DHCPv6 communication: UDP port 546 (client) and 547 (server).

 *Quote:*   

> Prefix delegation with DHCP is independent of address assignment with DHCP.  A requesting router can use DHCP for just prefix delegation or for prefix delegation along with address assignment and other configuration information.
> 
> A requesting router first creates an IA_PD and assigns it an IAID. The requesting router then transmits a Solicit message containing an IA_PD option describing the IA_PD.  Delegating routers that can delegate prefixes to the IA_PD respond to the requesting router with an Advertise message.
> 
> The requesting router may include prefixes in the IA_PDs as a hint to the delegating router about specific prefixes for which the requesting router has a preference.
> ...

 

http://tools.ietf.org/html/rfc3633

Also possibly helpful:

http://tools.ietf.org/html/rfc3315

http://tools.ietf.org/html/rfc3736

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