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How Does DHCPv6 Prefix Delegation Work?
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Joined: 23 Mar 2004
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Location: Idaho, USA

PostPosted: Sun Aug 12, 2012 4:36 pm    Post subject: How Does DHCPv6 Prefix Delegation Work? Reply with quote

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

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PostPosted: Mon Aug 13, 2012 2:52 am    Post subject: Reply with quote

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).
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.

When the requesting router has identified a delegating router, the requesting router uses a Request message to populate the IA_PDs with prefixes. The requesting router includes one or more IA_PD options in the Request message. The delegating router returns prefixes and other information about the IA_PDs to the requesting router in IA_PD options in a Reply message. The requesting router records the lifetimes for the delegated prefix(es) and uses the prefix(es) as described in the previous section.

Before the valid lifetime on each delegated prefix expires, the requesting router includes the prefix in an IA_PD option sent in a Renew message to the delegating router. The delegating router responds by returning the prefix with updated lifetimes to the requesting router.

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