Broadband connections and DHCPv6 Prefix Delegation in Windows Vista

Some Microsoft customers and partners have enquired about the support for Requesting-Router behaviour of DHCPv6 Prefix Delegation in Windows Vista. I have also seen attempts by some vendors to use prefix delegation to assign IPv6 addresses to Broadband/PPPoE interfaces. In this post, I’ll clarify our view of prefix delegation and outline the support available for it in Windows Vista.


RFC 3633 defines options for delegating IPv6 prefixes through DHCPv6. As the aptly termed phrase “requesting router” suggests, the prefix delegation mechanism is suitable for assigning a prefix to clients which will act as routers between networks.


The appropriate mechanism for assignment of addresses to PPPoE interfaces is through the use of IPv6CP to negotiate an interface-identifier and then using the prefix in a RA from the server to generate an IPv6 address. In the case of a broadband router which dials a PPPoE connection, we recommend that the router first obtain an IPv6 address for its PPPoE interface through this mechanism. It should then follow this with a DHCPv6 prefix delegation request on the PPPoE interface to obtain an IPv6 prefix from the service provider’s router. This prefix should then be used for Router Advertisements on its LAN interface.


In most cases, Windows Vista clients dialling a Broadband connection will use this connection for their own Internet connectivity. This is the reason that the Vista PPPoE client does not by default trigger DHCPv6 prefix delegation. Only when Internet Connection Sharing is enabled on Windows Vista, where the host needs to act as a router for other clients on a home network does Windows Vista trigger the requesting router behaviour of prefix delegation. In this scenario, the Windows Vista host will obtain the IPv6 prefix from the server and advertise it to other clients on the home network.


Santosh Chandwani

Windows Enterprise Networking


[This posting is provided "AS IS" with no warranties, and confers no rights.]


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