DHCPv6 Prefix Delegation

André Mamitzsch andre at mamitzsch.de
Thu Feb 13 21:41:48 UTC 2014


we are running DHCP 4.3.0 in v4 and v6 mode. V4 is working absolutely
fine, no issues there. With v6 and prefix delegation, we are
experiencing a small issue, or better, I need some clarification on the
seen server behavior and I' m not sure how the client should behave.


In the Solicit message, the clients sets the prefix-length value in
option 26 to a value of 56, so that's the preferred prefix length for a
delegated network to the client.

If the server is not configured to delegate /56 but /48, an error
message is send back to the client in the reply message and the client
never gets a prefix delegated.

I' m not sure whether the server is too strict here or the client simply
should always use 0 in prefix-length, so the server can decide which
length to delegate. Another option might be that the client steps
through different prefix lengths, so starting with /56 and then jumping
to /48 and then to 0 upon receving an error in the reply.

>From my point of view, at least that's the way I understood the RFC, the
prefix-length set to 56 represents the clients preference only,
nevertheless if the server does not support /56 but /48 is available, a
/48 prefix should be offered.

Am I wrong here ? The current workaround is to configure both prefix
lengths (56 and 48) in prefix6 statements.

Fortunately, I have not seen clients requesting anything else than /56,
/48 or 0, but this may change in the future. I' m a bit concerned about
possible interoperability issues.

I have been trying to find any references about this, previous
discussions, but had no luck so far.

Can somebody help me here ?



More information about the dhcp-users mailing list