Re: DHCP serving more than one subnet (longish)
From: John E. Malmberg (Malmberg_at_dskwld.zko.dec.compaq.hp)
Date: 10/23/03
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Date: Thu, 23 Oct 2003 21:43:22 GMT
Michael T. Davis wrote:
> I posted this a short while ago and received a few responses.
> I'm hoping that someone has a kind of "cookbook recipe" for this process.
> Since my original posting, we upgraded TCP/IP Services and VMS.
>
> ** updated original posting **
>
> Server Information
>
> Compaq TCP/IP Services for OpenVMS Alpha Version V5.3 - ECO 2
> on a AlphaServer 800 5/500 running OpenVMS V7.2-2
>
> JOIN Server Release 4.1.0f-DECsrc for Alpha with OpenVMS
> Copyright 1992-1998 Competitive Automation, Inc. All Rights Reserved.
>
> Problem
>
> We recently acquired a /24 subnet (net. mask 255.255.255.0) to
> dedicate to our DHCP clients. We still need to DHCP serve our "old"
> subnet, a /22 (net. mask 255.255.252.0). All of the potential clients
> are on the same physical network segment. How might I configure things
> to allow our VMS system to serve both subnets? Would I need to get
> folks higher up in the support chain (who manage the router which
> provides our Internet connectivity) involved?
>
> ** end of original posting **
>
> While we want to maintain DHCP service for the old subnet, we
> only need to provide for "permanent" leases there. We'd like to move all
> dynamically assigned IP addresses to the new subnet. I managed to get
> things to almost work, but the DHCP server wouldn't assign a client
> system to the old subnet, even if there's a permanent lease assigned
> for the system.
I have not worked with such things for at least three years now, so I
could have some of the concepts wrong.
Even if all the addresses are on one physical link, if you have two
discontigous subnets, all packets between them need to go through a
router. The configuration is sometimes called a router on a stick, and
it doubles the amount of traffic on the segment any time that systems in
the two subnets need to communicate with each other. If they do not
need to communicate with each other, then everything is fine.
Now the DHCP server needing to serve both subnets is a problem.
And I could very well be mistaken on this, but from what I can remember,
when a DHCP server is handling two subnets, it expects one subnet to be
behind a router. When the router forwards the DHCP packets (not a
default configuration) to the DHCP server, this is what tells the DHCP
server what subnet to allocate the addresses from, because it knows that
it came from the router. As I was not doing this, I did not study the
techniques in detail.
But with both subnets on the same segment as the DHCP server, the DHCP
broadcast packets are going directly between the client and the server,
and even if the router was also relaying them, it would lose the race
with the direct packets.
DHCP clients do not know their subnet until after they have received
that information from the DHCP server once. After they get an address,
they will continue to ask for it from the same DHCP server, and since
they will be using their designated address to make the request, it will
go through the router if needed.
> (From my testing, it appears that the DHCP server won't
> consider a subnet for service unless the server is configured to provide
> at least one IP address from that subnet as a dynamic address; is this
> correct?) In fact, while the DHCP server doesn't complain about the
> specification of the old subnet in its configuration, it doesn't seem to
> want to serve out _any_ addresses from it, unless a client already
> managed to acquire a lease from the old subnet before the server's
> configuration is changed.
My guess is that you have set the old subnet to go through a router and
back to your segment, and the new subnet does not.
Since all the new DHCP requests come in directly, they get sent to the
DHCP zone for the new subnet.
Renewals use the I.P. and subnet they were issued, so they go to the
expected place.
There may be a way to do what you want, but it would require studying
the rules for DHCP assignment much closer than what I have done.
It probably involves making the DHCP server think that all the addresses
are in one larger subnet, but to give out to the fixed addresses
different information than for the dynamic ones for the subnet and
network masks.
And I do not know if the DHCP server you are using has that ability
because I never had to study that when I used to work with DHCP servers.
>
> By the way, now that www.join.com has apparently been assigned to
> some purpose other than the support of the JOIN DHCP server, where are we
> supposed to find the documentation that used to be referenced there?
I have no idea what the JOIN DHCP server is or what the URL is, and
never heard of it before your posting.
Good luck,
-John
malmberg@dskwld.zko.dec.compaq.hp
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