Perhaps I should be more clear about this and say that this is the behaviour
of MICROSOFT DHCP clients. Here is the info from the Windows NT Resource
kit:
"Note: The client accepts the first offer it receives, regardless of
whether the offer came from a DHCP server on the local subnet or from a DHCP
server on a different subnet. ... In the case where the DHCP server is
unavailable or there is no available IP addressing information to lease to a
client computer, the client is unable to bind to TCP/IP."
An MS DHCP client may receive many DHCPOFFER's for its DHCPDISCOVER
broadcast. It will accept the first offer it receives (actually, the first
response it gets), and NACK all others. If the first response it gets is
negative, it will settle for that, and NACK anything from the other servers.
I have seen this (and sniffer traced it) in production. MS was unwilling to
call it a bug, and said the behaviour was by design and was RFC compliant.
Case was closed...
This was NT 4.0 Service pack 4 with Win98 clients. I dunno if they have
changed things since, but I doubt it.
Dale
[=`)
>From: "Donald B Johnson Jr" <[EMAIL PROTECTED]>
>To: "Dale Holmes" <[EMAIL PROTECTED]>, <[EMAIL PROTECTED]>,
><[EMAIL PROTECTED]>
>Subject: Re: Ip helper address
>Date: Mon, 11 Sep 2000 12:36:49 -0700
>
>I don't understand this, wouldn't the client accept the second offer by
>sending the seconds servers siaddr in the request packet. also DHCP
>standard
>says that nowhere must a client accept the first offer and then stop
>broadcasting. All servers will answer the clients DHCPDISCOVER broadcast
>with any help it can or can not offer. The first server does not tell the
>second server to shutup so as soon as the (second or 1nanosecond slower
>server) receives the broadcast it will it will send a DHCPOFFER packet and
>the client will reply with an DHCPREQUEST packet to the second server
>(using the siaddr field) that will be ack'd by the second server with an
>DHCPACK packet. This is all made quite clear in RFC 1541. So you can have
>two DHCP servers on the same segment you just don't know which one will
>serve the address to the client but both will try independent of each other
>and the client will ot stop trying after receiving after a nack from a
>server.
>Duck
>----- Original Message -----
>From: Dale Holmes <[EMAIL PROTECTED]>
>To: <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
>Sent: Friday, September 08, 2000 7:10 AM
>Subject: Re: Ip helper address
>
>
> >
> > You have 2 DHCP servers on the same subnet??? This is probably not a
>good
> > idea... it does not really provide redundancy or load balancing.
> > The DHCP client will issue a request and accept the first response that
>it
> > gets.
> >
> > If you split your scope such that half of your available addresses are
>on
> > one server and half are on the other, you will *NOT* see that half of
>your
> > clients use one server while half use the other. If for some reason one
> > server always replies a nanosecond earlier than the other, then all
>clients
> > will accept the response from that server. Once that server is out of
> > addresses, it will start sending nack's. The clients will start
>accepting
> > those nack's and will not request an address again, even though the
>other
> > DHCP server may have dozens of free addresses to offer.
> >
> > SO - in answer to your question, the ip helper address of 10.10.10.0
>will
> > allow your client's requests to reach all DHCP servers on that subnet,
> > HOWEVER they will only accept leases from the first server from which
>they
> > receive a response. Chances are that server will be the same one all the
> > time, even after it runs out of addresses to offer...
> >
> > You *could* set up your DHCP servers such that the scope on EACH ONE is
> > sufficient to offer leases to ALL of you clients, but that is probably a
> > less than efficient use of your address space.
> >
> > I hope that this helps...
> >
> > Dale
> > [=`)
> >
> > >From: "Dennis Bates" <[EMAIL PROTECTED]>
> > >Reply-To: "Dennis Bates" <[EMAIL PROTECTED]>
> > >To: [EMAIL PROTECTED]
> > >Subject: Ip helper address
> > >Date: Fri, 8 Sep 2000 08:10:44 -0500
> > >
> > >I am trying to put a statement on the remote router to allow the
>clients
>to
> > >obtain an IP address accross the WAN. I have used the ip
>helper-address
> > >command successfully. My problem is that i would like any of the DHCP
> > >servers at the central site to be able to service DHCP requests from
>the
> > >remote site. Do I have to use mutilple ip helper-address statements ?
>I
> > >have tried a helper address pointing to the subnet, but that does not
>seem
> > >to work. EX. i have DHCP servers at 10.10.10.10 and 10.10.10.11 do i
>have
> > >to
> > >use two seperate ip helper address statements or can i use ip
> > >helper-address
> > >10.10.10.0 ?
> > >
> > >
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