From: Anthony Pace (anthonypace@xxxxxxxxxxx)
Date: Tue Jul 23 2002 - 15:50:19 GMT-3
Andre,
You are right that it does require twice as many addresses to have the
DHCP servers operating independently and I would also conceed that I
only tried 3 or 4 vendors DHCP servers which "shared the ranges" before
giving up and falling back on the "2 server separate range" method. It
was also several years ago that I became sold on this method. I
remember using Checkpoint's DHCP and a couple of others that were
supposed to work as you described below but it was problematic and
caused workstations to be down hard; where as the 2 server method, in a
worst case scenario required a workstation to renew the address. What
is the DHCP server that "works in real life "? Is it the Cisco Router
DHCP? or a How big are the networks? What happens when communication
problems occur? I would be interested because the 2 server method is
only feasible with private addresses due to the waste.
Anthony Pace
On Tue, 23 Jul 2002 09:50:33 +0200, "Andri Bersvendsen"
<an-bersv@online.no> said:
>
> I am sorry to say that I do not agree.
>
> The model of operation described in RFC2131 enables multiple,
> independent DHCP servers on a network. It require that a DHCP client is
> prepared to receive multiple responses to its broadcast messages. I
> described this last part in an earlier e-mail to this group when I
> wrote
> that the first reply received from a server will be handled other
> replies from other servers will be discarded.
>
> In my example I used a shared DHCP database for the two DHCP servers
> but
> in fact they do not have to share any database at all - and they do not
> have to communicate together.
> A DHCP server is supposed to test if IP address is in use or not when
> it
> chooses an IP address that it will use for a client. This is normally
> done with PING.
> So if you have two DHCP servers with the same scope then the two
> servers
> might/possibly want to give out a IP address that the other server
> already have given out to a client, but it will not be done because the
> PING will see that the address is in use. A new address is then used
> until it find one that is available.
> So the RFC say that you can have multiple independent DHCP servers with
> the same scope. And it works in real life networks.
>
> I do not find a solution with two (or more) DHCP servers with different
> scopes as a optimal solution. The reason is that this will waste IP
> addresses, and it will force the clients to change IP address if the
> DHCP server that provided its address go down.
> You must make the scopes on the servers large enough to handle all the
> clients. So two servers must use twice as many IP addresses than one.
>
> Lets say that server #1 provided the client a address. Now server #1 is
> down.
> The client want to renew its leasing time for the IP address but get no
> answer from server #1.
> It will get a DHCPNAK from server #2 because the address requested is
> out of the scope.
> Then the client have to start the DHCP process with DHCPDISCOVER. The
> result is that the client will get a new IP address. This is not
> optimal
> if the client was doing a FTP session or a video conference etc. You
> loose the sessions.
>
> If they had the same scope server #2 would have looked at the request
> from the client and the address would not have been marked as used in
> its DHCP database (not shared database) so the server would then have
> sent out a DHCPACK and updated its database.
>
> If they had a shared database server #2 would have found the clients
> MAC
> address in the database and if the requested IP address is the same as
> in the database it will send out a DHCPACK.
>
>
> >Another thing to consider about the 2 DHCP servers with mutually
> >exclusive address rages is that they are truly fault tollerant as they
> >are not communicating or depending on each other for any thing. Any
> >DHCP solutions where the two need to "share" ranges or communicate run
> >the risk of having that process "break down"
> >
> >Anthony Pace
> >
> >
> >
> >On Mon, 22 Jul 2002 12:55:47 +0200, "Andri Bersvendsen"
> ><an-bersv@online.no> said:
> >
> >
> >>This is how the DHCP database stored on the TFTP server looks like.
> >>This sample is from my network at home and it have two routers that is
> >>set up as DHCP servers.
> >>
> >>*time* Jul 22 2002 11:36 AM
> >>*version* 1
> >>!IP address Type Hardware address Lease expiration
> >>192.168.13.101 id 0100.d0ba.846a.4e Jul 23 2002 08:25 PM
> >>192.168.13.103 id 0100.50da.5a39.6f Jul 24 2002 11:32 AM
> >>192.168.13.116 id 0100.3080.62ab.b7 Jul 23 2002 08:25 PM
> >>192.168.13.124 id 0100.40d0.1432.26 Jul 22 2002 09:42 PM
> >>192.168.13.125 id 0100.3080.62ab.19 Jul 23 2002 10:02 PM
> >>
> >>
> >>!IP address Interface-index Lease expiration Server IP
> >>address Vrf
> >>*end*
> >>
> >>
> >>
>
>
>
-- Anthony Pace anthonypace@fastmail.fm-- http://fastmail.fm: send your email first class
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