Advice on redundant DHCP Server configuration with reservations

I have taken over a network with ~150 servers (Mixed Windows 2000 & 2003)
and 50~workstations (Mixed Windows 2000 and XP) with all servers and
workstations running on static IP addresses. The network is spread across a
number of VLANs and everything is running without problem, but we have
recently had to change our primary and secondary DNS servers which resulted
in a large amount of manual work on every box. Subsequently we would like
to look at deploying a redundant DHCP configuration with reservations for
all of the servers and probably all of the clients as well, but are
struggling on the best way to achieve this.

If we were to deploy two DHCP servers with either a 50/50 or 80/20 share of
each subnet can anyone clarify how reservations are used. For example using
the settings below can anyone confirm if DHCP Server 1 were to fail, would
DHCP Server 2 hand-out the reserved addresses to Server 1, 2 and 3 even
though those addresses are actually part of DHCP Server 2's exclusion list ?

Can anyone suggest a better method we should consider to achieve a redundant
dhcp deployment ?

Thanks,
Alex.


DHCP Server 1:
192.168.1.11 - 192.168.1.250
Exclusion: 192.168.1.119 - 192.168.1.250
Reservations: server 1 - 192.168.1.11
server 2 - 192.168.1.12
server 3 - 192.168.1.13

DHCP Server 2:
192.168.1.11 - 192.168.1.250
Exclusion: 192.168.1.11 - 192.168.1.118
Reservations: server 1 - 192.168.1.11
server 2 - 192.168.1.12
server 3 - 192.168.1.13

VLans in use:
192.168.1.0
192.168.2.0
......
192.168.14.0

<snippage>
> Can anyone suggest a better method we should
> consider to achieve a redundant dhcp deployment ?

I'd use subnetting and routing in addition to VLANs; what
I mean is that you may setup your server so that they'll be
sitting on a different subnet and reachable through a router
at this point you'll have a full subnet for your server and a
full range (w/o any need for reservations) for your clients

going back to DHCP fault-tolerance; the usual approach is
to setup things using the 80/20 principle, that is, having a
subnet with N addresses, you throw away the reserved ones
and then assing the remaining ones so that one DHCP server
will serve the 80% of the remaining addresses and the other
will serve the other 20% .. but.. there's another approach too
that is .. the 100/100 no, I'm not kidding although the idea
can't always be applied, is some cases it's feasible and will
allow you to always have your network fully served

the idea is to change the netmask of the DHCP subnet so that
you'll have TWO times the addresses you need, at this point
you may split those addresses amongst two DHCP servers
each server will so give out 50% of the whole subnet, but since
even the 50% of the subnet will cover the 100% of the needed
IPs, your network will still be FULLY covered; then if you want
to make things more complex and add some more tolerance
you may even use FOUR DHCP servers and set then up to
use the 80/20 rule on each half of the subnet

HTH

--

* ObiWan

Microsoft MVP: Windows Server - Networking
http://www.microsoft.com/communities/MVP/MVP.mspx
http://italy.mvps.org