Advice on redundant DHCP Server configuration with reservations

[Guest Alex]

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

[Guest ObiWan [MVP]]

Re: Advice on redundant DHCP Server configuration with reservations

 

 

<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 :D 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