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How do I configure Windows XP to choose between one of as two DHCP servers in one (same) LAN?

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Just curious. Why do you have two DHCP servers in same LAN? –  Saurabh Barjatiya Jul 7 '09 at 13:24
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You would want two servers for redundancy, for example. If one goes down the other will still be around to hand out addresses while you get the other one back up. –  squillman Jul 7 '09 at 13:27
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If you have two servers for redundancy, why would you want XP to choose only one to use? :) –  Ernie Jul 7 '09 at 17:41

7 Answers 7

If you've got two (2) DHCP server computers in the same layer 2 broadcast domain, clients are going to be (mostly) randomly assigned addresses by either server computer. The one that answer's the client's DHCPDISCOVER reuqest first is going to end up assigning the address to the client.

I'm not sure this is a great idea. You're adding what is, essentially, a non-deterministic element to your network. If you want redundancy in DHCP, consider using a program to monitor the "health" of the primary DHCP server and, in the event of failure, bring up the secondary DHCP server.

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Please see the existing question that directly answers this issue

http://serverfault.com/questions/27175/split-your-dhcp-scope-among-multiple-servers

These are not client side changes

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I am not aware of any way to specify a DHCP server from a network client. My question to you would be - why would you want to? In an AD, you can have multiple redundant DHCP servers, and you can configure them to manage the same scope without overlapping addresses. You can also configure reservations and several other things. If you need to have different DNS or WINS servers for some reason, the machines that require the exceptions can have manually configured DNS or WINS entries but still use DHCP.

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AD does not handle DHCP like this. In fact, AD has nothing (intrinsically) to do with DHCP. They authorize themselves every minute, but there is no DHCP redundancy in AD. Are you thinking of clustered DHCP servers? –  Izzy Jul 7 '09 at 13:48
    
There are many ways to implement DHCP. However, if you run DHCP on an AD server, it has to be authorized to distribute addresses. You can have several servers with the DHCP role installed, and they can all be configured for the same scope. This is redundant but not clustered. However, when you configure this, you also have to configure non-overlapping exclusion ranges. You put the full range on both servers, then exclude a range from one server and a different range on the other (about 80/20 split). They won't distribute duplicate IP's, but they will allow a renewal in the exclusion range. –  Jes Jul 7 '09 at 15:33
    
+1, This is the correct way to setup two DHCP server on the same network (unless you're running a DHCP server that coordinates redundancy, ISC's DHCPd supports failover now). Configure the same scope with different exclusions on each, or two different scopes (either way works); also they should be split 80/20 or 50/50 (most use the former, I've seen the latter work just as well though). –  Chris S Nov 22 '10 at 16:26

I don't think it is really recommended to have two DHCP servers on one lan...

If it is for redundancy one option is to use cluster services so that the dhcp files are stored and referenced by which ever server is running the dhcp service.

another solution would be to put you separate lans in vlans or similar so that only the client that are on that lan with each dhcp service will receive the correct dhcp request

hope that helps

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I think that it should be perfectly valid to have more than one DHCP server on the same LAN. In our case we want it because we have two gateways and we want all the settings to be received automatically by the clients but have some clients utilizing one gateway and some the other gateway (due to different characteristics of the network beyond the gateway).

As is mentioned in another thread, a server-ranking solution was proposed in 1997 (see http://tools.ietf.org/html/draft-ietf-dhc-sso-00) but does not seem to have been implemented.

It should be perfectly possible for client utilities to provide a selection, based on DHCP server host name or IP address. Clients receive a DHCPOFFER from all DHCP servers that respond to a DHCPBROADCAST, and then the client sends a DHCPREQUEST. These DHCPREQUESTs could be filtered according to pre-established rules, with a fall-back of responding to the first DHCPOFFER receieved if no DHCP server passes the filter.

In fact, I have seen a number of occasions where problems have been caused on small networks by installation of a second DHCP server (the default configuration of all ADSL routers, for example). Client rules for filtering could prevent these problems.

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if you want some clients use 1 gateway and others to use another gateway, having 2 DHCP servers with "client utilities" is not the way to accomplish this... –  August Nov 22 '10 at 13:43

Sounds like you're trying to load-balance your gateways by evenly assigning them to routers via DHCP.

Really the optimum solution is to do the load balancing on the gateway device itself. Your LAN should only have one gateway, and that gateway device should have three interfaces, an "inside" one connected to your LAN, and two "outside" ones connected to your two external gateways.

You can then flexibly configure things as you need on the gateway device without having to involve any individual machines on the LAN.

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Control Panel, Network Connections, Local Area Connection, Internet Protocol, Properties button, Advanced button, WINS tab, Add, Enter your DHCP IP Address in question

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WINS <> DHCP. WINS is for name resolution, not IP address assignment. –  squillman Jul 7 '09 at 13:28

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