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I want to implement a redundancy solution. I have two subnets all windows XP machines - each with their own DHCP server - joined by a bridge.

Currently - all machines get DHCP leases from the one DHCP server with the other disabled.

For when the bridge goes down - I want to split my DHCP scope among multiple servers. How can I do this on Windows XP client machines?

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4 Answers 4

up vote 11 down vote accepted

The most common way to do this is to split your IP range between the 2 DHCP servers and have both running at the same time. Clients will then recieve an address lease from whichever server responds first. In the event of the bridge failing, those with leases from the server on their own subnet will be fine, any with leases from the other subnet will get a new lease from the other server when they renew.

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You'll need a DHCP server on both sides of the bridge to implement redundancy properly. XP Clients are not servers.

I suppose there's possibly some software out there that will do it to some degree but it probably won't integrate with AD correctly and so-forth (and what happens when the "nominated" workstation is broken or just switched off?).

How long do you expect the bridge to be offline? It might be simpler to extend the DHCP lease length a bit, and IIRC clients can just continue to use their currently allocated address if they can't contact the DHCP server.

If you want to implement DHCP redudancy with 2 DHCP servers, its easy enough. Just think of a scope thats potentially twice the size you need it to be for all workstations, and allocate half each of it to the two servers so there is no duplication of address ranges.

Clients will get IP addresses from whatever DHCP server responds fastest to a request normally, and with the bridge down this will simply just be the server on their side of the bridge.

In the remote office, you could get a low power / low cost device to act as your "2nd" DHCP server and shove that into whatever cupboard or cabinet you currently have the bridge hiding in. It doesn't have to be a full price server with all the trimmings just to do this.

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I believe the ISC dhcpd (which comes with most Linux distributions) can do server redundancy and pool-sharing, to the level that leases are synchronized between the two servers.

Never done it myself though

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Yes it can. We do that with anycasting for redundancy over geographically separate campuses. –  Jason Tan Jun 18 '09 at 17:08

I found an intersting solution at http://windowsdevcenter.com/pub/a/windows/2004/04/13/DHCP%5FServer.html:

If you're really paranoid about DHCP availability on your network, you can also configure a third DHCP server that mirrors the configuration of your 80 percent server, if you're using the 80/20 rule, or one of your 50 percent servers, if you're using 50/50. Once you've configured this server, however, keep it disconnected from the network as a hot backup DHCP server. Then if your main DHCP server goes down you can bring the backup one online to take its place. In this scenario you might ask, "Why keep the 20 percent server if you have a hot backup?" The reason is, what if your 80 percent server dies during the night and you're not around to flip the switch and connect your backup to the network? Unless you have sophisticated network management software that can handle this situation by bringing your spare 80 percent server online, some of your clients may be out of luck if they want to connect to the network before morning comes.

Here's another interesting approach: configure both DHCP servers on your network with identical scopes. Now, normally this would cause problems because you might end up having the same address leased by both servers, causing clients to hiccup. However, if you enable address conflict detection on your DHCP servers and set it to 2, then each DHCP server will test an address three times to make sure it's not already being used before leasing it to a client, and this will prevent one server from leasing an address that the other server has already leased. The only downside to this approach is that it will make DHCP traffic on your network a bit more noisy, but DHCP traffic is so minimal anyway this will have only marginal impact on overall network performance. But why not set this to one attempt instead of two attempts? Safety--you don't want a network glitch that causes brief packet loss to ruin your setup. To configure address conflict detection on a Windows 2000 DHCP server use the Advanced tab of the properties sheet for the server in the DHCP console.

I'll test if this 'conflict detection' is made by ping or something else.

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