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So I change jobs recently and in this new workplace I've found that we've actually got two DHCP servers. In the very same subnet, and they both have the same scope. I did not set up this but I see there are a lot of IP address conflicts, which must be because they aren't connected to one another in any way. So, this is probably the reason right?

I did however see that there are settings in the DHCP console which is called "Server connection bindings" and there are IP adresses that can be entered there. The IP entered is however the same IP address of itself, the same as 127.0.0.1. I don't know if they should point to one another though.

Now, the way I see it, only one DHCP is needed. OR, we could split the scopes. But I'm not really sure if that's necessary at all really. I mean, we only have a couple of hundred computers in the domain, as well as a bunch of iPhones and other smart phones.

So, am I way off here or should one of them be disabled? Or should I split the scopes?

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There's no problem with having 2 DHCP servers. In fact, it gives you some redundancy in case one of them goes down. It just needs to be setup correctly. Make sure they both have a different range of addresses to hand out and the same reservations created on each.

Of course, if you don't need or want the redundancy and uptime, then just turn one of them off. That's probably the easiest thing to do.

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  • I think this would probably be the best for now :) Commented Sep 5, 2011 at 8:46
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The obvious and wrong answer is:

"If they are on the same network segment, you should only have one."

You can have multiple DHCP servers (e.g. for redundancy) so long as the ranges don't overlap, but I've never done it myself. There probably also exists DHCP software that can be set up in a failover configuration, so that only one is active at a time.

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  • As I thought. Seems like common sense really. Ok, we'll take down one tonight then and see if the problem improves, which it probably will! :) Thanx! Commented Sep 5, 2011 at 7:17
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    Nonsense, hence the -1 vote. It's perfectly reasonable to have more than one DHCP server on the same network and/or subnet. It would be ludicrous to have a network where every other component was highly available/redundant brought to its knees because the one DHCP server was unavailable. However the DHCP servers should not be handing out the same addresses, that is correct, however its perfectly reasonable (and windows 2008 DHCP, for one example, has a wizard to do this easily) to split a scope so that one server hands out (say) x.x.x.1-120 and the other hands out x.x.x.121-240 Commented Sep 5, 2011 at 8:51
  • It's pretty harsh handing out a -1 not knowing the age and type of the OP's equipment. Given the OP indicated a heterogeneous hardware environment, the advice about 'general weirdness' is spot on. So I dished out a +1 :-) Commented Sep 5, 2011 at 17:51
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    @UnisoftDesign But, saying in the absolute that "you should only have one" isn't really accurate. Commented Sep 5, 2011 at 19:46
  • @Unisoftdesign - I'm comfortable with my vote. There's been no reasonable scenario where you can only have one DHCP server on a network that I've been aware of for years. The person configuring them simply has to know what they're doing and understand the implications of any choices they make (I concede that isn't always a given though!). As I implied in my original comment, do you seriously think that very very large network infrastructures get by with one sole DHCP server? Commented Sep 6, 2011 at 14:52
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Many modern DHCP servers come with an option to "Ping Check" before offering an IP address to a client. When enabled, this allows the DHCP server dodge the problem of giving a duplicate IP address.

Some servers also allocate a random IP address from available pool instead of a sequence to further reduce the chance of allocating a predictable duplicate address.

The "Ping Check before Offer" allows the Admin go creative and mix and match the configurations on clients. When he want to remember the IP of a specific client, he can just bypass the DHCP server and configure it himself.

This is no way a solid solution for redundant but works fine in most cases. Your mileage may wary though!

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A failover DHCPd pair can be set up as stated in Necronomicon, just set your parameters. Yes, IP counts as an address too, and peer address matches that of the other ISC DHCPd server. The failover clauses must be applied on both servers to avoid malfunctions.

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