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Aug. 14th, 2006 @ 02:44 pm A few questions about DHCP
Hello everybody, I want to ask a few questions about DHCP service in Windows Server 2003. We have primary and backup DHCP servers, both running Windows Server 2003 Enterprise Edition SP1 in a workgroup.

1. Can DHCP server lease an address if this address already exists in the network and set manually? Or I should exclude statically This potentially may cause IP conflict and network failure.
2. If one of the servers is down, what may happen?
3. If both servers are down and every machine runs XP SP2 and has an alternative configuration, will it use the alternative configuration? And when it will acquire an address from DHCP?
4. If both servers are available, which of them will be used for leasing an IP address?
5. As I know, primary and backup DHCP servers must have different scopes to avoid IP conflict. Is it true?

Thanks for the help.
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kkursor:
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From:stainsteelrat
Date:August 14th, 2006 01:13 pm (UTC)
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1. I'm not sure offhand if the DHCP server will check an address exists (you'd hope so!), but one option is to assign addresses statically based on MAC address. Or just exclude and assign at the PC.

2. One of the servers should be found.

3. Yes, it will use the alternate configuration. I'm fairly sure you'll have to reboot to get it to check the DHCP server again, but I can check.

4. I think it's just a case of which one is found quickest on the network. I don't think they load balance.

5. I'm not too au fait with primary and backups unfortunately. Are they aware of each other's existence? If not, then they'll definitely need different scopes.

I did just study this recently, but I'm forgetting it already unfortunately. I'll double check some of this though...
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From:meltedeyes
Date:August 14th, 2006 03:41 pm (UTC)
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DHCP servers will give out any address in the pool that it has not already assigned. If you set something statically the DHCP server will not know it and will hand out that address as it's not currently in it's assigned pool. I just ran into this at work last week.

Depending on how #4 works will determine #5. If sysstems only use the primary then there shouldn't be a problem. Also, if the Primary and the Secondary are aware of each other and communicate there shouldn't be problem.

I don't use windows for my DHCP needs, I use an old box with Linux that is easily replaceable should anything go wrong. Also check you lease time. Mine is set for 1 week, meaning I have a halthy time window to get the server back up before to many systems even know it's down. if your lease time is set for 5 min, outside of a heavy load on your server, you have no time window to fix it before the entire network goes offline.
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From:kkursor
Date:August 14th, 2006 05:57 pm (UTC)
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My lease time is 8 days. Our network consists of about 40 machines. And they are started and ended commonly one time a day.

A little question. Does the machine stay with its old IP or acquire the new one, if lease time didn't expire?

Thank you very much, you helped me greatly
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From:meltedeyes
Date:August 14th, 2006 06:18 pm (UTC)
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Generally, when the lease expires the computer will poll the DHCP server to request a new lease. As long as the server hasn't handed out the old address (say if the computer was turned off for a week while the emploee was on vacation) the server will re-lease the old address back to it.

You're welcome.
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From:kkursor
Date:August 14th, 2006 06:47 pm (UTC)
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You helped me greatly, thank you very much!
Where are you from?
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From:meltedeyes
Date:August 14th, 2006 06:54 pm (UTC)
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Depends on how far back you want to go. Currently Minnesota USA
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From:kkursor
Date:August 14th, 2006 07:14 pm (UTC)
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Wow, and I am from the great land of brown bears :) and good vodka
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From:meltedeyes
Date:August 14th, 2006 07:28 pm (UTC)
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I live pretty close to the Philips Vodka distillery. I've heard they export something like 80% of their vodka to Russia.
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From:kkursor
Date:August 19th, 2006 05:41 pm (UTC)
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I hear it at the first time =) We have the Moscow Plant "Cristall", which produces good vodka for internal and export sale =)
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From:network_nerd
Date:August 17th, 2006 01:43 am (UTC)
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The client will start requesting renewal when the lease is half up, so the lease can only expire if (a) the server is rebooted, or (b) the client is down for at least half the lease period (and usually this means longer than the entire lease period).

If either of those occurs, there is a *chance* that the server will lease the client's address out to some other client, and so the client's next request for a new lease will get a new address, or its request to renew will fail and it will be told to request a new one.

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From:meltedeyes
Date:August 17th, 2006 02:30 am (UTC)
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I knew I didn't know the whole story, and was in a rush, hence the generally. Thanks for filling me in.
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From:network_nerd
Date:August 17th, 2006 01:39 am (UTC)
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1. Can DHCP server lease an address if this address already exists in the network and set manually? Or I should exclude statically This potentially may cause IP conflict and network failure.

There is no "DHCP telepathy" by which the server can know that you've statically assigned that address, unless you tell it. Exclusions and reservations are mechanisms for telling the server information like this. Use them.

2. If one of the servers is down, what may happen?

Is the other server's scope big enough to hold all of your clients? If not, some clients may not be able to lease (or renew) addresses. If your lease time is long campared to the duration of the outage, this probably won't be a problem.

3. If both servers are down and every machine runs XP SP2 and has an alternative configuration, will it use the alternative configuration? And when it will acquire an address from DHCP?

I don't see a timeout on the alternative configuration; it might last until /renew or reboot.

4. If both servers are available, which of them will be used for leasing an IP address?

Both will offer; client will pick one, usually the one it heard from first (presumably closer or less busy).

5. As I know, primary and backup DHCP servers must have different scopes to avoid IP conflict. Is it true?

See #1. There's no magic telepathy. Server A doesn't know what leases Server B has outstanding.


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From:kkursor
Date:August 19th, 2006 05:43 pm (UTC)
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Thanx a lot =)