Trying to understand how DHCP works.

[Joeyp]

I'm trying to understand DHCP a little better. Why I could get an IP address conflict on a machine that is using DHCP.

OK, I have a PC(actually I've seen this a few time) that is set to use DHCP to obtain an IP address, but it receives a conflict message when I boot.
I thought that the whole reason for using DHCP was to avoid conflicts. If the DHCP server has a reservation for that machine and that IP address why is it assigned to another computer. If it's already assigned to another computer, and the problem computer is requesting an address through DHCP then why is the DHCP server trying to assign an IP address that's already in use?
The only thing that I can possibly think of is that the problem computer has a reservation in DHCP, but the IP address is staticly (sp) set on another computer. Seems a little far fetched, but I guess it's possible in a private network with multiple technician making setting changes on computers.
Any thoughts?

Thanks

Joe

 

[dingo987]

I am fairly new to DHCP, but with my luck I have had lots of problems already. What I would do is:
1. Find which IP address is causing the conflict
2. Find which machines are trying to grab the same IP address
3. Shut them down(from my experience a machine will try to authenticate an IP lease quickly after startup)
4. Create a new reservation (with MAC address) for whichever machine will get the static address
5. Startup that machine making sure the reservation took.
6. Startup the other machine that was trying to grab the same IP address
Viola!! Hopefully this will solve this problem. If I am forgetting something I'm sure someone will correct me.

 

[gunthnp]

are you runnig more than one dhcp what os are you using?

can't call ms dhcp a ture dhcp it follows non of the rules. If you have say 2 dhcp and they have an over lap in address and dhcp1 give out an address to computerA(and with NT they keep that address for a long time) and you have "lease" set for 21 days, ok now computerA gets turned off and now computerB talk to dhcp2 it get the same address now I turn computerA I get what you are seeing. So long and thanks for all the fish.

 

[sobak]

I've ran into that same issue with MS DHCP, right now I am using Lucent Quadritek DHCP/DNS server. The configuration of DHCP allows the DHCP server to ping an address before it gives it out. If it gets a response from the address then it goes for another one. I believe it is a little more overhead but eliminates the problem with others in my department assigning static addresses within my DHCP range.

david e

 

[kjonnnnn]

I have the same problem and have posted it on this forum. What use to work for me it this.

Release the IP address on the DHCP Client. Then shut down the computer.

On the DHCP server, delete the lease for that number.
Then power up the client computer. Sometimes this works.

 

[Zelandakh]

I get this occasionally when I am issuing DHCP to a Macintosh. It uses the IP address but does not get registered in the DHCP active reservations list.

This is almost certainly due to the MS DHCP server. To avoid it, replace it with a Lucent one

 

[Packeteer]

Well...

Ideally the DHCP server should ARP for the IP address it's about to assign. Then it should try to ping the IP address to ensure no one is using it...

I don't think MS does that <sigh>.... they've also had some problems with the expiration of their IP addresses.

Why don't you download the EtherPeek demo from

http://www.wildpackets.com

so you can watch the actual DHCP bootup sequence? That'll give you the definitive answer.

 

[Guest_imported]

Microsoft DHCP works perfectly well when everything is correctly configured. I've set it up on several country wide LAN/WAN installations without issue. The most likely cause of your problem is that there actually IS another device with a static IP address that's within your scope. Printers are often to blame because they require static addresses. The fact that MS DHCP doesn't ping addresses before assigning them is hardly an issue - even if it did, there's always the chance that the device with the duplicate address is not currently active.
As long as you instigate a proceedure to make sure that static addresses within your DHCP scope do not get used - you should be OK. It's usually some human error involved...