DHCP problem

ericb

Senior member
Nov 11, 1999
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I have a DHCP problem and was wondering what I'm missing. When we move a computer from one office to the next the computer continues to renew the old office IP range. If branch one is 172.16.2.x and branch two is 172.16.3.x the computer in branch two continues to renew as 172.16.2.x. This is after releasing the DHCP address and also flushing the dns.

These are XP Pro computers. There is only one DHCP server and it's running on Windows 2000 SP4. We also run into this problem when we take a laptop into a different branch.

I have checked and there are plenty of available IPs for each range (most branches have a range of 50 available and only 3-4 computers).

What am I missing to get the DHCP to use the correct branch range? I'm sure it's probably something simple but it's frustrating right now.
 

Smilin

Diamond Member
Mar 4, 2002
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What subnet mask?

Take a network trace at the DHCP server. See if the Discover frame is coming in from a broadcast source (source IP 0.0.0.0) or if it is a unicast frame from a particular IP (relay agent/router).

Post what you find.
 

ericb

Senior member
Nov 11, 1999
898
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0
Subnet mask for all ranges is 255.255.255.0

All offices are connected over MPLS using Cisco 1721 routers. Each router has the ip helper address of 172.16.1.5 (the DHCP server address) and each branch increments up (172.16.2.x, 172.16.3.x, etc).
 

Smilin

Diamond Member
Mar 4, 2002
7,357
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Check network trace, see where the discover packet is arriving from.

It's going to be one of three things:

1. Nothing. In this case a local addy will be issued, 172.16.1.x
2. A 172.16.x.x address. In this case a packet from that 172.16.x network will be issued.
3. A 172.16.1.x address. In this case it sounds like you have an misconfigured relay agent. This would result in a 172.16.1.x address being issued as well.

Also see if you are getting duplicate Discover packets arriving. You should see one and only one arrive from the client (or a relay on it's behalf).