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dhcp, router problem

Posted on 2011-02-23
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Last Modified: 2012-05-11
Managers,
using a W2K3 dhcp server delivering network data for a vlan. Since some days a couple of W'XP and W7 clients won't get settings from the dhcp server anymore. On the dhcp server, such clients produce a NACK with e.g. an ip 192.x.x.x. Connecting the client to another line, i.e. another connection on the router, settings may be delivered from the dhcp server. Setting the client then back to the troublesome line and deleting the lease, again no settings are deliverd by the dhcp server. Any idea, wether it is a problem of the router or the dhcp server and how to solve it?
TIA
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Question by:GeologyETH
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by:shadowmantx
ID: 34963498
Check your workstations if the DHCP Client is running and is set to automatic.
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by:rdhoore108
ID: 34967726
One thing I would try is assign a static IP address on a problematic client, and see if you have ethernet functionality then. If not, you have a network problem (wiring or bad router port/ports). If it does work, you have a DHCP problem.

I do not know the details of your network (are you reaching remote sites?), your rules on your router, or where your DHCP server is located, so the above suggestion may or may not be useful.
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by:GeologyETH
ID: 34968909
The problem started in a sudden, i.e. the clients are set to get the network settings via dhcp server. Moving a client with working network settings from the dhcp to a "troublesome" network line, the client still works. Disabling and reactivating the network on the client  leads to a RENEW of the lease on the dhcp server.
Now, deleting the lease of this client on the dhcp server, deactivating and reactiving the network connection on the client (remaining this client on the "troublesome" network line) , brings network setting to the client, e.g. 192.168.1.33, 255.255.255.0, 192.161.1.1 gateway, dhcp and dns). Nothing to see in the dhcp server's log.
Connecting this client then to a network line on which the dhcp server delivers settings to clients, The schp server's log shows NACK 192.168.1.33, ASSIGN 82.x.x.x immediately.

All network lines are configures to the same VLAN. But on some network lines, i.e. either routers or ports on a router doesn't give access to the dhcp server. What could be the problem?

Thanks in advance.
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rdhoore108 earned 500 total points
ID: 34969029
Is it possible that you suddenly have two dhcp servers on your network? Maybe your router's DHCP server has been enabled, or something else that was installed recently?
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by:GeologyETH
ID: 34969295
I have no access to the routers. Can I find dhcp servers in the network with a tool?
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by:rdhoore108
ID: 34969703
When a client needs an IP address, it sends a broadcast for a dhcp server. So, if you have more than one, there is no way to predict what will happen. Also, routers can be set to forward dhcp packets, which complicates things even more.

How many pc's do you have on your network? Are they all on one site? When you say router, is that only one router, or are there also one or more network switches inbetween?
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by:GeologyETH
ID: 34971194
It was an unauthorized WLAN router with default settings in the network. I found it with the tool dhcploc support tool for W'XP. Since unmanaged I could logon remotely and disable the dhcp server. Now everything is running fine again.
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by:rdhoore108
ID: 34971565
I'm glad you found the culprit. I didn't know about dhcploc. It seems very useful indeed. If only it could also tell who installed it, or in which office, it would make it even more useful ;-)
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