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DHCP issuing wrong scope

Posted on 2008-02-04
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Last Modified: 2012-05-05
I have 2 sites with seperate DHCP servers and scopes.  Both servers run Sever 2003, one is R2.

In one site, when a IP lease expires, it renews with an IP from the other location (R2 and across a VPN and slow).  Is there anything that would cause this?  

A simple /release and /renew works fine however is an unecessary step.  Is there a basic step that I have missed to point the workstaion to the appropriate DHCP server?  All workstation IP's are assigned automatically.
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Question by:RobWasho
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by:Aaron Street
ID: 20813653
waht are the two scopes??

and as you are vpn connections it works a bit different.

if the two scopes are in teh same sub net then they will update accross the VPN connection (or from which ever DHCP server response the quickest)
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by:Aaron Street
ID: 20813678
well depending how the VPN is set up. but if it is a persistance one then the computers and servers could be treating its as a single large flat network and not seeing the seperate site..

I what you want to do is block DHCP traffice being sent accross the WAN. routers do this by default but a VPN would get round this.. so you whould have to look at that set up..

but with out knowing your subnets and so forth its hard to picture your exact situation
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by:RobWasho
ID: 20813736
Bad explanation on my account...

What I need is one DHCP server to provide for one site and one for the other site.  This is in place but some requests go to the wrong DHCP server across the VPN which I need to stop.  What is the best way to stop this?

One server uses 10.0.0.x to other 10.0.1.x

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by:Aaron Street
ID: 20813790
whats the subnet mask?

the thing is with a VPN is that it can give the effect of having a flat network..

the first DHCP server to respond is used... theefor if the first server is busy its possible you may get the other DHCP server at the other site running the request..

the way to stop it is to block the DHCP requests being sent through the VPN you would need to look at your VPN set up to set it up though
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by:RobWasho
ID: 20813850
subnet mask on both sites is 255.255.255.0

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by:Aaron Street
ID: 20813882
yep i think in effact you are running two scopes over one logical network.. you need to check if your routers are forwarding DHCP traffic over the VPN connection. As  like i say routers normaly block these. but as the conection will be encrypted then they will probable be forwarded.. and like i say although you would expect them to use the DHCP server closer to them.. it is possible that sometimes the one the other side of the WAN will be used..

All i can say id check your VPN and make sure you block any DHCP traffic accross it.
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Aaron Street earned 2000 total points
ID: 20814364
DHCP works at layer 2 rember. so if your VPN is set to allow LAyer 2 (MAC address) packets accross it then you will have this problem. DHCP also does not care about network addresses or any IP information. ITS all run on layer 2 and MAC addresses.

a layer 2 dhcp request hits a dhcp server and if the DHCP server can it returns an ip address. becasue its layer 2 there is no way for the dhcp server to know what ip network a machine is on.

oh and make sure your routers on each network dont have DHCP relay/helper turned on on the routers.. this should not give you the problem your are experincing. however its work checking and if you have two DHCP servers (one on each site) then you would not want it turned on
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