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Is it possible to setup up remote dhcp server over T1 or fiber backbone?

Posted on 2008-11-13
Medium Priority
Last Modified: 2012-05-05
Right now I have seven separate windows 2003 servers at each building that handout DHCP, DNS, Filesharing, Active Directory.  Each of these buildings have a T1 connection with cisco routers.  In the near future I will have a fiber optic backbone at two building or depending on the cost have this at all seven buildings leased from AT&T.  I would like to know if its possible just to have one DHCP server at the main office and have it hand it out over the T1 or fiber optic backbone to each building. each building has 10.23.x.x something for IP Range.  Is it possible to have seven different scopes on one server then have the cisco router help at in sending the DHCP renew request back to the server at the office so the computers can get DHCP?
Question by:Comptekhs
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Expert Comment

ID: 22956283
DHCP is not routable as it works on broadcast.  To do this you woudl have to set your router up to forward DHCP broadcasts or install DHCP proxy servers on the you remote networks.  It is possible however.

Accepted Solution

takendown earned 500 total points
ID: 22956294
Yes...IP Helpers should let you accomplish this.  I am preparing to do the same thing right now and we are going to try to implement the microsoft recommended 70/30 rule for DHCP.  This would allow your clients at the remote sites to get addresses from the servers at their respective sites and from the Central Site. This  is a little harder to manage, but if the WAN link goes down then the clients can stay up and running at the remote site.

Good Luck!


Author Comment

ID: 22956438
Lets say I setup IP Helpers on each cisco router back to the central DHCP server.  Could I use one Network Interface Card on one DHCP server at the central office for all the buildings?  I have the following IP range for each building: (, (, (, (, (, (, (this IP is where the central DHCP server will be located
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Expert Comment

ID: 22959478
That is correct!  All of these pools would need to be defined on your centralized DHCP server of course.  I am currently doing this with ip helper addresses on vlan interfaces.  

Vlan 50 with IP helper and the IP on Vlan 50 being When DHCP broadcasts are forwarded via the help to the server the clients receive an IP in the subnet.

Vlan 60 with IP helper and the IP on Vlan 60 being When DHCP broadcasts are forwarded via the help to the server the clients receive an IP in the subnet.

These IP helpers live on a 3750 L3 Switch right now, but we are going to carry this over to our remote locations and use IP Helpers on what are now 1600 series routers, but will soon be 1800 series.  I can't find any documentation for you right now, but there is data in the DHCP Unicast created by the IP helper that allows the DHCP server to assign an IP out of the correct pool for the subnet the machine is in.

Maybe someone can help me out here with some documentation!


Expert Comment

ID: 22959627
Here is some good discussion.


And here


I just can't really find any technical documentation as to how it works.  It must be based on the source IP of the request, but I can't find anything specific to back me up.

I'll keep looking.


Expert Comment

ID: 22959684
Hey!  I found the RFC.


Look for this section.

   In any of these or other cases, the relay agent needs to be able to
   communicate to the DHCP server the subnet/link from which to allocate
   an IP address.  The IP address, which will communicate to the DHCP
   server the subnet/link information, cannot be used as a way to
   communicate between the DHCP server and the relay agent.

Cisco Routers acting as relay agents using the IP Helper commands follow this RFC :)

Hope this helps!

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