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How to configure a Cisco 2600 Router to forward DHCP requests.

Posted on 2008-10-16
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Last Modified: 2012-05-05
Can someone instruct, or point me to a link on how to have a Cisco 2600 router forward DHCP requests from clients.  I'll likely need to also do this on a Cisco 1700 as well.  Basically, I want the DHCP server at our main site to server as the DHCP for a remote location connected by a dedicated T1.
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Question by:Go-GBS
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by:JFrederick29
ID: 22730125
Use the "ip helper-address <dhcp server>" command on the LAN interface of the 2600/1700.

conf t
int fa0/1
ip helper-address x.x.x.x   <--the DHCP server at the main site
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by:Go-GBS
ID: 22731127
Question, the router located at the remote location points to a router at our main site, will I have to do this on both?
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by:JFrederick29
ID: 22731163
Nope, just on the "first hop" on the LAN as the router will unicast the DHCP request at that point so there is no broadcast forwarding necessary on any other router.
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Expert Comment

by:techsupportcpm
ID: 22731188
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Author Comment

by:Go-GBS
ID: 22731437
How do I view the configuration of the router, such as the interfaces to be sure which one is the LAN interface?
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Expert Comment

by:JFrederick29
ID: 22731467
On the remote LAN, determine the clients default gateway IP address.  Once you have that, do this on the router:

enable
show ip int brief

You will see the interface that has the IP address configured as the LAN clients default gateway.
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Author Comment

by:Go-GBS
ID: 22731558
Ok, I've determined that to be Ethernet0/0, so would that affect the command I use, like this?

conf t
int fa0/0 (as opposed to int fa0/1)
ip helper-address x.x.x.x
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Expert Comment

by:JFrederick29
ID: 22731584
Yes, exactly.  x.x.x.x is the IP address of your DHCP server.
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Expert Comment

by:JFrederick29
ID: 22731602
Or if it is truly "Ethernet0/0"

int ethernet0/0
ip helper-address x.x.x.x
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Author Comment

by:Go-GBS
ID: 22731715
When I type int ethernet0/0, I get the response, "% Invalid input detected at '^' marker.
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JFrederick29 earned 500 total points
ID: 22731733
Sorry, you need to go into config mode first:

enable
Password:
conf t
int ethernet0/0
ip helper-address x.x.x.x
end
wr mem   <--save changes
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Author Comment

by:Go-GBS
ID: 22731791
That did it, won't actually be able to test it today, but thank you very much for the help.
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Expert Comment

by:JFrederick29
ID: 22731810
No prob.  Let us know how it works out.
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Author Comment

by:Go-GBS
ID: 22811977
Sorry for the lack of an update, just waiting on a time to get over to the remote facility.
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Expert Comment

by:JFrederick29
ID: 22811994
No sweat.  Thanks for the update.
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