IP Helper on Vlan interface

Unanswered Question


Is it necessary to have the ip-helper address on the vlan interface?

We are having problems with thin-clients obtaining a dhcp address. This happens sporadically and to resolve we must make a reservation in dhcp then give the client a static address with the reservation address then change the client back to dhcp. I feel like this is a switch problem.

Any ideas or suggestions?

Thanks in advance,


I have this problem too.
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lamav Tue, 04/07/2009 - 10:56


If your DHCP server is located on the same subnet as the clients, then you do not need to have a helper address configured.

The purpose of the helper address is to forward the DHCP Discover packet, which is a L3 broadcast, from the client to the DHCP server when it sits on another vlan. A router will not forward broadcasts by default, so the helper address tells the router to forward the DHCP Discover packet in a unicast to the DHCP server specified in the helper address command.

Are you certain that the DHCP server itself is not experiencing problems? Maybe intermittent connectivity to the network?

Are you sure that the DHCP pool of addresses is not getting exhausted?




Thanks for info. I do not believe the dhcp is having problems. There are still plenty of addresses available and their are clients that are computers and they do not have this problem only the thin clients. My thinking was they act differently them the PC's. No one can seem to pin-point what the problem is. I want to make sure that my switches are not the cause.

will.cullen Tue, 04/07/2009 - 11:01

The ip-helper address command is used to forward dhcp requests to a dhcp server, so it should be put under the vlan interface that your thin clients are on, by default the ip-helper address command forwards the following udp port no:37,49,53,67,68,69,137 & 138. The first thing to try would be to see if you can ping the dhcp server from the switch, also you may want to see if under the ports that the thin clients are connected too, you have spanning-tree portfast, may be the thin clinets are sending the DHCP requests before the port is actually in a forwarding state, spanning-tree portfast used only be used on ports that have one host.

Hope this helps

lamav Tue, 04/07/2009 - 11:08


"The ip-helper address command is used to forward dhcp requests to a dhcp server, so it should be put under the vlan interface that your thin clients are on"

Will, that is not altogether accurate. You would have no reason to configure a helper address under a vlan interface when the DHCP server resides in the same vlan.



will.cullen Tue, 04/07/2009 - 11:15


Thats my fault for assuming that they're on different subnets/vlans :)

lamav Tue, 04/07/2009 - 11:35


It's no biggie. I just didnt want the OP to think that a helper address must ALWAYS be configured, regardless of where the DHCP server is ocated...



glen.grant Wed, 04/08/2009 - 04:36

Their refusal to turn on portfast on user ports could very well be your problem , this is one of the main reasons to use portfast to eliminate dhcp problems due to spanning tree timeout issues.


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