DHCP Relaying - Help

Unanswered Question
Jul 25th, 2007


I have a situation here which I need help on.

We have 3 VLAN's here. All 3 VLAN's are connected to a Catalyst switch and then to a router via E0.

We have DHCP servers installed on each VLAN which provides to each subnet/vlan.

We are shutting two of them down and we only want to use 1 DHCP server for all 3 VLAN's/Subnets.

I was told that I must configure the IP Helper configuration on the router's E0 interface which is connected to the switch pointing to the DHCP server.

Is there any additional steps required for this to work?

I was told by someone else that some relay configurations must be made to the VLAN's on the Catalyst switch to point to the correct DHCP server for requests?

Any ideas?


I have this problem too.
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Jon Marshall Wed, 07/25/2007 - 11:33

Hi Danny

Where are the routed interfaces for your vlans.

Lets say you have 3 vlans: 10,11,12

You have the DHCP server on vlan 10 with address

Under the routed interfaces for vlan 11 & 12 you need to add the command

ip helper-address

This should do it as long as you don't have access-lists applied to the routed interfaces.



danny9797 Wed, 07/25/2007 - 11:38

I see, makes sence... The helper commands are configured on the switches for the other 2 VLAN's...Thats what the second person told me as well.

So nothing has to be added to the E0 interface on the router, correct?

Jon Marshall Wed, 07/25/2007 - 11:45


As Jorge said if the router is routing for all the vlans you need to configure ip helper-address on router subinterfaces.

If vlan 11 & 12 (from previous example) have vlan interfaces on your switch then it is here you would add the ip helper-addresses. It all depends on where the routed interfaces for vlans 11 & 12 are.


JORGE RODRIGUEZ Wed, 07/25/2007 - 11:39

if that router is routing all three vlans, ensure ip helper-address is configure for each interface or subinterface on the router pertaining to that vlan, with this you should be all set.

Also ensure DHCP has all three subnet scopes.

danny9797 Wed, 07/25/2007 - 12:21

I see thanks a lot.

The router is not configured with subinterfaces for each VLAN. All of the VLAN configurations are on the catalyst so i'm going to apply the helper configurations on the VLAN interfaces on the switch.

Thanks again


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