DHCP does not pass through 3750 with VLAN

Unanswered Question
Mar 12th, 2007

Have a Cisco 3750 with a bunch of VLAN's configured for our vmware esx environment. If I plug in a client (esx ot windows) and make a DHCP request, the client never get a address. Have put a sniffer between host and swicth and reguests is seen but when sniffer is between the switch and the next one no dhcp request is seen.

It works fine when in switchport access mode but never in switchport trunk mode.

Is there anything to configure on the 3750 to allow dhcp requests to pass through? Thanks alot. Marten

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acomiskey Mon, 03/12/2007 - 15:36

Is the dhcp server on the same vlan as the client requesting address? If not you have to add ip helper-address in the corresponding vlan.

MUHAMMAD SHAHEEN Mon, 03/12/2007 - 15:37


You need to configure IP Helper address on every VLAN interface you have.

For example if your DHCP server (with ip add has a scope configured for Vlan 1, Vlan 5, Vlan 15, Vlan 20 etc

then on

Inteface Vlan 1

Ip address

IP helper-address

Inteface Vlan 5

Ip address

IP helper-address

and so on

This should resolve your problem provided All scopes are configured correctly on the DHCP server and ensure DHCP server is able to PING ALL VLAN interfaces to prove the connectivity.

Hopr This Helps


martentrogen Mon, 03/12/2007 - 15:55

We have set IP helper-address on the switch but there are no option to set it on every port. Why does it work in switchport access mode but never in switchport trunk mode? Is there anything else to enable "pass through" of DHCP requests? The sniffer never even see the request on the "other" side, between Switch1 and Switch2.

Client -> Switch1 -> Switch2 -> ....

Thanks for your help. Marten

acomiskey Mon, 03/12/2007 - 16:01

Are you saying the dhcp server is off switch 2 and there is a trunk between switches? Are you routing between these vlans?

martentrogen Mon, 03/12/2007 - 16:15

DHCP Server is connected to Switch2 but the sniffer never even see the request from Switch1. If I connect an ESX Server with a defined vSwitch with VLAN ID yyy and connect a VM to that vSwitch it all works fine. But when I'm not connected to a vSwitch with a VLAN ID, instead physically to the port I don't get through the switch. It's like the native VLAN never get access through the switch...or something. I'm not to good at communications configurations myself. That why I really appreciate your help. Marten

glen.grant Mon, 03/12/2007 - 16:32

If the 2nd switch is trunked to the first they should be able to pull an address as long as ip routing is turned on the 1st switch and you have ip helper addreses on the layer 3 SVI for those vlans . Also make sure you have portfast turned on your client access ports . You have to make sure the trunk is also setup and working correctly. You also have to make sure the dhcp server function is turned on the 1st 3750 .

martentrogen Mon, 03/12/2007 - 16:59

Thanks a lot guys for your help. Just a quick one, what is this "dhcp server function" and how do I turn it on? Could you also confirm what command turns on IP routing? Cheers Marten

acomiskey Mon, 03/12/2007 - 17:01

The switch itself can be a dhcp server. If you weren't using another machine. And ip routing command is "ip routing", haha.

martentrogen Mon, 03/12/2007 - 17:43

If I already have a DHCP Server on Switch2 as shown below

Client -> Switch1 -> Switch2 -> DHCP Server

do I still need to configure "dhcp server function"

Also, Glen, regarding "You have to make sure the trunk is also setup and working correctly"

Can I get a hint of how a working configuration looks, or would look? Thanks once again for all your help. Marten

acomiskey Mon, 03/12/2007 - 19:30

do I still need to configure "dhcp server function"

No, you do not.

kyawzawhtut Tue, 03/13/2007 - 00:25

Just want to add on.

1. Make sure there is no pruning and filtering on trunk link.

2. I believe you have already enable ip helper address. Can other PC from different VLAN receive the IP address?

The best is post your 2 switches config without any sensitive info. I am sure netpro can help you better by that way.





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