Cisco Support Community
Community Member

dhcp for vlans

hi all

Can anyone tell me how come my switch knows only to issue my dhcp pool to the relevant vlan, the vlan and dhcp pool are are the same subnet, but what if I didnt have a vlan in that subnet, would it not issue them ?


Re: dhcp for vlans

Hi Carl,

I'm not entirely sure on what you mean, however I'll explain a bit on how I see your question.

Basically a PC on a subnet will send a Layer2 Broadcast requesting an IP address and various other details. Therefore, you will find that in most cases the DHCP server should be on the same subnet to receive the broadcast frame.

All your switch does is forward frames (unicast, broadcast and multicast) across the same Vlans that are configured. Generally speaking switches do not forward to different vlans unless you have configured a multilayer switch.

The exception in the dhcp case is where you use a dhcp relay agent to forward dhcp requests across different subnets to a central dhcp server.

Please let me know if this makes sense or not. I'm not quite sure what you mean if you didn't have a vlan in that subnet.



Community Member

Re: dhcp for vlans

Hi Michael

thanks for that, I basically have 3 vlans with ip addresses on them in a layer 3 router, with 3 dhcp pools, how does the switch know only to forward the correct pool to the correct vlan ?

Re: dhcp for vlans

Hi Carl

AFAIK it does allocate based on the option 82 feature which has the required informations like port number,vlan id etc.,

for more info do refer this link..



Re: dhcp for vlans


you need to first of all configure DHCP server pool with respect to your vlans, with dhcp relay agent.


then you need to put ip helper address ( dhcp serever pool) on that particular vlan. exclude vlans ip address from the DHCP server.

I hope you this will solve your problem.

ip helper address command with convert broadcast to unicast request.


Community Member

Re: dhcp for vlans

The actual switch is providing the dhcp pools not an external server , so i dont think i need the ip helper command

CreatePlease to create content