Cisco – Configuring trunk port on Cisco Catalyst switch to serve DHCP settings for multiple VLANs

ciscotrunkvlan

I am trying to configure a trunk port to serve single DHCP settings for multiple VLANs. I tried the following configuration, but to no avail.

switchport mode trunk
switchport trunk allowed vlan 10,20,30
swithport trunk native vlan 10

I can only get vlan 10 to work with DHCP. I appreciate any help on the issue. Thank you.

Best Answer

Serving DHCP settings for multiple VLANs is not mainly a port trunk issue.

A DHCP client sends broadcast packets to find a DHCP server that will respond to this request.

When multiple VLANs are configured, broadcast traffic will not be forwarded to other VLANs. So your DHCP Client won't be able to find a DHCP Server on another VLAN.

The proper way to achieve what you want is to configure the ip helper-address on all your VLANs interfaces where you want DHCP to be available.

This commands allow the router to identify these broadcasts messages and turn them into unicast messages before forwarding them directly to your DHCP Server.

Basically :

interface Vlan20
   ip address 192.168.0.1 255.255.255.0
   ip helper-address 10.10.10.10
   ...
   ...
end

Where 10.10.10.10 is the ip address of your DHCP Server.

Below some references for further reading :

Related Topic