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New Member

DHCP not working through VLAN

I have a VLAN setup on a switch that assigns 192.168.6.x to a specific port on my switch. I then have a dhcp scope for that address range in my router config. DHCP works on 192.168.1.x but it does not on 192.168.6.x. I can assign a manual address at the 192.168.6.x port and acccess the Internet. When I try with DHCP, I don't see any leases being accessed from the router side. Would you think this is a router problem or a VLAN (switch) problem? Thanks for your advice.

7 REPLIES
Cisco Employee

Re: DHCP not working through VLAN

Please paste you router configuration. Also, a brief network diagram.

-amit singh

New Member

Re: DHCP not working through VLAN

Depends on your switches. If you are using a L3 switch, you can use the DHCP snooping feature. In order to enable this you will need to configure your VLAN interface with an "ip helper-address" of your DHCP server.

"[no] ip helper-address address

Defines a helper-address for a specified address. The helper-address defines the selective forwarding of UDP broadcasts, including BootP, received on the interface. The address argument specifies a destination broadcast or host address to be used when forwarding such datagrams."

You might want to look into the Routing Commands for your particular switch. I know on my 3750's that this is what we use:

> interface vlan 10

> ip helper-address XXX.XXX.XXX.XXX

Hope this helps.

New Member

Re: DHCP not working through VLAN

Since ur DHCP, is in other Vlan, in the Vlan 192.168.6.x, u must configure ip helper-address "ip of DHCP Server", in order to pass the broadcast from that Vlan to the specific ip address defined in the Vlan. (Usually DHCP Server's, RIS Servers, etc...)

Petr?nio

New Member

Re: DHCP not working through VLAN

look the picture-....

do you want to configurate a Router, DHCP and Vlan ?

its very easy!

but tell me

New Member

Re: DHCP not working through VLAN

The question is where is your layer 3 interface for the 192.168.6.x network? If the layer 3 interface is a subinterface on the router it needs to be configured mapped to the pariticular vlan on the switch.

interface GigabitEthernet0/0.1

description dot6 network

encapsulation dot1Q 100

ip address 192.168.6.1 255.255.255.0

If the layer 3 interface is on the switch then the DHCP scope needs to be configured on the switch.

New Member

Re: DHCP not working through VLAN

The issue that I found with our network was that even though the VLAN was setup on the port, the DHCP server did not hand out the correct IP. I had to end up making reservations for each machine in a ceritan VLAN. There is probably a better way to do it, but thats what I did.

New Member

Re: DHCP not working through VLAN

I should have said your layer 3 gateway for the 192.168.6.x network.

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