DHCP not working through VLAN

[jweldin]

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.

[Amit Singh]

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

-amit singh

[mike.pruett]

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.

[b.petronio]

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

[spichiger]

look the picture-....

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

its very easy!

but tell me

[griffijo]

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.

[mgibson]

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.

[griffijo]

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