DHCP not working through VLAN

Unanswered Question
Mar 15th, 2007
User Badges:

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.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Amit Singh Thu, 03/15/2007 - 04:43
User Badges:
  • Cisco Employee,

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

-amit singh

mike.pruett Thu, 03/15/2007 - 19:06
User Badges:

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 Fri, 03/16/2007 - 03:22
User Badges:

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...)


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

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


This Discussion