DHCP request through accesspoint and router

Question

This is how our setup is like:
Laptops => Cisco 1100 AP(static IP) => Wired LAN (Subnet 1) => Cisco Router => Wired LAN (Subnet 2) => DHCP server
What i would like is that the DHCP-request from our laptops is forwarded/relayed through the AP and Router to the DHCP-server and back.
We dont want to use the AP's internal DHCP server but centralise this.
At the moment i'v configured the AP through CLI and added the following command: ip dhcp-server xx.xx.xx.xx
Where xx is the IP of the DHCP server
On the Router i'v added the command ip helper-address xx.xx.xx.xx
Where xx is the IP of the DHCP server
Don't know if this is the correct way, but i do know it isn't working.
Some help is realy appreciated.
Grx,
Joris

Answer 1

You need at least the following connectivity for DHCP relay to work.
Between the DHCP relay agent and the DHCP server:
- UDP Port 67 <-> UDP Port 67
Between the DHCP server and the DHCP client:
- UDP Port 67 <-> UDP Port 68 (renewing/releasing a lease)
- ICMP echo (Most DHCP servers check if an IP is still free)
On the Cisco router you can use some debugging commands
to check that relaying works.
(This might produce a lot of output, careful on a production system.)
#term mon
#debug ip dhcp server packet
#debug ip dhcp server events
For Subnet 1 with the router's IP 10.0.0.1 and two configured
DHCP servers (192.168.0.1, 192.168.9.1) as "ip helper" a successfull
exchange looks like this:
DHCPD: DHCPREQUEST received from client 0100.aabb.bbcc.cc.
DHCPD: setting giaddr to 10.0.0.1.
DHCPD: BOOTREQUEST from 0100.aabb.bbcc.cc forwarded to 192.168.0.1.
DHCPD: BOOTREQUEST from 0100.aabb.bbcc.cc forwarded to 192.168.9.1.
DHCPD: forwarding BOOTREPLY to client 00aa.bbbb.cccc.
DHCPD: Forwarding reply on numbered intf
DHCPD: creating ARP entry (10.0.0.49, 00aa.bbbb.cccc).
DHCPD: unicasting BOOTREPLY to client 00aa.bbbb.cccc (10.0.0.49).
#undebug all
I would also try to ping the router's IP in Subnet 1 and a client
with a fixed IP in Subnet 1 from the server. (The DHCP server uses
this to check if a lease is actually free before handing it out)
Otherwise it's time for packet sniffing on the DHCP server
and the client. (snoop, tcpdump, ethereal)