DHCP Relay Problem - Server offering wrong address

[Burgal]

DHCP Server on windows 2003 server on vlan1
DHCP Relay on Dell 6024 aggregate switch with routing and multiple VLANs on different subnets


I have a multi vlan network with a DHCP server residing on vlan 1 with a subnet of 10.0.1.0/24.
I have vlan 10 with a subnet of 10.0.10.0/24

workstations on vlan 1 grab their IP just fine from the DHCP server with a scope of 10.0.1.0/24

If I place a workstation on vlan 10 with a subnet of 10.0.10.0/24 and watch the packets with omnipeek I see the workstation request a DHCP address. That request is forwarded to the vlan 1 subnet where the DHCP server receives and responds to the request. However, teh response is for the wrong subnet, and thus never is forwarded by the dhcp relay to the subnet where the requesting workstation resides.

so I've noticed a couple things here. The DHCP Relay doesn't always put's its gateway address in the DHCP request packet.

The DHCP server isn't matching the scope of the incoming request and thus returns the wrong scope's address.

the DHCP response is not forwarded back over DHCP relay on the Dell 6024 switch.

Any ideas?

 

[WhoKilledKenny]

I think you are on the right track here:
so I've noticed a couple things here. The DHCP Relay doesn't always put's its gateway address in the DHCP request packet.

The DCHP server needs to know which VLAN the request packet came from. Since I am not familiar with the switches you are using, I don't have a suggestion. But I do think that that is were the problem is comming from (the switch). Are all of your clients on the 10 VLan having the same issue?

 

[Burgal]

I haven't deployed any clients on vlan 10 due to this issue.

I'm using a dell 6024 gigabit switch with layer 3 routing. The switch only has 1 option for dhcp relay. Enabled, and DHCP server IP address.

I don't have any other things on other than IP routing. UDP Relay, and multicast relay are off.

 

[WhoKilledKenny]

I know that DHCP requests that come from other subnets are sent as a unicast broadcast, is there anything on the Router/or Switch that is blocking this type of broadcast?

 

[Burgal]

Not that I can tell.

And that is why there is a dhcp relay agent.
and the dhcp requests ARE being relayed. just not well...

I'll recheck the omnipeek log on the dhcp server side to reexamine the headers of the dhcp request.

 

[WhoKilledKenny]

Look at the Discover Packets, should have the IP helper in there...