Cisco 3550 switch as DHCP relay agent with Windows 2000 server dhcp server

     ------------CISCO 3550 SWITCH -------->Internet
                                            |                           main DHCP server          
                                   test DHCP server

I have a Cisco 3550 switch sitting on a network. The switch has two VLANs on it , and I'm trying to get clients on these VLANs to obtain their IP addresses form the DHCP server on the network.

The switch acts as a router for inter-vlan traffic.
There are no ACLs between the VLANs.
The Default gateway for the Switch is the FW and the FW routes back any traffic destined for the / networks that comes from the network to the switch IP address.

All this works fine.

The main DHCP server (a windows 2000 type) has three scopes on it, one for each of the subnets. I belive the switch should have a dhcp relay agent on it by default. However none of the clients on either of the VLANs can find the main DHCP server, clients on the network have no such problem.

As a test I configured a test DHCP server on the VLAN. Again, although clients on the same VLAN can get DHCP IPs. Clients on the VLAN cannot.

Do I need to configure the Switch in to forward DHCP requests? Or is my proposed scenario impossible?

Many thanks for any help given.

Who is Participating?
PennGwynConnect With a Mentor Commented:
> The Default gateway for the Switch is the FW and the FW routes back any traffic destined for the /
> networks that comes from the network to the switch IP address.

The DHCP relay needs to happen at the router between the VLANs.  It looks like you're trying to have the firewall do that, and ignoring the layer 3 capabilities of the switch -- although I don't see how that can actually work if you expect the switch's address to be able to deliver packets to the other VLANs.  (If the switch has VLAN interfaces on these networks, traffic to those networks will go there directly and NOT visit the firewall at all, since direct connections take precedence over the default route.)

Anyway, I believe "ip helper-address" is the command you want.  Add it to the VLAN interfaces of the 3550, telling them to forward broadcast DHCP requests to the server's address.  (The forwarded requests will carry the address of the interface that forwarded them, so the server will know which scope should respond.)

You need to add the following to your VLAN interfaces:

interface vlan3
ip helper-address 10.1.0.x  <---specify the DHCP server address.

Do the same thing for VLAN 4:

interface vlan4
ip helper-address 10.1.0.x
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mattdunn1264Author Commented:
IP helper-address was indeed the command I was after.. thanks

I was looking for answers for configuration of my catalyst 3550 and came across your problem.
I have same scenario at my company. (even same ip adreses)... I also configured separate VLANs and different DHCP scopes on my 2003 server, but I still can't get IP addresses on other VLANs but the one that server is on. I tried IP helper-address...
Is the problem at my cisco or something wrong with my DHCP configuration? :-(
I'm new at  cisco configurations (my first cisco switch)
Would it be too much if I asked you to send me configuration of your 3500 switch?

mattdunn1264Author Commented:

i cant send you the config but I'll try and help with a few pointers. You've probably already thought of it all anyway.

Is it just that the DHCP that isnt working? Can you access the or from the network on any port at all?Because anything on the network has a default gateway of the FW, you will have to set up a static route that tells traffic from this network that reaches the FW to go back to the switch.

On the switch itself, have you set up any access lists? you will need to permit DHCP (or bootps) protocol, port 67 on UDP i think, to be permitted from any to any, otherwise DHCP will be blocked.

On win 2k dhcp server, the scopes for other subnets are identical to scopes on the same subnet as the server, except for..

a different IP range (obviously) and a differnt defaut anything else you need to cater for that subnet.

Try using the Cisco CMS interface, you can download it from their site (its tough to find though) and use it as a gui to configue quite alot of your switch. You'll still have to use the command line for the ip helper-address.

Sorry I couldnt help more!

service DHCP
ip dhcp smart-relay
ip dhcp relay forward spanning-tree

interface GigabitEthernet0/11
 description To Gateway
 no switchport
 ip helper-address
 ip address X.X.X.X
 ip dhcp relay information trusted
 no shut

interface vlan 300
 description Staff_Vlan
 ip helper-address
 ip address
 no shut

this is what I did and it worked.  Now I cant seem to get it to pull it from the correct scope.  Ill keep u posted
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