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New Member

IP Helper on CAT 3524 XL

I have 2 CAT 3524 XL switches trunked together. I have 2 different subnets one on each switch that I want to have our DHCP server give out different address depending on which subnet they come from. I have IP Helper commands on each Vlan pointing to the DHCP Server. I can ping the DHCP server from within both switches, so I know the connectivity is there. I can get address from the DHCP Server on both Vlans but only from the one pool of addresses, not from the proper pool. I know this works and the servers work with the 4006 and 3550 Layer 3 switches, because I have them working.

Is this not possible with trunked Layer 2 switches? I do have a global command to point to the DHCP Server as well and the IP Helper command under each Vlan. Any help would be appreciated.


New Member

Re: IP Helper on CAT 3524 XL

Not sure about the specific configuration, but here goes...

Generally, the ip helper-address just tells the router where to forward L3 broadcast traffic too for network services. In this case its DHCP which is a default protocol forwarded by this command.

That said, when the router converts the broadcast to a unicast with the source being the router. So, the receiving application should see the source subnet being the VLAN GW and provide an address to the correct subnet.

From what I can tell, you have DHCP server on a vlan that is NOT on either of those configured with the switches. Since you are getting the IPs from the same pool off the DHCP server it would appear that the client ports are not in the VLAN you expect them to be... meaning the ports are in the same VLAN for your test because the GW for the VLAN servicing the DHCP request for both clients is providing an IP address within the scope defined for only one VLAN.

Look at the port vlan settings on the switch and make sure the client ports are in the right vlan for the other subnet.



Re: IP Helper on CAT 3524 XL


AFAIK, Cat3524XL are only L2 devices. They don't provide any L3 functionality.

IP helper command is a router command saying router where to forward some

received broadcast packets on an interface to the destination host. This configuration ensures that broadcasts from the DHCP client can reach the DHCP, DNS, and

TFTP servers and that broadcasts from the servers can reach the DHCP client.

So it has no sense to use it on L2 switches.

I beleive this command was forgotton in 3524 config by mistake and is doing nothing.

For your topology:

Let's say you are using VLANa and VLANb, DHCP server is placed in VLANa.

You need to connect each VLAN to one router interface (or subinterface while using trunk connection to "router on the stick").

Configure ip helper on that router interface connected to the VLANb using the IP address of the DHCP server.

This way the router will forward DHCP requests from VLANb to the DHCP server in VLANa.

We are using this configuration in our network and it works fine.

You also need to configure your DHCP server to recognize the requests which came from VLANb and reply with DHCP offers using different IP address pool. (I don't know how to do it in details, there was another specialist configuring our DHCP server.)

Here are good links explaining details: