DHCP through VLAN

[richard.gore]

Hey

I can't seem to find a solution to this problem.

Here's the situation.

I have a 2620 Cisco Router plugged into a 2924 Cisco switch by a trunk.

The vlan configuration works, I subdivide my router interface with dot1q and have virtual machines on different vlans and everything works perfectly.

The problem comes with a DHCP request.

Let's say that I have my local lan on the native VLAN 1. I create a VLAN 25 in the switch and create a fa0/0.25 in the router.

In the switch, I plug the cable modem from my ISP in a port on vlan 25

In the router, I go to fa0/0.25 and issue "ip address dhcp"

The DHCP request goes out, but never comes back. The problem seems to be in the switch because if I try the same thing with a virtual machine I have on a trunked VMWare ESXi server, I get the exact same results. I just don't get the IP address from the ISP.

The next step would be to monitor the port on which the cable modem is connected and sniff the packets to see if the DHCP request actually gets back through, but I thought one if you guys might have an idea before I go and do that.

Thanks!

Richard

[stephen.stack]

So, if you connect direct to the cable modem with say a laptop, you get an ip? Also, if you connect the same laptop to a switch port also on vlan 25 what are the results ??

Regards

Stephen

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[richard.gore]

Direct to the cable modem, no problem, I get an IP

In fact, right now, my cisco router is connected directly to the cable modem (not through the switch) and it gets an IP no problem.

I'll try connecting a laptop to another switch port also on vlan 25 because all I've tried right now is connecting a virtual machine through a trunk on VLAN 25 and I don't get an IP.