DHCP across inter-vlan trunk

Technowiz

Ok hopefully one of you can shed some light on this one for me. I have my lab set up as follows.


2600Router (router on a stick)
|
| ISL Trunk
|
3550Switch (4 vlans configured)
|
| 802.1Q Trunk
|
2950Switch (4 vlans configured same as on 3550 switch)


On vlan 1 of the 3550 I have a computer configured to provide DHCP to all 4 subnets. I have the 2600 router configured with the helper address so that DHCP broadcasts from all 4 subnets get sent to the DHCP server. This is working. If I connect to any of the 4 vlans on the 3550 I pick up a DHCP address from the DHCP server on vlan 1.

The trunk port from the 3550 to the 2950 is using port fa0/24 which is assigned to vlan 4. Not sure if that matters or not. I don't think it should.

Now the problem is when I plug into a vlan 1 port on the 2950 it works the same as plugging into the 3550. I get a DHCP assigned address and good communications between switches and vlans. However when I plug into a different vlan on the 2950 I don't get DHCP. At first I thought it was a problem with the trunk or vlan pruning or something. But then I statically configured the IP address and everything works great. So I know the trunk is good but for some reason the DHCP broadcasts on vlan2-4 of switch aren't getting to the server.

I'm thinking there is something missing in the configuration on my 2950 but can't put my finger on it. Any help is much appreciated.

Technowiz

Well I've been scratching around with this problem for the last couple hours and have finally figured out what is going on although I don't completely understand it.

I tried connecting the dhcp server to vlan 3 on the 3550 then plugged my laptop into the same vlan on the same switch. Picked up an IP address. Great. Plugged the laptop into vlan 3 on the 2950, no IP address. Hmmm... Put the dhcp server back on vlan 1 and connected the laptop to vlan 1 on the 2950 and it worked ok. Moved the laptop to vlan 2 of the 2950 and wait a minute! I got an IP address! Tried the next two vlans and nothing. Went back to vlan 2 and no dhcp address. What the heck is going on here!?

So then I notice something that I hadn't really been paying any attention to before. When I connect the laptop to the 2950 the port LED goes amber for a number of seconds and then turns green even if the laptop doesn't get an IP address. So I bring my laptops ethernet port down and back up to renew the DHCP request (ifdown/ifup) and this time I get a dhcp assigned address. Works on every vlan.

So long story short the dhcp request is timing out because of the time it takes for the switch port to come up 100% when connected. I still don't know what is going on when the light is amber and then turning green. I timed it and it took over 30 seconds from plugging it in until it turned green. When I plug in to a port on the 3550 it is green right away. I will try to do some googling on this but if this makes sense to anyone and they can explain it that would be great.