Vlan problems, ip connectivity failing on certain vlans

In article , Paul Groth wrote: :I've got a weird little problem. I've got about 50 switches (2950's, :3500's, 4006's) and about 40 different vlans which make up my network. :We have close to 1100 PC's on the network, we have segregated vlans :based on geographic location within the building.

:The problem we are having is in some cases when we move PC's or replace :PC's, the PC is unable to pick up an ipaddress on a specific VLAN (ie a :PC is on VLAN 37 is moved to a new location and the port is configured :to VLAN 37 but it is unable to obtain an IP address or a PC is replaced :with a brand new one but it still is not able to obtain an IP)

:The problem isn't DHCP related, as these devices are unable to :communicate with anything even when configured with a static IP.

How long at a time does it fail to work?

:Now the strange part, if I change the vlan that the port is configured :to use, the device will start working ie going from vlan37 to vlan446. :Also if after a couple of days/weeks I set the port back to the orignal :vlan it will work again..

Certainly an unusual problem.

The first thing that comes to mind for me is that it sounds as if one of the switches or routers is holding on to the ARP entry for too long, so the pakcets are not getting ot where they need to go. When you switch VLANs, the device enters a new broadcast domain and the fact that one of the switches is stuck with that MAC in the old broadcast domain becomes irrelevant. Eventually the stuck device times out the entry, and at that point you can switch back to the original VLAN (and original broadcast domain.)

"Stuck" is, of course, a relative term,that depens on how long the devices have been configured to hold on to ARP entries.

I would suggest re-checking the ARP timeouts on the switches.

Thanks for the reply Walter,

Yes it certainly is a strange problem. I like your theory, it sounds very probable. A little bit more information about our setup, the majority of our problems occur in 3 or 4 vlans. These 3-4 vlans have more devices then most of our other vlans. Also these VLANs are all routed through a specific 6000 (we have 2).

One other interesting thing is that if I change a device to a new vlan after it starts exhibiting problems, it can take take days/weeks before it will operate again on its original vlan.

I'm no expert on Cisco gear, however I do have enough knowledge to be a little dangerous :) , so can you suggest anywhere specifically I should start looking? I've had a look at the running configs on the switches and the 6000 and can't find any references to APR entries.

Thanks

Paul