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Community Member

possible solution to vlan change?


As I use 6509 DHCP services to support multi-vlan, when one user log on domain in one vlan using windows 98 platform, after he move the pc to another vlan he can not get IP and log on the domain again, even have already set 'portfast'. Only use WINIPCFG to release IP first, then get the IP - but it will confuse users.

Some expert told me that it might be related to windows platform: when network disconnected, windows would not take it as a release.

My question is, when network disconnected, can Cisco 6509 take it as a release? because if I use NT DHCP service (not Cisco 6509), the problem does not exist.



Community Member

Re: possible solution to vlan change?

I've had a similar problem. From what I've been able to find out, Windows 9x workstations don't release their IP address on shutdown, unless a Registry key is set to release it on shutdown. The default is to *not* release it, evidentally. Interestingly, not every Win9x workstation exhibits the problem. I had the problem on my workstation in fact. I found the key and flipped it, and I give up my address each time.

I don't think the DHCP server can control this (not totally sure though). The workstation has to tell the server he is releasing it.

More importantly though, each time a Win9x workstation boots, it *does not always* check to see if it still has a valid DHCP lease (i.e. it doesn't test the one it has to see if it's still good). Unless the workstation actually gave up the address, it *may* assume the one it has is still good.

The solution is to flip to "1" the registry key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\DHCP\ReleaseLeaseOnShutdown

...on each workstation. I suspect that will be the kicker.

Another cheap way out may be to set the lease to a really short time. The idea being that the lease would be ready to be renewed by the time the workstation is booted up again. That's kind of a crappy band-aid in my opinion though. JimW

Community Member

Re: possible solution to vlan change?

Appreciate for your suggestion, it works!

Shorter lease time would cause more traffic while changing registration would take too much time for thousands of clients.

I still have question, why there is no problem when I use NT DHCP service rather than C6509 DHCP service?


Community Member

Re: possible solution to vlan change?

Not sure why it isn't a problem with your NT DHCP server. I have seen the exact same thing you describe *with* NT servers in a customer account. Suspect you should look through the DHCP config on both the Cat and the NT server to see what the differences are. Another thing to do is set up a sniffer on a nice quiet VLAN and capture DHCP transactions between the client and the Cat, and another capture between the client and the NT server.

By the way, why are you not using the NT server for all the subnets? I've frequently heard suggestions that the Cat DHCP facility should only be used as a "last resort", not as the primary DHCP server.

Finally, you are right on the shorter lease time. I couldn't beleive this was the long term solution for the similar problem for the customer where I encountered this. More worrisome, though: Could you imagine thousands of clients slowly dying as their leases expire if the DHCP server went offline for, say, an afternoon? Sheesh!

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