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  Asked: Mar 11 2008   5:32 PM GMT
  Asked by: EastCoastGuy

Wireless and DHCP Leases


Windows Server 2003, DHCP, Wireless

I'm looking for a recommendation on DHCP and a Windows wireless IAS network using 802.11x.

We currently set clients to expire leases every 5 days. However on the wireless network this causes them to drop off the network and re-authenticate if the lease expires mid day.

Any suggestions on the best way to handle this.

I'm using Win2003 with Xp sp2 clients and Meru AP's.

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Last Answered: Mar 11 2008  7:30 PM GMT  by Jlees




A client machine being setup for DHCP itself shouldn't be causing the user to drop off the network and certainly not reauthenticate. A 5 day lease should be fine.

DHCP clients attempt to renew their lease at HALF of the lease duration, so in your case they would attempt to renew the lease in 2.5 days. If the DHCP server is unavailable, or doesn't respond, the computer continues to use the lease until another half of the remaining time has lapsed-- or roughly 75% of the lease duration. At this point it waits until the lease expires and attempts a renewal of the lease, if the renewal isn't successful, again becasue the server doesn't respond, then the client drops off the network and starts the DHCP process from the beginning.... DORA.

Discovery - Basically, finding a DHCP server to service the client, via MAC braodcast from the client to the network.
Offer - an offer from a server with IP addresses, via MAC broadcast from the server to the client.
Request - A request by the client for a specific IP that was offered by a specific DHCP server sent to the server from the client.
Acknowledgement - an Acknowledgement that the IP is the clients from the server to the client.

It's possible there are collisions on the network, or some other factor not allowing some responses to get back to the client from the DHCP Server if the server is available during these time periods. Try looking for events in the clients event log stating it tried to renew a lease and what happened when it did so.
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EastCoastGuy  |   Mar 12 2008  8:40PM GMT

Good point. I’m looking into Collisions on the network as the cause.

thanks.

 

Labnuke99  |   Mar 14 2008  12:52PM GMT

If it is the wireless clients only, I wonder if it has something to do with the client authentication process with the access point. How is that setup? What encryption is used on the connection? Why would DHCP be the only affected traffic? Try installing the Microsoft network monitor software on the client if Windows and capture traffic (you can capture filter for just DHCP if you wish, but other traffic may hold the key here). I would then take the capture to Wireshark for analysis.

You will always have collisions in the wireless network. It is a shared medium like the old hub days. The difference is how in wired networks, it is called carrier sense / collision detection where in the wireless it is called carrier sense / collision avoidance.

Has Meru been able to offer any ideas why this is happening?

Good luck.