PXE IP Lease not expiring

[jmcelfresh]

I'm having a problem with PXE not releasing the IP Address after boot. This is with a WYSE 3150 Thin Client that uses the Helper Address to locate the TFTP or Wyse Device Manager's FTP Image. Does anyone have any ideas on how to force the release of the secondary address being held by PXE or to modify the lease time by policy?

192.168.10.98 0080.6439.0a07 Jun 01 2010 09:53 PM Automatic
192.168.10.99 0100.8064.390a.07 Jun 01 2010 09:54 PM Automatic

 

[jmcelfresh]

Wow! No takers? Come on guys, just some guesses would help at this point. I'm at a total loss.

According to the Cisco documentation DHCP supports three mechanisms for IP address allocation:
• Automatic allocation—DHCP assigns a permanent IP address to a client.
• Dynamic allocation—DHCP assigns an IP address to a client for a limited period of time (or until
the client explicitly relinquishes the address).
• Manual allocation—The network administrator assigns an IP address to a client and DHCP is used
simply to convey the assigned address to the client.

So, are the mechanisms something you setup or it is a negotiation between the client and the DHCP server unless it is manual?

Can you setup a policy on your router for certain MACs that would have a shorter lease time?

 

[burtsbees]

In your first post, I myself had no idea what this had to with Cisco routers. I also have no idea what a WYSE 3150 Thin Client is, much less why it uses a helper address to locate a TFTP/FTP image.

In your second post, I guess I could gather that your thin client is set to PXE boot and get a dhcp address from a Cisco router. Okay, so then it does. Then it boots and won't release the address? Why would it need to release the address in the first place?

There is nothing in the dhcp config of a Cisco router that can tell the client to release its address---the Cisco simply responds to DHCPDISCOVER packets with DHCPOFFER packets. If the thin client sends out additional DHCPDISCOVER packets after booting for some reason, it has nothing to do with Cisco...

You can create a dhcp pool for a specific client and set parameters for that specific client (cleanup timers, lease, etc). Is that what you mean?

/

tim@tim-laptop ~ $ sudo apt-get install windows
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package windows...Thank Goodness!

 

[jmcelfresh]

Thanks bustsbees for a response. Cleanup times might be exactly what I'm looking for. Can you elaborate on this a little more?

Just a FYI response to the rest of your comments:
1) "why it uses a helper address to locate a TFTP/FTP image."

The Helper Address used in by a Cisco DHCP server is for IP forwarding in such cases as BOOTP. In this case, I'm using it to locate a TFTP server that checks for updated images to firmware, as it was designed.

2) "I guess I could gather that your thin client is set to PXE boot and get a dhcp address from a Cisco router."

The type of PXE device is not really relevant in this case. I only included this information for those that may be experienced with Thin Client technologies.

3) "Why would it need to release the address in the first place?"

Cisco said it best, 'Dynamic allocation—DHCP assigns an IP address to a client for a limited period of time (or until the client explicitly relinquishes the address).' PXE does not use the IP after the client is fully booted and should be releasing the address.

4) "There is nothing in the dhcp config of a Cisco router that can tell the client to release its address".

DHCP does control the lease times of the IP Addresses it leases. You can see from the run command of any Cisco DHCP server. Mine reads:
ip dhcp pool DHCP-Pool
network 192.*.*.0 255.255.255.0
default-router 192.168.*.*
domain-name ******.net
dns-server 192.*.*.* 192.*.*.*
-->lease 30

 

[burtsbees]

I do know what IP Helper is in a layer 3 device, since layer 3 devices do not forward broadcasts. It looked like you configured the thin client with some sort of helper address deal, so I was questioning that.

As far as telling the client to release the address---yes, lease times can be configured, etc. But I meant that once the device boots and no longer uses the address it got via PXE, there is nothing on the router end to tell it to release it, like "OK, now you're booted---release the PXE address, send me a DHCPDISCOVER packet, and I'll hook you up with an ip address that you can use".

Why does the thin client ask for another address after it boots? WHy can't it use the PXE address? The solution pretty much hinges on that. There is nothing you can do in the router that will help---cleanup time interval is the time it waits to clean up expired bindings, so no bearing on the problem whatsoever. Setting a small lease will do nothing nor will limiting the dhcp bindings per interface---the client always asks for a second address no matter what, it seems, which is the problem, and so the lease time, etc. will only affect whatever leases it gives out. If it has two addresses, and the lease expires on both, it will only need one, so it will only ask for one and receive only one---the original would still be in the client. This is a WYSE 3150 Thin Client problem.

To increase/decrease the lease time, I would first create a separate pool just for that client, and specify the client...

router>en
router#conf t
router(config)#ip dhcp pool thin
router(dhcp-config)#lease 0 0 1

that sets the lease for one minute---you get the picture...

router(dhcp-config)#client-identifier xxxx.xxxx.xxxx (MAC address)

or it may be...

router(dhcp-config)#hardware-address xxxx.xxxx.xxxx

now I think you can do the

router(dhcp-config)#network 10.1.1.23 255.255.255.255

to configure 10.1.1.23 to be the offered address, or a small pool of addresses within the LAN subnet. If you make it one address, maybe it will get the same address twice!lol If you make it a small pool, just make sure you do

router(config)#ip dhcp excluded-add 10.1.1.20 10.1.1.45

that will exclude 10.1.1.20 through 10.1.1.45

HTH, good luck. But I would check to see why that client is failing to release the first address---it couldn't hurt, but it doesn't help...

/



tim@tim-laptop ~ $ sudo apt-get install windows
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package windows...Thank Goodness!

 

[jmcelfresh]

Hey, thanks Burtsbees.

I agree that the client should be releasing the address. I have over a 100 of these little suckers on this LAN. At my other locations, PXE releases the address just fine. PXE grabs an IP Addresses and releases one after 5 minutes (plus 1 minute to discover the binding changes).

Then one day, my address pool was being used up by PXE bindings. 'Before you ask, yes I have deleted the pool and reconfigured it again. I took one of my Thin Clients over to one of my other locations and it worked fine. This has had me scratching my head. I going to try the "ip dhcp binding cleanup interval 10" and see how that works out.

I appreciate the help and comments. I'll post back to let you know the outcome. Thanks again.

 

[burtsbees]

OK...what is passing out the DHCP addresses? I am assuming a Cisco router, but what model? Can you post

router#sh ver

and

router#sh run

from the working location and the non-working locations, and mark each as "WORKING" and "NON-WORKING"?

/

tim@tim-laptop ~ $ sudo apt-get install windows
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package windows...Thank Goodness!