If you’ve spent any time dealing with Config Mgr OS deployments using PXE you’ve probably had machines fail to connect to PXE service point with errors such as “TFTP download failed” or “unable to connect to PXE server “. Believe it or not this is by design – especially if you are using unknown computer support. This is most likely due to a previous task sequence being advertised to that machine. By default Config Mgr will cache the PXE advertisement for 60 minutes (Default setting) and then it will expire. Most of the time you can right click the collection the collection that the machine sits in and select “Clear last PXE advertisement” but sometimes you have to go as far as restarting the WDS services. Microsoft published a hotfix under KB969113 for Config Mgr SP1 that will reduce the interval from 60 minutes to 2 minutes and in SP2 the cache expiry can be set in the registry.
On an x86 machine the located under:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SMS\PXE
On an x64 machine the registry key is under:
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\SMS\PXE
Create a new DWORD value CacheExpire and set the desired value in seconds. Be careful if you have R2 installed and unknown computer support enabled because if you set the cache expire value too low and you are using a mandatory task sequence your machine will continuously start the same task sequence every time it reboots. There is a good post on the System Center Configuration Team blog that details PXE cache expire behaviour - http://blogs.technet.com/b/configmgrteam/archive/2010/05/27/pxe-cache-expire-behavior-in-configuration-manager-2007-sp1-and-sp2.aspx
