Great article over The Configuration Manager Team Blog.

Read the original post here.

If a Configuration Manager client has received and started an advertised operating system deployment task sequence, attempting to start another operating system deployment advertisement within a certain period of time might result in the deployment being ignored and failed to start. In this case, the client ignores the PXE boot request and falls back to the local hard disk when booting up.

This is an expected behavior in both Configuration Manager 2007 SP1 and SP2 based on the value of the PXE cache expire setting, which controls how long before a PXE advertisement expires. The default value is set to 60 minutes. This setting is useful when unknown computer support in R2 is turned on. If the value is too short, an unknown computer running a mandatory operating system deployment will repeatedly begin the same advertisement when rebooting in WinPE. For deployments that do not use unknown computer support, setting the value to a shorter period of time can help them quickly recover from failures that occur during operating system deployment and restart the advertisement.

For Configuration Manager 2007 SP1 hotfix KB969113 will set the default value from 60 minutes to 2 minutes. Configuration Manager 2007 SP2 improved functionality to make this value configurable in a registry key. The default value is 60 minutes in Configuration Manager 2007 SP2.

To set the registry key for the cache expire value for the PXE service point role, open the following registry key on the computer with PXE service role installed:

On a x86 machine


On a x64 machine the registry key is under WOW6432Node


Create a new DWORD value CacheExpire, and set the desired value in seconds. For example, if you prefer the cache expired in 2 minutes, set the value to 120. If the value is set to 0, then the default 60 minutes value is used.

Note: Setting the value in CacheExpire is only supported in ConfigMgr 2007 SP2.