DHCP for pxe

Im DHCP on a slesOES2 box and running zen7 on a sles box. I have set the
imaging policy so It will only enter into pxe when I hold crl-alt but when
all services are up on the servers the workstations go into pxe every
time(with out key hold). I think the error is in the dhcp.conf. Exacty how
to I need to set the DHCP up to allow PXE. All ive done at the moment is
done a simple subset and added filename and next-server options.