I don't normally post stuff this long but I think it might be the solution to the problem below.

"There is one more issue with RIS & DHCP in Switched
Networks. The villain is the SPANNING TREE PROTOCOL! What
happens is this. When the PXE boot rom comes up and starts
looking for a DHCP server, it does so within 10-15 seconds
of the system booting up. Spanning tree takes 50-60
seconds to move a port to forwarding state from the time
it is powered up. PXE typically times out within 60
seconds. You can verify this when your PXE starts looking
for a DHCP server. The switch port will be glowing Amber
or Red instead of "all ok" Green.

"This is by design, to prevent loops in a switched network.
The solution, ask your network administrator to disable
Spanning tree on the ports your RIS clients are using
go or
hit the pause button as soon as PXE starts up. Wait for
one minute or till he switch port of the client PC glows
green and then proceed. You will sail through.

"Believe me, I was tearing out my hair on this till I got
my CCNP and gained some insights into the wonderful world
of switched networks".