DHCP Cisco switches

I am having a problem with imaging to a netware server across a cisco 3500
XL switch. We recently had a 3com switch go bad and had to replace it
with a cisco switch. We can get DHCP from the PXE when booting but when
the Zen client boots it cannot get DHCP, and the problem seems to be with
all of the cisco switches not just the Cisco 3500 xl.

Thanks
Dave.