Here is some config I did some time back on a core Cisco switch for PXE boot'ing. These are the commands I put in on a 6500:
ip helper-address pxeServerIPAddress
ip helper-address dhcpServerIPAddress
ip dhcp relay information trusted
the only other command you might need for multicast is
ip pim dense-mode
On the 6500 Core switch:
!
interface Vlan102
ip address 172.16.21.1 255.255.255.0
ip pim sparse-dense-mode <-----For Multicast Mode
!
interface Vlan103
ip address 172.16.22.1 255.255.254.0
ip helper-address 172.16.21.11 <-----DHCP server
ip helper-address 172.16.20.19 <-----PXE server
ip pim sparse-dense-mode <-----For Multicast Mode
ip igmp query-interval 125
