Bellow you have Scott Morris's answer:
"PXE (Preboot eXecution Environment) searches for a boot server as well as a DHCP server if an IP is not preconfigured. The information returned from DHCP normally provides things like an IP address, netmask, default gateway and any other desired options. In the PXE world, one of the options is an Remote Installation Services (RIS) server. If the PXE boot code REQUIRES this option and it’s not being handed back, then the information may be ignored. (It’s option 60, in case you care!)
That’s potential problem No. 1, but if you aren’t seeing the broadcasts forwarded to begin with, then this isn’t your issue.
Another issue depending on the PXE software/requests is if the router views this as a "malformed" DHCP request, it won’t forward it. Some PXE vendors do not put an IP address field into their DHCP discovery packet. Normally, in a workstation that doesn’t know who it is, this field will be there and contain "0.0.0.0" as the entry. If not present, a Cisco router will regard this as an invalid packet and not forward it.
This is potential problem No. 2 and seems more likely what you’re running into. I read somewhere that when vendors use an LanClient Control Manager (LCCM) server instead of the RIS that this seems to be a common issue and there is no workaround other than putting the DHCP server on the same subnet as the clients.
A third thing that you may be running into would be in your switch configuration. If your switch is doing spanning tree calculations, you’re not forwarding any traffic at all within the first 50 seconds and that may exceed the timeouts in your PXE setup. On your switch’s ports, try using "spanning-tree portfast" to jump immediately to a forwarding state.
Do this on the ports your hosts are connected to. Keep things simple and sane!
Answering your question though about running a DHCP server and forwarding at the same time, the answer would be no. You can configure it all you want, but if you have a DHCP server running on the router, it’ll process the received broadcast first to the service and the helper-address will never engage. At least that has been my experience with setting things up.
While it’s nice that your router/switch can be a DHCP server for your network, it’s really not recommended! You can always get a spare PC and install some free flavor of Unix on it to run DHCP services. Even if you don’t know anything about Unix, it’ll take perhaps an hour worth of searching on the Web for step-by-step instructions, and you’ll have a much more robust environment. Then, as you go from office to office, the setup will be the same, except for the unique scope information.
You might also try asking your computer/NIC vendor about any issues they know about with remote DHCP servers. There may be other workarounds based on their particular drivers and coding.
Hope that helps!"
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