I had a segment a network recently into a few Vlans. This meant the new Vlan was on a different subnet to the WDS server so machines were not getting the PXE Traffic.

1. Make sure the WDS is routable from the Vlan ( Create a route enable firewall) 
2. Enable a second IP Helper Address with the WDS server. You might already have an IP Address if your DHCP server is on a different Subnet, but you can have multiple.

PXE does not come with a dedicated boot protocol. It is simply DHCP packets extended with additional DHCP options. It’s formerly known as the bootstrap protocol. If a PXE-enabled network card sends out an DHCP discover package, it will add DHCP option 60, which includes the string “PXEClient:Arch:xxxxx:UNDI:yyyzzz”. Then it waits for DHCP offers.

It will only respond if it gets a DHCP offer including option 60 which means: I am PXE capable and able to send out boot server and boot file information.

The DHCP offer can be splitted into two independent packages, coming from different servers. The DHCP server can send out the DHCP offer containing the clients IP address and the PXE server can send out the DHCP offer containing the option 60.