PXE clients cannot obtain IP address when on different VLAN
I've setup a VLAN for client PC's, with my L3 switch as the default gateway and an IP helper setup. PC's work fine with this, they obtain an IP address from the correct scope, and all is well.
But I'm trying to PXE boot a PC in order to get into MDT and it just won't get an IP address. If I move it to the original VLAN it works fine.
I was expecting a few problems booting into MDT but I at least expected to be able to obtain an IP address first! It just waits and waits and eventually times out. I don't understand why Windows can, but PXE boot can't. Any ideas?