I recently spent some time investigating an issue with a new Fusion drive Mac Mini (model Macmini7,1) that would not Netboot. Eventually we narrowed it down to my HP 2915-8G switch. The Mac would attempt to use a TFTP package size of >2000 which would cause packets to fragment and eventually the netboot would fail but this didn't occur with other switch models or same switch models with older firmware.
Today one of our network engineers found that HP have posted that there is a problem with DHCP snooping on their HP5500 switches that causes Netboot to fail and it can be fixed by turning off DHCP snooping or by setting the MTU size on the Netboot server to a maximum of 1460. I am not sure if that is possible on NetSUS but we were able to Netboot after turning off DHCP snooping even though my switch is not the model noted in the article.
We are experiencing the exact same behavior only with Palo Altos and their load balancing. My network guys have suggested I change the MTU size to something 1500 or less on the NetBoot Server. Mine is 10.11.6 OS X Server (version 5.x).
This information was very useful in tracking down unreliable NetInstalls with an iMac18,1.
We have Cisco 3750X switches at the edge in the dept with DHCP snooping enabled and the one iMac18,1 Mac was very unreliable when booting from the network. Sometimes it would fail before the tftp stage, be very slow at transferring the booter file or even fail with a no entry sign. Setting --blocksize 1460 in the tftp server config has solved the problem.