[2.0a3] Can't commission too many machines at a time

Trying to commission 40 machines at a time, I saw a weird issue. The machines are unable to PXE boot and hence, they can't commission. The output that's shown in the console log ends with:

CLIENT MAC ADDR: 74 D4 35 89 BC 86 GUID: 86BC8935 D474 8001 E311 EEA180BAF14E
PXE-E53: No boot filename received

PXE-M0F: Exiting Intel Boot Agent.

Basically, it seems that the machine has successfully DHCP'd , but was unable to get PXE information. This may be related to:

2016-03-19 12:08:15+0800 [ClusterClient,client] RemoteOriginReadSession starting on 52088
2016-03-19 12:08:15+0800 [ClusterClient,client] Starting protocol <tftp.bootstrap.RemoteOriginReadSession object at 0x7fe30c170588>
2016-03-19 12:08:17+0800 [TFTP (UDP)] Datagram received from ('192.168.10.34', 2070): <RRQDatagram(filename=b'pxelinux.0', mode=b'octet', options=OrderedDict([(b'tsize', b'0')]))>
2016-03-19 12:08:17+0800 [-] RemoteOriginReadSession starting on 44086