Failed to restart TFTP issue

[richard_chen]

I am to an issue when iPXE load undionly.kpxe, it will show "Failed to restart TFTP",
1. bellow is my detail environment:
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1) have about 1000 NoteBook in my network
2) have three DHCP&iPXE server
3) These NB all with USB LAN card

2. issue like bellow info(I don't know how to upload picture as attachment, so...)
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Realtek USB Ethernet controller (xHCI) v2.10AD(11/24/16)
CLIENT MAC ADDR:00 E0 4C 62 C6 26 GUID:44454C4C-3800-1056-8053-C4C04
CLIENT IP:100.110.153.105 MASK:255.255.0.0 DHCP IP:100.110.3.4
Downloaded WDSNBP from 100.110.3.4 WIN-RCLVTQ9LUFK
Press any key to cancel network boot service
Architecture:x64
Contacting Server: 100.110.3.4.............................................
Failed to restart TFTP.
TFTP download failed.
PXE-M0F:Exiting PXE ROM.
No Boot Device Found. Press any key to reboot the machine

3. Condition for issue happend
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1) there lots of NB in network, if there have 100 NB, it can boot normal
2) more than DHCP&iPXE servers, if only have one DHCP&iPXE, it can boot normal
3) only happend with USB Ethernet, if NB with onborad Ethernet will boot normal
4) only happend with iPXE boot, WDS boot normal

I use Wireshark get the Packets, I found NB can get boot\x86\wdsnbp.com by TFTP, but after NB show "Contacting Server: 100.110.3.4............", NB show error like above info, and during NB show "Contacting Server: 100.110.3.4............", there have only two type packets, one is "DHCP request", the other is "DHCP ACK", who can hep me with this issue??

Many thanks,
Richard_Chen