What are possible causes for error PXE-E99 during MAAS machine commissioning

When trying to commission a node in MAAS, the following is displayed on the target's screen after boot:

Booting from PXE Device 1: Integrated NIC 1 Port 1 Partition 1
>>Start PXE over IPv4. Station IP address is 10.22.20.191 Server IP address is 10.22.20.48 NBP filename is bootx64.efi NBP filesize is 1169992 Bytes Downloading NBP file... PXE-E99 Unexpected network error.
Boot Failed: PXE Device 1: Integrated NIC 1 Port 1 Partition 1

Any idea what causes this? Nothing notable in the maas logs.

TCP Dump from MAAS:

17:45:03.279587 IP 10.22.20.191.1721 > 10.22.20.48.69: 41 RRQ "bootx64.efi" octet tsize 0 blksize 1468
17:45:03.285344 IP 10.22.20.48.53395 > 10.22.20.191.1721: UDP, length 29
17:45:04.285873 IP 10.22.20.48.53395 > 10.22.20.191.1721: UDP, length 29
17:45:05.286848 IP 10.22.20.48.53395 > 10.22.20.191.1721: UDP, length 29
17:45:06.287664 IP 10.22.20.48.53395 > 10.22.20.191.1721: UDP, length 29
17:45:07.288373 IP 10.22.20.48.53395 > 10.22.20.191.1721: UDP, length 29
17:45:08.289217 IP 10.22.20.48.53395 > 10.22.20.191.1721: UDP, length 29
17:45:08.298572 ARP, Request who-has 10.22.20.191 tell 10.22.20.48, length 28
17:45:09.298576 ARP, Request who-has 10.22.20.191 tell 10.22.20.48, length 28
17:45:10.298575 ARP, Request who-has 10.22.20.191 tell 10.22.20.48, length 28

1 Answer

The MAAS server has two NICs: 1 for management/gui/ipmi (eth0) and 1 for PXE (eth1).

There was an IP address conflict with the MAAS nic used for PXE (eth1) and some other host on the network. The IP conflict did not prevent DHCP assignment but caused the TFTP exchange of bootx64.efi to fail.

This was not initially obvious since MAAS was being accessed successfully through eth0 and DHCP was assigning addresses through eth1.