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Put vmlinuz and initrd.img at /tftpboot/xcat/image/x86.
Put rootimg.gz the same place you put the listener.py script (/install/image/x86/installer_files for me)
The installer image uses NFS to mount the image store. In the past, it mounted the management node. It has now been updated to be able to mount any NFS export available on the install network. Two additional items need to be added to the xCAT site table: