On Alpha Systems, there is no option to boot from a tape nor from a 3.5"
diskette. All Alpha systems are supposed to have access to a CD-ROM drive
for software distribution, either as a local device or attached to an
InfoServer. The CD-ROM boot method provides a "real" OpenVMS environment from
which you can do backups or other operations.
Another option, if you have room on a spare data disk, is to install OpenVMS
with minimal options to a disk and boot that.