HP recommends
that you use the CLUSTER_CONFIG procedure to define port allocation
classes. If this is not possible (for example, if you are booting
a private system disk into an existing cluster), you can use the SYSBOOT
SET/CLASS command to assign port allocation classes to shared SCSI
ports. For example, if port PKB is connected to a SCSI bus that
another node has assigned port allocation class 152, you would enter
the following
command:
SYSBOOT> SET/CLASS PKB 152
Be sure that the DEVICE_NAMING parameter is set to 1 to enable
new device-naming; for example:
SYSBOOT> SET DEVICE_NAMING 1
To deassign a port allocation class, enter the port name without
a class number; for example:
HP OpenVMS System Manager's Manual, Volume 1:... 
Starting Up and Shutting Down the System
