We've got a configuration where some media servers are on isolated networks
and cannot talk to each other.
Though the NetBackup master server and robotic control server have multiple
network connections and are able to communicate to all media servers. The
master acts as the volume database host.
Scan host by design is not restricted to run on a certain media server only
and can change to any media server.
It can be priotized by using MH_SCAN_ABILITY = scan_factor variable, but it
can not be disabled.

1. I wonder if there is a way to run scan host on the master server only ( I
guess one way is to run vmdareq as a cron job).
2.  What's happen if the scan host functionality moves to a server which is
not 'seen' by other media server?
 Will the master / volume database server down the drive or move scan host
to a different server? 

Regards,

Glen Vassileff