NetBackup servers must use a host name that is compliant with RFC 1123 and RFC 952
Starting with NetBackup 8.0, all NetBackup server names must use a host name that is complaint with RFC 1123 ("Requirements for Internet Hosts - Application and Support") and RFC 952 ("DOD Internet Host Table Specification") standards. These standards include the supported and unsupported characters that can be used in a host name. For example, the underscore character ( _ ) is not a supported character for host names.
More information is available about these standards and about this issue: