This attribute determines if this stp instance is to act as the stp instance
for the backbone or an stp instance for a non-backbone media
configuration. There can exist only one Backbone stp instance per
virtual router. If the attribute domainStpTbState is set to a value on and
Backbone bridge ports have been provisioned, there should exist a
Backbone stp instance. Please note that it is not permissible to assign the
instance identifier of this stp instance as domain number of any access
bridge port. In case of stp instances that are non-Backbone, the instance
identifier is indicative of the domain number the stp is operational in.