This attribute specifies the bridge port's configuration. It indicates if the
bridge port is operating as an access port or as a backbone port. An
access bridge port does not encapsulate a MAC frame and does not
preserve domain information in the encapsulation, whereas a backbone
bridge port encapsulates the MAC frame and optionally preserves
domain information in the encapsulated frame.A backbone bridge port
is typically used to transport frames across a high bandwidth backbone
network to another bridge unit for further processing/forwarding of the
frame.