The number of the port selected as the backbone segment
port when the corresponding value of a3ComSysBridgeMode is
is expressMode(1). The value of this object can range
from 1 to the corresponding value of a3ComSysBridgePortCount,
however, some bridges may support only a subset of ports
as backbone ports. If the corresponding value of
a3ComSysBridgeMode is not expressMode(1), the value of this
object is ignored.