Indicates this port's role on the corresponding
Spanning Tree instance.
disabled -- this port has no role on this MST instance.
root -- this port has the role of root port on this MST
instance.
designated -- this port has the role of designated
port on this MST instance.
alternate -- this port has the role of alternate port
on this MST instance.
backUp -- this port has the role of backup port on this
MST instance.
master -- this port has the role of master port on
this MST instance.