The MAC address used by this bridge when it must
be referred to in a unique fashion. It is
recommended that this be the numerically smallest
MAC address of all ports that belong to this
bridge. However it is only required to be unique.
When concatenated with dot1sCistBridgePriority or
dot1sMstiBridgePriority a unique BridgeIdentifier
is formed which is used in the Spanning Tree Protocol.