Indicates the mode of operation of this bridge. The
following operation modes are possible:
customerBridge(1) - Indicates the bridge is a MAC
Bridge as specified by IEEE Std 802.1D or a VLAN
Bridge as specified by IEEE Std 802.1Q.
providerBridge(2) - Indicates the bridge is an
S-VLAN bridge or Provider Edge Bridge.
providerBackboneBridge(3) - Indicates the bridge is a
Backbone Core Bridge or a Backbone Edge Bridge.