auto(1). If the device does not implement the spanning-tree
protocol, or if exactly two interfaces are connected to the
forwarding function, then it will act as if `forward(2)' has
been specified. Otherwise, it will act as if `process(4)' has
been specified.
forward(2). Received spanning-tree messages are broadcasted
to all interfaces except the interface where the message was
received. No further processing is done.
drop(3). Received spanning-tree messages are dropped without
any further processing.
process(4). Received spanning-tree messages are processed
according to the spanning-tree protocol.