In STP, a link fault is the loss of connectivity on a port directly
connected to the switch. A link fault will occur on a switch root port. Loss of
connectivity is determined by the absence of BPDUs on the link providing access
to the root switch.
A link fault can occur as a result of the following:
- The media toward the root switch is physically disconnected
- The media toward the root fails
- The switch port at the other end of the root port fails, is disabled or is
shutdown
- Any event in software or hardware that causes BPDUs not to arrive from the
root switch on the port that a switch considers its root port
In the example, Switch A experiences a link fault. It is no longer
receiving BPDUs over the link to the root switch.