This object records the prior spanning-tree port state (disabled, blocking, listening, learning, forwarding, or broken) of the port referenced by dot1dStpPortState in the same notification. Comparing this previous-state value with the current dot1dStpPortState lets an admin see exactly what transition triggered the trap, such as a port moving from forwarding to broken. For example, seeing forwarding(5) here alongside a current state of broken(6) tells the admin a previously healthy forwarding port has just failed.