This read-only value reports the port's live forwarding state under a specific MST instance: disabled, listening, learning, forwarding, blocking, or unknown, mirroring stpPerPortInfoState but per instance. An admin can see the same physical port simultaneously forwarding for one instance's VLANs while blocking for another's, since each instance computes its own tree. For example, a port showing forwarding(4) under instance 1 but blocking(5) under instance 2 confirms that single physical link is carrying instance 1's VLANs while sitting in standby for instance 2's, spreading traffic across the network's redundant paths.