This counts cells discarded on this port due to buffer overflow specifically within the traffic-shaping (VP shaping) process, rather than general congestion elsewhere on the port. A rising count here on this specific port points directly at the traffic shaper's buffer being undersized or its configured rate, see pportTrafficShaperPeak and pportTrafficShaperCredit, being too aggressive for the offered load, distinguishing this from a general egress congestion problem like pportOutDiscardsCell. A shaper configured on this port with a low pportTrafficShaperCredit value that then shows climbing pportVpshapingDiscardCellCount during bursty traffic would suggest raising that credit value to better absorb bursts without discarding cells.