This Counter32 counts packets a VSL port failed to send locally, for example due to running out of internal buffers or other on-board errors. Because the VSL carries the traffic that keeps the two chassis of a virtual switch synchronized, a rising value here signals internal resource exhaustion on that port rather than a network-side problem. An admin seeing this counter climb might check buffer utilization on the module hosting the VSL port before the link starts dropping inter-chassis traffic.