This 16-bit counter tracks how many times, since the last reset, a frame transmission on this port could not complete because not all the data had been supplied to the port hardware in time. Unlike an overrun (a receive-side CPU bottleneck), an underrun is a transmit-side timing problem, often pointing to the source feeding this port rather than the port itself. For example, underrun errors appearing on a port feeding a slow upstream data source would suggest the source, not this node, is failing to keep pace with the port's transmit clock.