This counter tracks frames not transmitted or retransmitted due to a Transmit DMA underrun, which the MIB explains happens when the internal transmit FIFO 'runs dry' from lack of PCI bandwidth during a frame's transmission. This is an internal hardware-resource-contention symptom rather than a network-layer problem -- the router's own bus/PCI subsystem couldn't feed data to the Ethernet transmitter fast enough, more likely under simultaneously heavy satellite receive and Ethernet transmit activity competing for the same internal bus. A technician noticing this counter climb during periods when the unit is also processing a high satellite data rate near its capacity ceiling would treat it as evidence the router's internal bus is the bottleneck, not the Ethernet cable or switch.