This blog posts examines the two different queues on a Cisco router interface which are key to understanding QoS behaviors. One of the points that many Cisco QoS texts fail to make clear is that a typical Cisco network device interface possess a software queue and a hardware queue for outbound traffic transmissions. When we read that a particular QoS tool will engage and work in times of congestion only, this is a reference to packets overflowing the hardware queue into the software queue. The software queue is where the magic of Quality of Service happens. In fact, one of the QoS mechanisms called Link Fragmentation and Interleaving (LFI) directly addresses the split between these two queues. LFI ensures that large packets (Jumbograms) do not clog the hardware queue. These large packets are chopped up (fragmented) so that higher priority, smaller packets (like voice) can be interwoven between the fragments.  In the interest of sounding really clever, the hardware queue is typically referred to as the Transmit Ring or TX-Ring. I forever refer to it as simply HQ for the Hardware Queue.