When an interface on a router cannot transmit a packet immediately, the packet is
queued, either in an interface Tx ring, or the interface output hold queue, depending
on the switching path used. Packets are then taken out of the queue and eventually
transmitted on the interface.
If the arrival rate of packets to the output interface exceeds the router’s capability
to buffer and forward traffic, the queues increase to their maximum length and the
interface becomes congested. Tail drop is the router’s default queuing response to
congestion. When the output queue is full and tail drop is in effect, all packets
trying to enter (at the tail...