Hardware Queue
Logical interfaces, such as subinterfaces and tunnel interfaces, use the hardware
queue of the physical interface; they do not have their own transmit
queues.
The number of packets a TxQ can hold depends on the speed of the interface.
It is automatically determined, and that length is typically fine but can
be tuned if desired. Most devices use the tx ring-limit command for tuning
TxQ size.
Lowering the Tx ring size lessens the length of time a packet waits in the
FIFO queue, and it increases the chance of a packet hitting the software
queue. However, too short of a TxQ results in router resource use, as the
CPU must be interrupted each time the Tx ring requests a packet from a
software queue. A decrease in ring size means an increase in interrupts.
Increasing the TxQ size is usually not recommended when using QoS,
because it decreases the use of the software queue.
Use the show controllers interface command to find information about the
transmit ring size.