When traffic needs to egress from an interface, it is put to an output queue.

When a traffic shaper is shaping egress traffic going out of an interface, it puts the traffic in an output queue also.

So can i say

  1. The only difference between having a shaper or not is just that the shaper controls how much traffic is allow to egress out of the queue/interface?
  2. If an interface outbound queue is full, it doesn't matter if a shaper is in place or not, tail packets will still be drop. - right?

1 Answer 1


A standard hardware queue is FIFO (First In, First Out), and all the traffic going through the interface passes through this one queue.

Based on your previous question, you probably want to shape based on traffic type, giving priority to things like VoIP. This involves setting up multiple software queues, and giving each queue a certain period of time to send to the hardware queue. A priority queue, like for VoIP, will always get first precedence, up to it defined bandwidth, as long as there is something in the priority queue.

Also, tail drop can cause TCP global synchronization (a bad thing). There are mitigations like RED (Cisco WRED) that randomly drop packets in a queue to prevent the queue from filling and causing tail drop.

  • thanks for your reply. so there are actually 2 different kind of queues. where does software queue resides then ? cisco.com/c/en/us/support/docs/quality-of-service-qos/… -> "Buffers packets up to the length of the queue" -> leads me to think it is the hardware queue
    – Noob
    Jul 11, 2017 at 2:02
  • No the hardware queue is part of the interface hardware. Routers uses DRAM for other queues. There may be commands to show this, for instance, Cisco has the show memory commands that show the memory used for I/O. Even with software queues, you have a limit, and you must balance the size to the queues. There are some complex formulae for figuring this, and Cisco routers have a hard limit (varies by model) on how many packets a queue, or the total of all your queues, can hold.
    – Ron Maupin
    Jul 11, 2017 at 2:08
  • i believe the " Output queue: 0/40 (size/max)" in show int is thus showing the hardware queue and its memory is not taken from the dram" ?
    – Noob
    Jul 11, 2017 at 2:55
  • @Noob all ports in a Cisco or other major network company switch or router has dedicated memory per port group per ASIC.
    – user36472
    Jul 11, 2017 at 8:03

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