So, I am getting deep into my CCIE study and I just read about the TX-ring-limit interface command. I have used QoS a bit to oversubscribe software buffers to prevent frame loss on switches and whatnot, can you use the "TX-ring-limit" in a similar manner to decrease the likelihood of frame loss during bursty traffic on a LAN?

Additionally I understand the following:
1) TX-ring-limit should be set to either a value of 1 or 2 for a low speed interface such as a T1 serial
2) Higher values can be configured on higher speed interfaces
3) Setting a high value can introduce jitter on a link because the software queue of a IOS device will send the packets to the hardware queue where they are simply FIFO

Has anyone experimented with setting this to a higher value on ethernet LAN interfaces to allow the hardware queue to "hold on to frames" during a bursty traffic period, rather than drop them?

  • 4
    Speaking personally, I hope the site doesn't embrace dedicated certification tags like ccie. I took the liberty of editing, but if you feel strongly about it, let's raise a question in Network Engineering Meta
    – This
    Dec 18 '13 at 15:47
  • I would agree with not having certification tags, if we allow class questions it should not matter that someone is rasing a question because of a cert they are studying for
    – fredpbaker
    Dec 18 '13 at 21:46
  • +1, no tag required, it's not relivant
    – jwbensley
    Dec 19 '13 at 11:39
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Dec 20 '20 at 18:35

The TX-Ring is the hardware interface queue and is FIFO. QOS does NOT begin to operate in a router until the TX-Ring is full. The tradeoff is a small TX-Ring causes QoS to cut in quicker (good when you want to use LLQ for VoIP) at the cost of CPU, a large TX-Ring uses less cpu (don't need to service the queue as much) but will increase jitter (remember that the queue is Fifo) for voip applications. This is an important VoIP tuning knob on the older cisco routers for the above reasons.

It does not impact drops because TX-Ring only exists in routers and when the tx ring is full traffic is queued in router memory.

  • "It does not impact drops because TX-Ring only exists in routers and when the tx ring is full traffic is queued in router memory." The ability to set the tx-ring exists on switches as well. Do you have any input regarding this tuning on the LAN? Dec 19 '13 at 14:37
  • I would not as concerned about LAN tuning, on a 256Kb lan connection a Fifo queue would play havoc on VoIP latency and jitter, at LAN speeds that effect is a lot less pronounced. Most switches that I am aware have multiple hardware queues that are serviced in a round robin fashion the deal with TX-Ring is that it is the only queue
    – fredpbaker
    Dec 19 '13 at 16:15

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