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Does tuning the tx-ring limit make sense when outbound shaper is configured


I'm facing the following issue:

Cisco 891 <---1Gb,fulld,ethern---> Cable modem <---10 mbps upload ---> ISP

I have a policymap with a shaper and a child policy map configured like this on the wan interface of the 891 (Giga0):

  Policy Map QOS_WANOUT_PM

    Class class-default

      Average Rate Traffic Shaping

      cir 9400000 (bps)

      service-policy QOS_WANOUT_CHILD_PM

So my CIR is shaped to 9,4 mbps.

Does adjusting the tx-ring limit to a lowed value than 256 (default) makes sense in a situation like this? My guess is that its not since the tx-ring will never get full with the shaper attached.

Anyone any idea's?


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Re:Does tuning the tx-ring limit make sense when outbound shaper

I could be wrong, but I have a feeling the hardware queue size is determined by interface speed. So let's say you decrease the hardware queue size to 5 this would cause more traffic to be put in memory which wouldn't be optimal. Once again I don't know the answer, so I'm taking a guess.

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Re: Does tuning the tx-ring limit make sense when outbound shape


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It depends on you Bc and Be.  (In your case, as you're using shape average, Be shouldn't apply.)

The shaper allows ingress to egress traffic at line rate and/or might dequeue at its maximum PPS rate.  A burst that's "fits" within Bc (or if shape peek Bc+Be) could queue in the tx-ring.

Considering the performance capacity of an 891 and assuming you don't have LAN ingress bandwidth that oversubscribes the egress gig, I doubt there would be the need for deep tx-ring.

BTW, some of the later IOSs are supposed to minimize tx-ring when a service policy is applied.  (This to minimize FIFO tx-ring queuing, before packets are placed into an interface policy [without shaper].)  Are you sure tx-ring size is still 256?

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