TX-ring limit PA-A3 and drop rate

Unanswered Question


I'm trying to adjust the TX-ring limit on a vbr-nrt vc 20Mbit/s. We are using fancy queuing with RED enabled in the default class. We also run a voice class so we want to keep jitter and and delay to a minimum.

When I use a TX-ring limit of 12 for example, the drop rate in the class-default is somewhere around 30-200kbit/s, is this normal ?

I've been trying to find a best practice for the TX-ring setting but the Cisco documentation just says "small to keep delay down, but large enough not to affect throughput".

Is a TX-ring limit of 12 to low for a VC at this speed ?

I have this problem too.
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Paolo Bevilacqua Thu, 07/19/2007 - 02:30


you should not set TX limit unless directed by the TAC or as corrective action to a know problem. In particular, every time you have QoS in form of LLQ, the TX limit should automatically be set to 1, as this will allow IOS software to have best control on queueing on the interface or VC. Any higher value would in fact defeat the purpose of a priority queue.

Hope this helps,please rate post if it does!

Okay - why we had it there in first place was to correct for very high delays we faced when we did a Mainframe data migration with a very aggressive datamoving application that caused high delays.

Can I somehow see that it's adjusted to 1 ?

Looking at the interface it seems selfadjusting to 768 when no ring limit is configured ?

sh atm pvc 2/81 :

VC TxRingLimit: 786 particles

Paolo Bevilacqua Thu, 07/19/2007 - 03:15

That is very much an pragmatic choice. Now, in the past the rule was that QoS would set appropriate TX limit automatically. Perhaps you have a PA-A6 and not PA-A3+?


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