TX-ring limit PA-A3 and drop rate

[danne]

Hi,

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 ?

[paolo bevilacqua]

Hi,

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!

[danne]

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]

Hi, particles are buffers generally sized 576 bytes. Do you have a service-policy attached to the PVC? Which router are you using?

Please look at this document:

http://cisco.com/en/US/tech/tk39/tk824/technologies_tech_note09186a00800fbafc.shtml#topic6

Hope this helps, please rate post if it does!

[danne]

It's a 7206 NPE-G1. And yes there is a service policy attatched to it with one strict priority class, two interactive and default class.

I did study the document, that lead to my first question about a best practice. But it sounds like best practice might be to leave it as default... if no one complains.. ;)

[paolo bevilacqua]

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+?

[danne]

It's a PA-A3+ for sure.. I'll have to do some more investigations / tests.