2960S Drops and QoS

[arthurlove]

We are upgrading a customer network and have found that users of a particular application [which is very bursty/bandwidth hungry] have experienced a marked performance drop. I see lots of output drops on the interfaces. This is only happening on 2960S-48 the rest of the users on 2960PSTL [all PoE] are not having nearly as bad a time.

I have tried various QoS configs with little success. I have seen on some other blogs that the 2960S has less buffers than the rest of the family. An idea of this is true?

Removing QoS or upping the users to 1Gb cures the performance problem.

TIA

[Leo Laohoo]

What kind of drops?  Input or Output drops?

Where is this application residing?  On a personal PC or a server?

[arthurlove]

Output Drops the App is on a server on the same switch [the 2960S] the users are all @ 100Mb. Users on another switch linked @ 1Gb to the 2960S are not having this issue. the qos on both switches is the same. We have serveral sites with this topology and to the man they are having the same problem. Removing qos or uping the users on the 2960S and the problem goes away. Removing qos is not an option at this stage.

[paolo bevilacqua]

 Removing qos is not an option at this stage.

But it should, because you gain absolutely nothing (except drops) from having it.

[Leo Laohoo]

Output Drops the App is on a server on the same switch [the 2960S] the users are all @ 100Mb.

Post the output to the following commands:

1.  sh interface ;

2.  sh contr e ; and

3.  sh contr u

[paolo bevilacqua]

Yes, it is know that QoS configurations causes drops on switches. Do not use.

[Joseph W. Doherty · Accepted solution]

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Posting

I believe the 2960 series divides up hardware buffering resources when QoS is enabled.  If all traffic remains in just one queue, with its hardware buffering resource reduced, it often encounters drops that were not encountered before.

If you treat all traffic alike, there's no need to enable QoS, and because of what I've just described, enabling QoS can be detrimental.