G'day!
this week I enabled "mls qos" on an 2960cg 8-Port office switch in preparation for a more detailed QOS implementation. However it didn't take long until some users attached to this switch complained about an application no longer working.
Upon further research I realized that disabling mls qos would restore the function of the application. I did a remote span with wireshark to understand what was going on, but have yet to find an explanation. The application in question is sending a roughly 7500 byte UDP packet to a remote server. Of course the network stack on the client pc is fragmenting the big packets in 6 fragments. As soon as mls qos is enabled I am unable to see the last 2 fragments of each UDP packet, which obviously causes the application to fail.
I am aware that by enabling QOS the output queue gets slashed in 4 pieces and increased output drops are to be expected, however the switch is not showing any output drops and there is certainly no congestion at all. Is there any explanation for this behaviour? Is this expected? I tried with IOS 15.2(2) as well as 12.2.(55). Any ideas what to do next to narrow down the problem?
Right now I am thinking about the need for fragmentation reassembly for a correct remarking, but a google search for QoS and fragmentation did not come up with any useful results.
Regards and happy weekend
Pille
