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This issue is specific to Cat9500 High Performance and Cat9600 switches that are configured with egress QoS policy that has Priority Queue configured with policer and is operating in Shared mode. This issue is only seen if there is bursty traffic going out through such a priority queue and spikes in the traffic rate are above the configured rate-limiting value. The symptom of this issue is that the average interface rate is well below the configured rate-limiting value for the priority queue, but there are Total Output Drops incrementing continuously on such an interface. These drops are seen under the QpolicerDrop column in the output of the show platform hardware fed [switch] qos queue stats interface command. The application performance is suffering due to a portion of the traffic stream being dropped by the switch. Applications that implement proper packet pacing [and thus generate rather flat traffic profile as opposed to bursty traffic pattern] do not exhibit such a problem. This issue cannot be confirmed by looking at the CLI outputs alone. Detailed analysis of the problematic data steam is needed. SPAN capture of the incoming traffic [that is enqueued in either PQ1 or PQ2 in Shared mode on the egress interface where drops are seen] is needed. I/O graph needs to be reviewed at 100ms/10ms/1ms granularity to see if there are spikes/bursts in traffic above the configured rate-limiting value.
- Cat9500 High Performance [C9500-24Y4C // C9500-48Y4C // C9500-32QC // C9500-32C] or Cat9600 - egress QoS policy with Priority Queue that has policer configured - Priority Queue is operating in Shared mode - bursty traffic that is subject to policer in Priority Queue
- modify the application that is generating bursty traffic to implement frame pacing and thus eliminate bursts - change the Priority Queue from Shared to Shaped mode which will allow it to absorb higher bursts [might slightly increase latency and/or jitter]
Whether a priority queue is operating in Shared or Shaped mode can be determined by looking at the output of the show platform hardware fed [switch] qos queue config interface command or by simply looking at the QoS configuration. A sample configuration of PQ level 1 in Shared mode: ! class-map CM_DSCP_EF match dscp ef policy-map PM class CM_DSCP_EF priority level 1 police rate percent 20 interface Twe1/0/9 service-policy output PM ! Cat9500H_PE#show platform hardware fed active qos queue config interface Twe1/0/9 Asic:0 Core:1 DATA Port:28 GPN:109 LinkSpeed:0x12 AFD:Disabled FlatAFD:Disabled QoSMap:0 HW Queues: 224 - 231 DrainFast:Disabled PortSoftStart:3 - 31500 BufferSharing:Disabled DTS Hardmax Softmax PortSMin GlblSMin PortStEnd QEnable ----- -------- -------- -------- -------- --------- ------- 0 1 4 600 5 600 0 0 0 0 1 42000 En 1 1 0 0 6 28800 1 900 1 900 1 42000 En 2 1 0 0 0 0 0 0 0 0 1 42000 En 3 1 0 0 0 0 0 0 0 0 1 42000 En 4 1 0 0 0 0 0 0 0 0 1 42000 En 5 1 0 0 0 0 0 0 0 0 1 42000 En 6 1 0 0 0 0 0 0 0 0 1 42000 En 7 1 0 0 0 0 0 0 0 0 1 42000 En Priority Shaped/shared weight shaping_step sharpedWeight -------- ------------- ------ ------------ ------------- 0 1 Shared 50 0 0 <========= PQ1 in Shared mode 1 7 Shared 50 0 0 2 0 Shared 10000 0 0 3 0 Shared 10000 0 0 4 0 Shared 10000 0 0 5 0 Shared 10000 0 0 6 0 Shared 10000 0 0 7 0 Shared 10000 0 0 ! A sample configuration of PQ level 1 in Shaped mode: ! class-map CM_DSCP_EF match dscp ef policy-map PM class CM_DSCP_EF priority level 1 percent 20 interface Twe1/0/9 service-policy output PM ! Cat9500H_PE#show platform hardware fed active qos queue config interface Twe1/0/9 Asic:0 Core:1 DATA Port:28 GPN:109 LinkSpeed:0x12 AFD:Disabled FlatAFD:Disabled QoSMap:0 HW Queues: 224 - 231 DrainFast:Disabled PortSoftStart:3 - 31500 BufferSharing:Disabled DTS Hardmax Softmax PortSMin GlblSMin PortStEnd QEnable ----- -------- -------- -------- -------- --------- ------- 0 1 4 600 5 600 0 0 0 0 1 42000 En 1 1 0 0 6 28800 1 900 1 900 1 42000 En 2 1 0 0 0 0 0 0 0 0 1 42000 En 3 1 0 0 0 0 0 0 0 0 1 42000 En 4 1 0 0 0 0 0 0 0 0 1 42000 En 5 1 0 0 0 0 0 0 0 0 1 42000 En 6 1 0 0 0 0 0 0 0 0 1 42000 En 7 1 0 0 0 0 0 0 0 0 1 42000 En Priority Shaped/shared weight shaping_step sharpedWeight -------- ------------- ------ ------------ ------------- 0 1 Shaped 1275 255 0 <========= PQ1 in Shaped mode 1 7 Shared 50 0 0 2 0 Shared 10000 0 0 3 0 Shared 10000 0 0 4 0 Shared 10000 0 0 5 0 Shared 10000 0 0 6 0 Shared 10000 0 0 7 0 Shared 10000 0 0 !