Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.0P1 - Page 254

By default the total available buffer for PFC is 6.6 MB and when you configure dynamic ingress buffering, a minimum of least 52 KB per queue is used when all ports are congested. By default, the system enables a maximum of two lossless queues on the S4820T platform. This default behavior is impacted if you modify the total buffer available for PFC or assign static buffer configurations to the individual PFC queues. Behavior of Tagged Packets The below is example for enabling PFC for priority 2 for tagged packets. Priority (Packet Dot1p) 2 will be mapped to PG6 on PRIO2PG setting. All other Priorities for which PFC is not enabled are mapped to default PG - PG7. Classification rules on ingress (Ingress FP CAM region) matches incoming packet-dot1p and assigns an internal priority (to select queue as per Table 1 and Table 2). The internal Priority assigned for the packet by Ingress FP is used by the memory management unit (MMU) to assign the packet to right queue by indexing the internal-priority to queue map table (TABLE 1) in hardware. PRIO2COS setting for honoring the PFC protocol packets from the Peer switches is as per above Packet-Dot1p->queue table (Table 2). The packets come in with packet-dot1p 2 alone are assign to PG6 on ingress. The packets come in with packet-dot1p 2 alone use Q1 (as per dot1p to Queue classification - Table 2) on the egress port. • When Peer sends a PFC message for Priority 2, based on above PRIO2COS table (TABLE 2), Queue 1 is halted. • Queue 1 starts buffering the packets with Dot1p 2. This causes PG6 buffer counter to increase on the ingress, since P-dot1p 2 is mapped to PG6. • As the PG6 watermark threshold is reached, PFC generats for dot1p 2. Configuration Example for DSCP and PFC Priorities Consider a scenario in which the following DSCP and PFC priorities are necessary: DSCP Expected PFC Priority 0 - 5, 10 - 15 20 - 25, 30 - 35 1 2 To configure the aforementioned DSCP and PFC priority values, perform the following tasks: 1 Create class-maps to group the DSCP subsets class-map match-any dscp-pfc-1 match ip dscp 0-5,10-15 ! class-map match-any dscp-pfc-2 match ip dscp 20-25,30-35 2 Associate above class-maps to Queues Queue assignment as below. NOTE: Although, each port on the S4810, S4820T, and S5000 devices support 8 QoS queues, you can configure only 4 QoS queues (0-3)to manage data traffic. The remaining 4 queues (4-7) are reserved for control traffic. Table 17. Queue Assignments Internal- 0 1 2 3 4 5 6 7 priority Queue 0 0 0 1 2 3 3 3 254 Data Center Bridging (DCB)