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 perqueue is used when all ports are congested.This default behavior is impacted if you modify the total buffer available for PFC or assign static buffer configurations to the individual PFCqueues.Behavior of Tagged PacketsThe below is example for enabling PFC for priority 2 for tagged packets. Priority (Packet Dot1p) 2 will be mapped to PG6 on PRIO2PGsetting. 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 asper 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 rightqueue 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 ismapped to PG6.• As the PG6 watermark threshold is reached, PFC generats for dot1p 2.Configuration Example for DSCP and PFC PrioritiesConsider a scenario in which the following DSCP and PFC priorities are necessary:DSCP 0 – 5, 10 - 15 20 – 25, 30 – 35Expected PFCPriority1 2To configure the aforementioned DSCP and PFC priority values, perform the following tasks:1 Create class-maps to group the DSCP subsetsclass-map match-any dscp-pfc-1match ip dscp 0-5,10-15!class-map match-any dscp-pfc-2match ip dscp 20-25,30-352 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 4QoS queues (0-3)to manage data traffic. The remaining 4 queues (4-7) are reserved for control traffic.Table 19. Queue AssignmentsInternal-priority0 1 2 3 4 5 6 7Queue 0 0 0 1 2 3 3 3Data Center Bridging (DCB) 265
