Behavior Of Tagged Packets; Configuration Example For Dscp And Pfc Priorities - Dell S4048–ON Configuration Manual

Although the system contains of space for shared buffers, a minimum guaranteed buffer is provided to all the internal and external
ports in the system for both unicast and multicast traffic. This minimum guaranteed buffer reduces the total available shared buffer
to . This shared buffer can be used for lossy and lossless traffic.
The default behavior causes up to a maximum of 6.6 MB to be used for PFC-related traffic. The remaining approximate space of 1
MB can be used by lossy traffic. You can allocate all the remaining 1 MB to lossless PFC queues. If you allocate in such a way, the
performance of lossy traffic is reduced and degraded. Although you can allocate a maximum buffer size, it is used only if a PFC
priority is configured and applied on the interface.
The number of lossless queues supported on the system is dependent on the availability of total buffers for PFC. The default
configuration in the system guarantees a minimum of 52 KB per queue if all the 128 queues are congested. However, modifying the
buffer allocation per queue impacts this default behavior.
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.
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 0 – 5, 10 - 15
Expected PFC 1
Priority
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
!
20 – 25, 30 – 35
2
Data Center Bridging (DCB)
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