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Dynamic ingress buffering enables the sending of pause frames at different thresholds based on the number of ports that
experience congestion at a time. This behavior impacts the total buffer size used by a particular lossless priority on an interface.
The pause and resume thresholds can also be configured dynamically. You can configure a buffer size, pause threshold, ingress
shared threshold weight, and resume threshold to control and manage the total amount of buffers that are to be used in your
network environment.
Buffer Sizes for Lossless or PFC Packets
You can configure up to a maximum of 4 lossless (PFC) queues. By configuring 4 lossless queues, you can configure 4 different
priorities and assign a particular priority to each application that your network is used to process. For example, you can assign
a higher priority for time-sensitive applications and a lower priority for other services, such as file transfers. You can configure
the amount of buffer space to be allocated for each priority and the pause or resume thresholds for the buffer. This method of
configuration enables you to effectively manage and administer the behavior of lossless queues.
Although the system contains 9 MB 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 7,787 KB. 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 generates 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 20 – 25, 30 – 35
Expected PFC
Priority
1 2
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