Dell S4048T, Руководство по настройке

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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 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.
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