Administrator Guide

• If priority group 1 or 2 has free bandwidth, (20 + 30)% of the free bandwidth is distributed to priority group 3.
Priority groups 1 and 2 retain whatever free bandwidth remains up to the (20+ 30)%.
Strict-priority
groups:
If two priority groups have strict-priority scheduling, traffic assigned from the priority group with the higher
priority-queue number is scheduled first. However, when three priority groups are used and two groups have
strict-priority scheduling (such as groups 1 and 3 in the example), the strict priority group whose traffic is mapped
to one queue takes precedence over the strict priority group whose traffic is mapped to two queues.
Priority-Based Flow Control Using Dynamic Buffer
Method
Priority-based flow control using dynamic buffer spaces is supported on the switch.
In a data center network, priority-based flow control (PFC) manages large bursts of one traffic type in multiprotocol links so that it does
not affect other traffic types and no frames are lost due to congestion. When PFC detects congestion on a queue for a specified priority,
it sends a pause frame for the 802.1p priority traffic to the transmitting device.
Pause and Resume of Traffic
The pause message is used by the sending device to inform the receiving device about a congested, heavily-loaded traffic state that has
been identified. When the interface of a sending device transmits a pause frame, the recipient acknowledges this frame by temporarily
halting the transmission of data packets. The sending device requests the recipient to restart the transmission of data traffic when the
congestion eases and reduces. The time period that is specified in the pause frame defines the duration for which the flow of data packets
is halted. When the time period elapses, the transmission restarts.
When a device sends a pause frame to another device, the time for which the sending of packets from the other device must be stopped
is contained in the pause frame. The device that sent the pause frame empties the buffer to be less than the threshold value and restarts
the acceptance of data packets.
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 12 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 9.5
MB. 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. By default, the system enables a maximum of 1 lossless queue on the switch.
This default behavior is impacted if you modify the total buffer available for PFC or assign static buffer configurations to the individual
PFC queues.
Data Center Bridging (DCB)
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