Buffer Sizes For Lossless Or Pfc Packets; Behavior Of Tagged Packets - Dell S6100 Configuration Manual

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 64 lossless (PFC) queues. By configuring 64 lossless queues, you can configure multiple 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 4 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 3399
KB. This shared buffer can be used for lossy and lossless traffic.
The default behavior causes up to a maximum of 3088 KB to be used for PFC-related traffic. The remaining approximate space of 311 KB
can be used by lossy traffic. You can allocate all the remaining 311 KB 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 45 KB (for 10G) per queue if all the 64 queues are congested. However, modifying the buffer
allocation per queue impacts this default behavior.
The following table lists the PFC buffer space required for one lossless queue corresponding to the port speeds supported by the system:
Table 16. Buffer Space Required
Interface Speed
10G
25G
40G
50G
100G
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).
246 Data Center Bridging (DCB)
PFC Buffer Space required for one Lossless Queue
45 KB
94 KB
94 KB
161 KB
161 KB