Users Guide

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 buer to be less than the threshold value and restarts the
acceptance of data packets.
Dynamic ingress buering enables the sending of pause frames at dierent thresholds based on the number of ports that experience
congestion at a time. This behavior impacts the total buer size used by a particular lossless priority on an interface. The pause and resume
thresholds can also be congured dynamically. You can congure a buer size, pause threshold, ingress shared threshold weight, and
resume threshold to control and manage the total amount of buers that are to be used in your network environment.
Buer Sizes for Lossless or PFC Packets
You can congure up to a maximum of 64 lossless (PFC) queues. By conguring 64 lossless queues, you can congure 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 le transfers. You can congure the amount of buer space to be
allocated for each priority and the pause or resume thresholds for the buer. This method of conguration enables you to eectively
manage and administer the behavior of lossless queues.
Although the system contains 4 MB of space for shared buers, a minimum guaranteed buer is provided to all the internal and external
ports in the system for both unicast and multicast trac. This minimum guaranteed buer reduces the total available shared buer to 3399
KB. This shared buer can be used for lossy and lossless trac.
The default behavior causes up to a maximum of 3088 KB to be used for PFC-related trac. The remaining approximate space of 311 KB
can be used by lossy trac. You can allocate all the remaining 311 KB to lossless PFC queues. If you allocate in such a way, the performance
of lossy trac is reduced and degraded. Although you can allocate a maximum buer size, it is used only if a PFC priority is congured and
applied on the interface.
The number of lossless queues supported on the system is dependent on the availability of total buers for PFC. The default conguration
in the system guarantees a minimum of 45 KB (for 10G) per queue if all the 64 queues are congested. However, modifying the buer
allocation per queue impacts this default behavior.
The following table lists the PFC buer space required for one lossless queue corresponding to the port speeds supported by the system:
Table 16.
Buer Space Required
Interface Speed PFC Buer Space required for one Lossless Queue
10G 45 KB
25G 94 KB
40G 94 KB
50G 161 KB
100G 161 KB
This default behavior is impacted if you modify the total buer available for PFC or assign static buer congurations 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.
Classication 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).
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Data Center Bridging (DCB)