Reference Guide

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. By default, the system enables
a maximum of two lossless queues on the S4820T platform.
This default behavior is impacted if you modify the total buffer available for PFC or assign static buffer
configurations to the individual PFC queues.
Interworking of DCB Map With DCB Buffer Threshold
Settings
DCB map functionality is supported on the S4820T platform.
The dcb-input and dcb-output configuration commands are deprecated. You must use the dcp-map
command to create a DCB map to configure priority flow control (PFC) and enhanced transmission
selection (ETS) on Ethernet ports that support converged Ethernet traffic.
Configure the dcb-buffer-threshold command and its related parameters only on ports with either
auto configuration or dcb-map configuration. This command is not supported on existing front-panel
interfaces or stack ports that are configured with the dcb-input or dcb-output commands. Similarly, if
the dcb-buffer-threshold configuration is present on a stack port or any interface, the dcb-input or dcb-
ouput policies cannot be applied on those interfaces.
Example: When the dcb-buffer-threshold policy is applied on interfaces or stack ports with the dcb-input
or dcb-output policies, the following error message is displayed:
%Error: dcb-buffer-threshold not supported on interfaces with deprecated
commands
Data Center Bridging (DCB)
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