Dell PowerSwitch S4820T 9.50.1 Configuration Guide for the S4820T System - Page 311

All the PFC-related settings such as the DCB input and output policies or DCB maps are saved in the DCB application and the Differentiated Services Manager (DSM) application. All of these configurations can be modified only for interfaces that are enabled for DCB. The DCB buffer configurations are also saved in the DCB and DSM databases. 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 dcbouput policies cannot be applied on those interfaces. Data Center Bridging (DCB) 311