Dell PowerConnect 8100 CLI Reference Guide - Page 931

42 Priority Flow Control Commands NOTE: Fiber Channel over Ethernet (FCoE) commands are only supported on the PC8024 and PC8024F switches. CLI commands and Dell OpenManage Switch Administrator pages are not available for other switch models. Priority Flow Control (PFC) provides a means of pausing frames based on individual priorities on a single physical link. By pausing the congested priority or priorities independently, protocols that are highly loss sensitive can share the same link with traffic that has different loss tolerances with less congestion spreading than standard flow control. The priorities are differentiated by the priority field of the 802.1Q VLAN header. PFC is standardized by the IEEE 802.1Qbb specification. PFC uses a new control packet defined in 802.1Qbb and therefore is not compatible with standard flow control. An interface that is configured for PFC will be automatically disabled for 802.3x flow control. When PFC is disabled on an interface, the flow control configuration for the interface becomes active. Any flow control frames received on a PFC configured interface are ignored. Each priority is configured as either drop or no-drop. If a priority that is designated as no-drop is congested, the priority is paused. Drop priorities do not participate in pause. By default there are no priority classifications configured and PFC is not enabled. While several no-drop priorities may be configured on a supporting system, the actual number of lossless priorities supported on a given system is a function of the switch chips packet buffer, the maximum supported MTU size, pause delay, the media type and the total number of ports enabled for lossless behavior. In order to guarantee lossless behavior, the switch chip must send a pause message prior to exhausting its available packet buffer and have sufficient buffer to absorb the delay. In order to accomplish this, it must reserve enough memory (headroom) to handle the max delay in processing the pause packet. The maximum number of lossless priorities per interface is two. The headroom is only used for guaranteeing lossless behavior. There must be enough dynamic memory to handle the typical work load of the switch in Priority Flow Control Commands 931