DELL 8132 ユーザーズマニュアル

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 

2CSPC4.X8100-SWUM102.book  Page 931  Friday, March 15, 2013  8:56 AM