DELL N3000 User Manual

Configuring Data Center Bridging Features

Ordinarily, when flow control is enabled on a physical link, it applies to all 

traffic on the link. When congestion occurs, the hardware sends pause frames 

that temporarily suspend traffic flow to help prevent buffer overflow and 

dropped frames.
PFC provides a means of pausing individual priorities within 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. 
This feature is used in networks where the traffic has differing loss tolerances. 

For example, Fibre Channel traffic is highly sensitive to traffic loss. If a link 

contains both loss-sensitive data and other less loss-sensitive data, the loss-

sensitive data should use a no-drop priority that is enabled for flow control.
Priorities are differentiated by the priority field of the IEEE 802.1Q VLAN 

header, which identifies an IEEE 802.1p priority value. These priority values 

must be mapped to internal class-of-service (CoS) values.
The PFC feature allows you to specify the CoS values that should be paused 

(due to greater loss sensitivity) instead of dropped when congestion occurs on 

a link. Unless configured as no-drop, all CoS priorities are considered non-

pausable (“drop”) when priority-based flow control is enabled until no-drop is 

specifically turned on.

PFC uses a control packet newly defined in IEEE 802.1Qbb and, therefore, is 

not compatible with IEEE 802.3 Annex 31B flow control. An interface that is 

configured for PFC is automatically disabled for flow control. When PFC is 

disabled on an interface, the flow control configuration for the interface 

becomes active. Any IEEE 802.3 Annex 31B link-layer 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. You must configure the same no-drop priorities and 

enable VLAN tagging for the no-drop priorities across the network to ensure 

end-to-end lossless behavior.