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39. What is 802.1Qbb Priority Flow Control (PFC)?


Priority-based flow control (PFC), IEEE standard 802.1Qbb, is a link-level flow control mechanism. The flow control mechanism is similar to that used by IEEE 802.3x Ethernet PAUSE, but it operates on individual priorities. Instead of pausing all traffic on a link, PFC allows you to selectively pause traffic according to its class.

Standard Ethernet does not guarantee that a packet injected into the network will arrive at its intended destination. Reliability is provided by upper-layer protocols. Generally, a network path consists of multiple hops between the source and destination. A problem arises when transmitters send packets faster than receivers can accept them. When receivers run out of available buffer space to hold incoming flows, they silently drop additional incoming packets. This problem is generally resolved by upper-layer protocols that detect the drops and request re-transmission.

Applications that require reliability in Layer 2 must have the flow control that includes feedback from a receiver to a sender regarding buffer availability. Using IEEE 802.3x Ethernet PAUSE control frames, a receiver can generate a MAC control frame and send a PAUSE request to a sender when a specified threshold of receiver buffer has been filled to prevent buffer overflow. Upon receiving a PAUSE request, the sender stops transmission of any new packets until the receiver notifies the sender that it has sufficient buffer space to accept them again. The disadvantage of using Ethernet PAUSE is that it operates on the entire link, which might be carrying multiple traffic flows. Some traffic flows do not need flow control in Layer 2, because they are carrying applications that rely on upper-layer protocols for reliability. PFC enables you to configure Layer 2 flow control selectively for the traffic that requires it, such as Fibre Channel over Ethernet (FCoE) traffic, without impacting other traffic on the link.

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