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BP1037 Transitioning to DCB with EqualLogic PS series Storage Solutions 10
2.3.3 Congestion Notification (CN)
Congestion Notification is a mechanism for managing congestion throughout a DCB fabric or domain.
Ideally, that fabric would consist of interconnected switches and end-devices that all conform to the
same settings for PFC, ETS and CN. Frames in a fabric that are conforming to CN will be “tagged” with
a Flow Identifier. CN then relays messages between two types of devices called Congestion Points
(CPs) and Reaction Points (RP) to control the flows. CPs are generally switches that have the capability
to determine that they are experiencing congestion. Once detected, a CP then sends a CN message to
the originating RP. When an RP receives the CN message, it begins to throttle the output for the
designated flow until the CN messages stop.
This mechanism is a way of moving the congestion from the core of the network to the edge. CN is
generally more effective for longer lived traffic flows, as opposed to small bursts of traffic. For instance
a switch would likely use PFC Pause frames to immediately slow the traffic, but if it determines over
time that the traffic is continuing to flow at a high rate, then it can begin using CN to throttle the
whole thing down.CN can work hand-in-hand with PFC to control congestion and overruns
throughout the DCB fabric.
Figure 3 Congestion notification