Addendum

PG:1 TSA:ETS BW:50 PFC:ON

Priorities:3

Commands

dcb-map — creates a DCB map to configure PFC and ETS parameters and applies

the PFC and ETS settings on Ethernet ports.

Priority-Based Flow Control Using Dynamic Buffer

Method

Priority-based flow control using dynamic buffer spaces is supported on the S4810, S4820T, S6000, and

MXL platforms.

In a data center network, priority-based flow control (PFC) manages large bursts of one traffic type in

multiprotocol links so that it does not affect other traffic types and no frames are lost due to congestion.

When PFC detects congestion on a queue for a specified priority, it sends a pause frame for the 802.1p

priority traffic to the transmitting device.

Pause and Resume of Traffic

The pause message is a mechanism that is used by the sending device to inform the receiving device

regarding a congested, heavily-loaded traffic state that has been identified. When the interface of a

sending device transmits a pause frame, the recipient acknowledges this frame by temporarily halting the

transmission of data packets. The sending device requests the recipient to restart the transmission of data

traffic when the congestion eases and reduces. The time period that is specified in the pause frame

defines the duration for which the flow of data packets is halted. When the time period elapses, the

transmission restarts.

When a device sends a pause frame to another device, the time for which the sending of packets from

the other device must be stopped is contained in the pause frame. The device that sent the pause frame

empties the buffer to be less than the threshold value and restarts the acceptance of data packets.

Dynamic ingress buffering enables the sending of pause frames at different thresholds based on the

number of ports that experience congestion at a point in time. This behavior impacts the total buffer size

used by a particular lossless priority on an interface. The pause and resume thresholds can also be

configured dynamically. You can configure a buffer size, pause threshold, ingress shared threshold

weight, and resume threshold to control and manage the total amount of buffers that are to be used in

your network environment.

All the PFC-related settings such as the DCB input and output policies or DCB maps are saved in the DCB

application and the Differentiated Services Manager (DSM) application. All of these configurations can be

modified only for interfaces that are enabled for DCB. The DCB buffer configurations are also saved in the

DCB and DSM databases.

Buffer Sizes for Lossless or PFC Packets

You can configure up to a maximum of 4 lossless (PFC) queues. By configuring 4 lossless queues, you

can configure 4 different priorities and assign a particular priority to each application that your network is

used to process. For example, you can assign a higher priority for time-sensitive applications and a lower

priority for other services, such as file transfers. You can configure the amount of buffer space to be

allocated for each priority and the pause or resume thresholds for the buffer. This method of

configuration enables you to effectively manage and administer the behavior of lossless queues.

Although the system contains 9 MB of space for shared buffers, a minimum guaranteed buffer is provided

to all the internal and external ports in the system for both unicast and multicast traffic. This minimum

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Data Center Bridging (DCB)