VSC7440 datasheet(313/444 Pages) MICROSEMI

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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

296

Figure 95 •

Down-MEP Injection from AFI

3.33

Priority-Based Flow Control (PFC)

Priority-based flow control (PFC, IEEE 802.1Qbb) is supported on all ports for all QoS classes. The

device provides independent support for transmission of pause frames and reaction to incoming pause

frames, which allows asymmetric flow control configurations.

3.33.1

PFC Pause Frame Generation

The queue system monitors the congestion per ingress and egress queue. If a flow control condition is

asserted, the queue system forwards the congestion status to the disassembler. The disassembler

stores the congestion status per (port, priority). Based on the congestion status, the disassembler will

transmit PFC frames for each port.

3.33.1.1 Queue System

The queue system has a set of dedicated PFC watermarks. If the memory consumption of a queue

exceeds a PFC watermark, a flow control condition is asserted, and the updated congestion status is

forwarded the disassembler. For information about the configuration of the PFC watermarks, see Priority-

Based Flow Control (PFC), page 296.

3.33.1.2 Disassembler

The disassembler (DSM) stores the congestion status per queue (per port, per priority) and generates

PFC PDU based on the information. For more information about the available PFC configuration

registers, see PFC Pause Frame Generation, page 274.

The following illustration shows the algorithm controlling the generation of PFC frames for each port. The

8-bit congestion state from the queue system is fc_qs. TX_PAUSE_VAL and PFC_MIN_UPDATE_TIME

are configurable per port. The TX_PAUSE_VAL configuration is shared between LLFC and PFC.

When a priority becomes congested, a PFC PDU is generated to flow control this priority. When half of

the signaled timer value has expired, and the queue is still congested, a new PFC PDU is generated. If

other priorities become congested while the timer is running, then PFC_MIN_UPDATE_TIME is used to

speed up the generation of the next PFC PDU. The timer is shared between all priorities per port.

Every PFC PDU signals the flow control state for all priorities of the port. In other words, the

priority_enable_vector of the PFC PDU is always set to 0x00ff, independent of whether or not a priority is

enabled for flow control. For disabled priorities, the pause time value is always set to 0.

REW

ANA

DEVCPU

ASM

Up-MEP injection point

SQS

DSM

taxi9

taxi0

VD1

port 0

port 52

port m

devcpu

OAM_MEP

Loopback path

Down-MEP point to port

AFI

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