VSC7511 datasheet(35/324 Pages) MICROSEMI

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Functional Descriptions

VMDS-10488 VSC7511 Datasheet Revision 4.2

(MAC_FC_CFG.FC_LATENCY_CFG). The benefit of this approach is that the queue system is not

risking being overloaded with frames due to a non-complying link partner.

In half-duplex mode, the MAC does not react to received pause frames. If the flow control condition is

asserted by the ingress queues, the industry-standard backpressure mechanism is used. Together with

the ability to retransmit frames after excessive collisions

(MAC_HDX_CFG.RETRY_AFTER_EXC_COL_ENA), this enables non-dropping half-duplex flow

control.

3.2.1.6

Priority-Based Flow Control (IEEE 802.1Qbb)

The device supports priority-based flow control on all ports for all QoS classes. The following table lists

the specific registers associated with priority-based flow control.

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

pause frames, which allows asymmetric flow control configurations.

The device obeys received pause frames per priority (ANA::PFC_CFG.RX_PFC_ENA) by pausing the

egress traffic according to the timer values specified in the pause frames. Transmission of frames

belonging to QoS class n is paused if bit n is set in the priority_enable_vector in the incoming pause

frame. The pause time for QoS class n is given by time[n] from the pause frame. The link speed must be

specified in order to evaluate the pause times (ANA::PFC_CFG.FC_LINK_SPEED).

The transmission of priority-based pause frames is triggered by assertion of a flow control condition in

the ingress queues caused by the memory consumption for a priority for an ingress port exceeding the

BUF_Q_RSRV_I watermark. For more information about the watermark, see Table 96, page 122. The

MAC handles the formatting and transmission of the priority-based pause frame. The following

configuration options are available:

•

Transmission of priority-based pause frames per QoS class

(QSYS::SWITCH_PORT-MODE.TX_PFC_ENA).

•

Pause timer value used in transmitted priority-based pause frames

(SYS::MAC_FC_CFG.PAUSE_VAL_CFG). All congested priorities use the same pause timer value.

Uncongested priorities use pause timer value 0.

•

Source MAC address used in transmitted priority-based pause frames (MAC_FC_MAC_HIGH_CFG,

MAC_FC_MAC_LOW_CFG).

•

Priority protection mode (QSYS::SWITCH_PORT_MODE.TX_PFC_MODE), which enables that

when a priority congests and causes a pause frame to be sent, then the pause frame will also pause

all lower priorities.

All transmitted priority-based pause frames have the priority_enable_vector set to 0xFF, independently of

whether a priority is enabled for flow control. However, the pause timer value, time[n], is always 0 for

disabled priorities.

Table 7 •

Priority-Based Flow Control Configuration Registers

Description

Replication

SYS::MAC_FC_CFG

Flow control configuration

Per port

DEV::MAC_FC_MAC_LOW_CFG

LSB of SMAC used in pause frames

Per port

DEV::MAC_FC_MAC_HIGH_CFG

MSB of SMAC used in pause frames

Per port

ANA::PFC_CFG

Configuration of Rx priority-based flow

control per priority.

Per port

QSYS:SWITCH_PORT_MODE

Configuration of Tx priority-based flow

control per priority

Per port

DEV::PORT_MISC.FWD_CTRL_ENA Enabling forwarding of priority-based pause

frames to analyzer.

Per port

ANA::CPU_FWD_BPDU_CFG

Disable forwarding of priority-based pause

frames beyond the analyzer

Per port

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