VSC7511 datasheet(136/324 Pages) MICROSEMI

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Functional Descriptions

VMDS-10488 VSC7511 Datasheet Revision 4.2

121

ANA:PORT:POL_FLOWC. Common for all port policers, POL_HYST.POL_FC_HYST specifies a

hysteresis, which controls when the policer can re-open after having closed.

To improve fairness between small and large frames being policed by the same policer,

POL_HYST.POL_DROP_HYST specifies a hysteresis that controls when the policer can re-open after

being closed. By setting it to a value larger than the maximum transmission unit, it guarantees that when

the policer opens again, all frames have the same chance of being accepted. This setting only applies to

policers working in drop mode.

The current fill level of the dual leaky buckets can be read in POL_PIR_STATE and POL_CIR_STATE.

The unit is 0.5 bits.

3.11

Shared Queue System

The device includes a shared queue system with one ingress queue per ingress port and one egress

queue per QoS class per egress port per ingress port. The queue system has 224 kilobytes of buffer.

Frames are linked into the ingress queues after frame analysis. Each egress port module selected by the

frame analysis receives a link to the frame and stores the link in the appropriate egress queue selected

by mapping various frame properties to a queue number. The transfer from ingress to egress is

extremely efficient with a transfer time of 12.8 ns per frame copy (equivalent to a transfer rate from

ingress to egress of about 50 Gbps for 64-byte frames and about 1 terabit per second (Tbps) for

1518-byte frames). Each egress port module has a scheduler that selects between the egress queues

when transmitting frames.

The following illustration shows the shared queue system.

Figure 36 •

Queue System Overview

Resource depletion can prevent one or more of the frame copies from the ingress queue to the egress

queues. If a frame copy cannot be made due to lack of resources, the ingress port’s flow control mode

determines the behavior as follows:

•

Ingress port is in drop mode: The frame copy is discarded.

•

Ingress port is in flow control mode: The frame is held back in the ingress queue and the frame copy

is made when the congestion clears.

For more information about special configurations of the shared queue system with respect to flow

control, see Ingress Pause Request Generation, page 126.

Egress Transmitter

Egress Port Module (1 per port)

Ingress Queues

(1 per port)

Egress Queues

Frame Analysis

(Find QoS class and

set of destination

ports)

Queue 0.0

Queue 0.1

Queue 95.11

Ingr. Queue 0

Ingr. Queue 1

Ingr. Queue 11

One or multiple copies

or discard

MAC Tx

Rx MAC

VSC7511 Datasheet(PDF) 136 Page - Microsemi Corporation

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