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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

212

frames/(8 × (64 + 20) ns) = 1,523 million frames per second. The forwarder can at maximum process

156 million frames per second. As a consequence, the ingress queues fill up and tail dropping is

inevitable.

The highlights of the modes are listed in the following table.

3.24.3.1 Forward Pressure

The worst-case switching capacity in the frame forwarder is 78 million frames per second. This

corresponds to 52 Gbps line rate with minimum-sized frames.

However, if a large share of these frames have multiple destinations, and are all close to the 64 byte

minimum frame size, the processing speed is insufficient to process the destination copy one at a time.

The consequence is that the ingress queues start to fill. The ingress queues contain both high and low

priority traffic, unicast, and multicast frames. To avoid head-of-line blocking effects from the ingress

queues filling up and eventually leading to tail dropping, a forward pressure system is in place. It can

discard frame copies based on the size of the ingress queues. Forward pressure is configured in the

QSYS::FWD_PRESSURE registers, where each port is allowed only a certain number of copies per

frame if the number of pending transmit-requests in the port’s ingress queue exceeds a threshold.

The following table lists the registers associated with configuring forward pressure.

3.24.3.2 Destination Selection

The forwarder adds references to the egress queues by evaluating the destination selection information

shown in the following table from the analyzer.

The destination set selects specific egress ports, and the CPUQ, mirror, and learn-all fields are indirect

frame destinations and must be translated to transmit requests through the frame copy configuration

Table 139 •

Statistics Modes

Statistics Mode

Operation

Ingress-focused

A frame switched to N ports and discarded to M ports will update the discard statistics for the

ingress port by one, only if N = 0 and M >0. The forwarder will use N cycles to process the

frame.

Egress-focused

A frame switched to N ports and discarded to M ports will update the discard statistics for the M

egress ports by one. The forwarder will use N+M cycles to process the frame.

Table 140 •

Forward Pressure Configuration Registers Overview

Description

Replication

QSYS::FWD_PRESSURE.FWD_PRESSURE

Configures forwarding pressure limits per port.

Per port

QSYS::MMGT_IQ_STAT

Returns current ingress queue size.

Per port

QFWD::FWD_PRESSURE.FWD_PRESS_DROP

_CNT

Counts number of frame copies discarded due to

this feature. Value is total discards for all ports in

common.

None

Table 141 •

Analyzer Destination Selection

Field

Function

Destination set

Set of ports to receive a normal switched copy.

All ports except the internal extraction ports have a bit in this mask.

CPUQ

Set of CPU queues to receive a copy.

Mirror

Request mirror copy.

Learn all

Request learn copy to stack ports.

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