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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

211

Frames discarded at this stage are discarded without considering the frame’s destination ports. This kind

of discarding is called tail dropping. It is an indication of head-of-line blocking and is not usually desired.

For information about how it can be avoided by properly configuring the congestion control system, see

Congestion Control, page 213.

The pause frame and discard mechanisms are activated when a threshold for the individual port’s

memory use is reached and when a threshold for the total use of memory is reached.

The following table lists the configuration registers involved in the buffer control.

The following table lists available status information.

3.24.3

Forwarding

The frame forwarder processes frames at the head of the per-port ingress queues by adding references

to the egress queues for each of the frames’ destination ports. The references points from the egress

queues to the stored frames. The forwarder can add from 78 million to 156 million references per second.

The ingress queues are selected by a weighted round-robin search, where the weights are configured in

QFWD::SWITCH_PORT_MODE.FWD_URGENCY.

Each frame is processed as either a drop-mode frame or a flow control frame, which influences how the

congestion control works. For a drop-mode frame, the congestion control can discard frame copies while

for a flow control frame, it lets the frame stay at the head of the ingress queue blocking further processing

of frames from the port until the congestion situation is over. A frame is processed as a drop-mode frame

if one of the following conditions is met:

•

The ingress port is configured to be an ingress drop port.

•

The egress port is configured to be an egress drop port.

•

The frame itself is analyzed to be a drop frame.

In terms of discarding frames, the forwarder can work in an ingress-focused statistics mode or an egress-

focused statistics mode. The mode is configured per ingress port in QFWD::STAT_CNT_CFG.

In the ingress-focused mode, a frame can be discarded towards multiple destinations simultaneously.

The discard statistics is only incremented if the frame is discarded towards all destinations.

In the egress-focused mode, the discard statistics is incremented for each copy of the frame being

discarded at the expense of the forwarder only discarding one frame copy per cycle. As a result, the

forwarder can become oversubscribed. For example, if two 10G ports are multicasting 64-byte frames at

wire-speed to ten destinations, the total amount of frame copies per second is 20 × 10

Table 137 •

Buffer Control Registers Overview

Description

Replication

QSYS::ATOP

Maximum allowed memory use for a port before tail dropping is

activated.

Per port

QSYS::ATOP_TOT_CFG

Total memory use before tail dropping can be activated.

None

QSYS::PAUSE_CFG

Thresholds for when to start and stop port flow control based on

memory use by the port.

Per port

QSYS::PAUSE_TOT_CFG

Total memory use before flow control is activated.

None

QFWD::SWITCH_PORT_MODE

Enable drop mode for a port.

Per port

Table 138 •

Buffer Status Registers Overview

Description

Replication

QSYS::MMGT_PORT_USE

Current consumption for a specific port

None

QSYS::MMGT_PORT_VIEW

Selection of port to show usage for

None

QSYS::MMGT

Number of free words in packet memory

None

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