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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

217

3.24.4.3 Weighted Random Early Discard (WRED) Sharing

In this mode, the shared resource accounts close at random. A high consumption gives a high probability

of discarding a frame. This sharing method only operates on the egress memory consumption.

There are three WRED groups, each having a number of ports assigned to it. Each WRED group

contains 24 WRED profiles; one per QoS class and per DP = 1, 2, 3. Each WRED profile defines a

threshold for drop probability 0% and a threshold for drop probability 100%. Frames with DP = 0 value

are considered green and have no WRED profile. Green frames should always be allowed.

Memory consumptions within a WRED group are tracked per QoS class, but across DP levels, however,

green frames affect the drop probability of yellow frames.

The WRED groups with each eight QoS classes are shown in the following illustration. It is also possible

to use fewer groups or different sizes for each QoS classes.

Figure 59 •

WRED Sharing

The following illustration shows an example of the WRED profiles for a QoS class within a WRED group.

Thresholds are shown for DP = 2 only.

Figure 60 •

WRED Profiles

The WRED sharing is operating as an egress algorithm. The following table shows the configuration

setting.

QoS reservation

(QRES::RES_QOS_MODE)

Enable for all.

Table 146 •

WRED Sharing Configuration

Configuration

Setting

Reservation thresholds

These can be set to any desired value, except for ingress per port reservation. The

algorithm requires these to be zeroed.

Table 145 •

Per Priority Sharing Configuration Settings (continued)

Configuration

Setting

DP = 0 limit

WRED Group 0

WRED Group 1

WRED Group 2

d res

ource

WRED group

consumption for

QoS class

Drop probability

DP=

100%

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