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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

215

For PFC purposes, the congestion controller contains an additional accounting sheet of the ingress

memory consumption. The sheet has its own set of thresholds. The result of the accounting is whether to

send PFC pause frames towards the link partner.

The combined rule for allowing a frame copy is if both of the following are true:

•

Either ingress or egress memory evaluation must allow the frame copy.

•

Either the ingress or egress reference evaluation must allow the frame copy.

When the reserved accounts are closed, the shared accounts spanning multiple ports are checked.

There are various rules for regarding these accounts as open or closed. These rules are explained in the

following through three major resource modes, which the congestion controller is intended to be used in.

Figure 56 •

Reserved and Shared Resource Overview

The resource modes are defined by the way the shared resources are distributed between classes of

frames. Note, that there are many options for configuring a different resource mode than the modes

described in the following. This is outside the scope of this document.

3.24.4.1 Strict Priority Sharing

This resource mode considers higher QoS classes as more important than lower QoS classes. The

shared area is one big pool shared between all QoS classes. Eight thresholds define the limit for each of

the eight QoS classes. If the total consumption by all QoS classes exceeds one of the thresholds, the

account associated with the particular QoS class is closed.

Figure 57 •

Strict Priority Sharing

prio 0

prio 1

prio 6

prio 5

prio 4

prio 3

prio 2

prio 7

prio 0

prio 1

prio 6

prio 5

prio 4

prio 3

prio 2

prio 7

prio 0

prio 1

prio 6

prio 5

prio 4

prio 3

prio 2

prio 7

prio 0

prio 1

prio 6

prio 5

prio 4

prio 3

prio 2

prio 7

Packet Memory

Egress

Reservation

Ingress

Reservation

Port 1

Port 0

Port 1

Port 0

are

d R

eso

urc

QoS class 0 limit

QoS class 7 limit

QoS class 6 limit

QoS class 5 limit

QoS class 4 limit

QoS class 3 limit

QoS class 2 limit

QoS class 1 limit

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