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Functional Descriptions

VMDS-10488 VSC7511 Datasheet Revision 4.2

122

3.11.1

Buffer Management

A number of watermarks control how much data can be pending in the egress queues before the

resources are depleted. There are no watermarks for the ingress queues, except for flow control,

because the ingress queues are empty most of the time due to the fast transfer rates from ingress to

egress. For more information, see Ingress Pause Request Generation, page 126. When the watermarks

are configured properly, congested traffic does not influence the forwarding of non-congested traffic.

The memory is split into two main areas:

•

A reserved memory area. The reserved memory area is subdivided into areas per port per QoS

class per direction (ingress/egress).

•

A shared memory area, which is shared by all traffic.

For setting up the reserved areas, egress watermarks exist per port and per QoS class for both ingress

and egress. The following table lists the reservation watermarks.

All the watermarks, including the ingress watermarks, are compared against the memory consumptions

in the egress queues. For example, the ingress watermarks in BUF_Q_RSRV_I compare against the

total consumption of frames across all egress queues received on the specific ingress port and classified

to the specific QoS class. The ingress watermarks in BUF_P_RSRV_I compare against the total

consumption of all frames across all egress queues received on the specific ingress port.

The reserved areas are guaranteed minimum areas. A frame cannot be discarded or held back in the

ingress queues if the frame’s reserved areas are not yet used.

The shared memory area is the area left when all the reservations are taken out. The shared memory

area is shared between all ports, however, it is possible to configure a set of watermarks per QoS class

and per drop precedence level (green/yellow) to stop some traffic flows before others. The following table

lists the sharing watermarks.

Table 96 •

Reservation Watermarks

Description

Replication

BUF_Q_RSRV_E Configures the reserved amount of egress buffer per QoS

class per egress port.

Per QoS class per

egress port

BUF_P_RSRV_E Configures the reserved amount of egress buffer shared

among the egress port’s allocated egress queues.

Per egress port

BUF_Q_RSRV_I Configures the reserved amount of egress buffer per

ingress port per QoS class across all egress ports.

Per ingress port per

QoS class

BUF_P_RSRV_I

Configures the reserved amount of egress buffer per

ingress port shared among the eight QoS classes.

Per ingress port

Table 97 •

Sharing Watermarks

Description

Replication

BUF_PRIO_SHR_E Configures how much of the shared memory area that egress

frames with the given QoS class are allowed to use.

Per QoS class

BUF_COL_SHR_E

Configures how much of the shared memory area that egress

frames with the given drop precedence level are allowed to

use.

Per drop

precedence

level

BUF_PRIO_SHR_I

Configures how much of the shared memory area that

ingress frames with the given QoS class are allowed to use.

Per QoS class

BUF_COL_SHR_I

Configures how much of the shared memory area that

ingress frames with the given drop precedence level are

allowed to use.

Per drop

precedence

level

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