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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

107

3.16.1.5 QoS, DP, and DSCP Classification

This section provides information about the functions in the QoS, DP, and DSCP classification. The three

tasks are described as one, because the tasks have functionality in common.

The following table lists the registers associated with QoS, DP, and DSCP classification.

The basic classification provides the user with control of the QoS, DP, and DSCP classification

algorithms. The result of the basic classification are the following frame properties:

•

The frame’s QoS class. This class is encoded in a 3-bit field, where 7 is the highest priority QoS

class and 0 is the lowest priority QoS class. The QoS class is used by the queue system when

enqueuing frames and when evaluating resource consumptions, for policing, statistics, and rewriter

actions. The QoS class is carried internally in the IFH field IFH.VSTAX.QOS.CL_QOS.

•

The frame’s DP level. This level is encoded in a 2-bit field, where frames with DP = 3 have the

highest probability of being dropped and frames with DP = 0 have the lowest probability. The DP

level is used by the MEF compliant policers for measuring committed and peak information rates, for

restricting memory consumptions in the queue system, for collecting statistics, and for rewriting

priority information in the rewriter. The DP level is incremented by the policers if a frame is

exceeding a programmed committed information rate. The DP level is carried internally in the IFH

field IFH.VSTAX.QOS.CL_DP.

•

The frame’s DSCP. This value is encoded in a 6-bit fields. The DSCP value is forwarded with the

frame to the rewriter where it is translated and rewritten into the frame. The DSCP value is only

applicable to IPv4 and IPv6 frames. The DSCP is carried internally in the IFH field IFH.QOS.DSCP.

The classifier looks for the following fields in the incoming frame to determine the QoS, DP, and DSCP

classification:

•

Priority Code Point (PCP) when the frame is VLAN tagged or priority tagged. There is an option to

use the inner tag for double tagged frames (VLAN_CTRL.VLAN_TAG_SEL).

•

Drop Eligible Indicator (DEI) when the frame is VLAN tagged or priority tagged. There is an option to

use the inner tag for double tagged frames (VLAN_CTRL.VLAN_TAG_SEL).

•

DSCP (all 6 bits, both for IPv4 and IPv6 packets). The classifier can look for the DSCP value behind

up to three VLAN tags. For non-IP frames, the DSCP is 0 and it is not used elsewhere in the switch.

•

Destination MAC address (DMAC) for L2CP frames. Layer 2 control protocol frames that are

redirected to the CPU are given a programmable QoS class independently of other frame properties.

The following illustration shows the flow chart of basic QoS classification.

Table 69 •

QoS, DP, and DSCP Classification Registers

Description

Replication

ANA_CL:PORT:QOS_CFG

Configuration of the overall classification flow

for QoS, DP, and DSCP.

Per port

ANA_CL:PORT:PCP_DEI_MAP_CFG

Port-based mapping of (DEI, PCP) to (DP,

QoS).

Per port per DEI per

PCP (16 per port)

ANA_CL::DSCP_CFG

DSCP configuration per DSCP value.

Per DSCP (64)

ANA_CL::QOS_MAP_CFG.DSCP_REWR_VAL

DSCP rewrite values per QoS class.

Per QoS class (8)

ANA_CL::CPU_8021_QOS_CFG

Configuration of QoS class for Layer 2

control protocol frames redirected to the

CPU.

Per Layer 2 protocol

address (32)

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