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Functional Descriptions

VMDS-10488 VSC7511 Datasheet Revision 4.2

3.7.3

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 a significant amount of 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 algorithm.

The result of the basic classification are the following frame properties, which follow the frame through

the switch:

•

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 frame’s DP level. This level is encoded in a 1-bit field, where frames with DP = 1 have the

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

level is used by the dual leaky bucket 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 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 classifier looks for the following fields in the incoming frame to determine the QoS, DP, and DSCP

classification:

•

Port default QoS class and DP level. The default DSCP value is the frame’s DSCP value. For non-IP

frames, the DSCP is 0 and it is not used elsewhere in the switch.

•

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_CFG.VLAN_INNER_TAG_ENA). Both S-tagged

and C-tagged frames are considered.

•

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_CFG.VLAN_INNER_TAG_ENA). Both S-tagged

and C-tagged frames are considered.

•

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

up to two VLAN tags.

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

Table 28 •

QoS, DP, and DSCP Classification Registers

Description

Replication

ANA.PORT.QOS_CFG

Configuration of the overall classification

flow for QoS, DP, and DSCP.

Per port

ANA:PORT:QOS_PCP_DEI_MAP_CFG Mapping from (DEI, PCP) to (DP, QoS).

Per port per

DEI per PCP

ANA::DSCP_CFG

DSCP configuration per DSCP value.

Per DSCP

ANA::DSCP_REWR_CFG

DSCP rewrite values per DP level and QoS

class.

Per DP and

per QoS

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