KSZ9563R datasheet(34/226 Pages) MICROCHIP

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KSZ9563R

DS00002419D-page 34

 2017-2018 Microchip Technology Inc.

4.4.4

QUALITY-OF-SERVICE (QOS) PRIORITY SUPPORT

The device provides quality-of-service (QoS) for applications such as VoIP. There are multiple methods for assigning

priority to ingress packets. Depending on the packet prioritization method, the packet priority levels are mapped to the

egress queues for each port. Each port can be configured for 1, 2, and 4 egress queues, which are prioritized. The

default is 1 queue per port.

When configured for 4 priority queues, Queue 3 is the highest priority queue and Queue 0 is the lowest priority. Likewise,

for a 2-queue configuration, Queue 1 is the highest priority queue. If a port is not configured as 2 or 4 queues, then high

priority and low priority packets have equal priority in the single transmit queue.

There is an additional option for every port to select either to always deliver packets from the highest priority queue first,

or use weighted round robin queuing amongst the multiple queues. This is described later in Section 4.4.13, "Schedule

and Rate Limiting".

4.4.4.1

Port-Based Priority

With port-based priority, each ingress port is individually classified as a specific priority level. All packets received at the

high-priority receiving port are marked as high priority and are sent to the high-priority transmit queue if the correspond-

ing transmit queue is split into 2 or 4 queues.

4.4.4.2

IEEE 802.1p-Based Priority

For IEEE 802.1p-based priority, the device examines the ingress packets to determine whether they are tagged. If

tagged, the 3-bit PCP priority field in the VLAN tag is retrieved and used to look up the “priority mapping” value. The

“priority mapping” value is programmable.

Figure 4-4 illustrates how the 802.1p priority field is embedded in the 802.1Q VLAN tag.

4.4.4.3

IEEE 802.1p Priority Field Re-Mapping

This is a QoS feature that allows the device to set the “User Priority Ceiling” at any ingress port. If the ingress packet's

priority field has a higher priority value than the default tag's priority field of the ingress port, the packet's priority field is

replaced with the default tag's priority field.

4.4.4.4

DiffServ (DSCP) Priority (IP)

DiffServ-based priority from the DSCP field in the IP header can be used to determine packet priority. The 6-bit DSCP

value is used as an index to a set of registers which translate the 6-bit DSCP value to a 2-bit value that specifies one of

the 4 (or 2) queues. These registers are fully programmable.

4.4.4.5

ACL Priority

The Access Control List (ACL) Filtering feature can also be used to assign priority to received packets. This is discussed

in Section 4.4.17, "Access Control List (ACL) Filtering".

FIGURE 4-4:

802.P PRIORITY FIELD FORMAT

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