Layer 2 802.1p prioritization

By setting a new 802.1p priority value, QoS allows you to control the priority of outbound packets moving through the switch. The Layer 2 802.1p priority setting in a packet header determines the outbound port queue to which the packet is sent.

By default, the switches covered in this guide have eight outbound traffic queues (0 through 7). A lower-numbered queue has a lower outbound priority; a higher-numbered queue has a higher outbound priority. Packets are transmitted from the switch port on the basis of their queue assignment and whether any higher queues are empty. (To increase bandwidth, you can reconfigure the switch to use four or two outbound queues.)

Configuring a new 802.1p priority value allows you to set the outbound priority queue to which a packet is sent. For example, you can configure an 802.1p priority of 0 through 7 for an outbound packet. When the packet is sent to a port, the QoS priority determines the outbound queue to which the packet is assigned as shown in the following table.

802.1p priority settings and outbound queue assignment

802.1p priority setting

Outbound port queue

1 and 2

Low priority (1, 2)

0 or 3

Normal priority (3, 4)

4 and 5

Medium priority (5, 6)

6 and 7

High priority (7, 8)

If a packet is transmitted in an untagged-VLAN environment, the 802.1p priority settings in the preceding table control only the outbound queue to which the packet is sent on the local switch. Because no VLAN tag is used, an 802.1p priority value is not added to the 802.1Q field in the packet header for use by downstream devices.

However, if your network uses only one VLAN and does not require VLAN-tagged ports, you can preserve 802.1p priority settings in outbound traffic by configuring the ports on links between devices on which you want 802.1p priorities to be honored as tagged VLAN members.

If a packet is transmitted in an 802.1Q VLAN-tagged environment, the QoS-configured 802.1p setting is also added to the VLAN packet header as an 802.1p priority for use by downstream devices and applications.

In an 802.1Q VLAN environment with VLAN-tagged ports, if QoS is not configured on the switch but is configured on an upstream device, the priorities carried in the packets determine the outbound port queue on which packets are forwarded.

Mapping 802.1p priorities to outbound port queues on the switch and downstream devices

Configured 802.1p priority

Outbound port queue in the switch

802.1p priority added to tagged VLAN packets exiting the switch

Queue assignment in downstream devices with:

8 queues

4 queues

2 queues

1

Queue 1

1 (low priority)

Queue 1

Queue 1

Queue 1

2

Queue 2

2

Queue 2

0

Queue 3

0 (normal priority)

Queue 3

Queue 2

3

Queue 4

3

Queue 4

4

Queue 5

4 (medium priority)

Queue 5

Queue 3

Queue 2

5

Queue 6

5

Queue 6

6

Queue 7

6 (high priority)

Queue 7

Queue 4

7

Queue 8

7

Queue 8

NOTE:

You can reconfigure the QoS queue setting to change the number of outbound port queues in the switch from eight (default) to four or two queues.