Overview of QoS settings

QoS settings operate on two levels:

• Controlling the priority of outbound packets moving through the switch: 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 8 queues Outbound port 4 queues Outbound port 2 queues 1 1 1 1 2 2 0 3 2 3 4 4 5 3 2 5 6 6 7 4 7 8

  (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 forwarding queues in the switch.)
• Configuring a priority for outbound packets and a service (priority) policy for use by downstream devices:
  • DSCP Policy: This feature enables you to set a priority policy in outbound IP packets. (You can configure downstream devices to read and use this policy.) This method is not dependent on VLAN-tagged ports to carry priority policy to downstream devices, and can:

    • Change the codepoint (the upper 6 bits) in the ToS byte.

    • Set a new 802.1p priority for the packet.

    (Setting DSCP policies requires IPv4 inbound packets.)

  • 802.1p priority rules: An outbound, VLAN-tagged packet carries an 802.1p priority setting that was configured (or preserved) in the switch. This priority setting ranges from 0 to 7, and can be used by downstream devices having up to eight outbound port queues. Thus, if packets within the switch move at the four priority levels shown in the table above, they still can carry an 802.1p priority that can be used by downstream devices having more or less than the four priority levels in the switches covered in this guide. Also, if the packet enters the switch with an 802.1p priority setting, QoS can override this setting if configured with an 802.1p priority rule to do so.
    NOTE:

    If your network uses only one VLAN (and therefore does not require VLAN-tagged ports), you can still preserve 802.1p priority settings in your traffic by configuring the ports as tagged VLAN members on the links between devices you want to honor traffic priorities.

    Rule and policy limits: A large number of 802.1p priority rules and/or DSCP policies are allowed in any combination. For example, for the 2540 switch 6000 are allowed.

You can configure a QoS priority of 0 through 7 for an outbound packet. When the packet is then sent to a port, the QoS priority determines which outbound queue the packet uses:

QoS priority settings and operation 802.1p priority setting Outbound port 8 queues Outbound port 4 queues Outbound port 2 queues 1 1 1 1 2 2 0 3 2 3 4 4 5 3 2 5 6 6 7 4 7 8

If a packet is not in a VLAN-tagged port environment, then the QoS settings in the table above control only to which outbound queue the packet goes. Without VLAN tagging, no 802.1p priority is added to the packet for downstream device use. But if the packet is in a VLAN-tagged environment, then the above setting is also added to the packet as an 802.1p priority for use by downstream devices and applications (shown in the table below). In either case, an IP packet can also carry a priority policy to downstream devices by using DSCP-marking in the ToS byte.

Mapping switch QoS priority settings to device queues Priority setting Outbound port queues in the switch 802.1p priority setting added to tagged VLAN packets exiting the switch Queue assignment in downstream devices with: 8 queues 3 queues 2 queues 1 Queue 1 1 (low priority) Queue 1 Queue 1 Queue 1 2 2 Queue 2 0 Queue 2 0 (normal priority) Queue 3 Queue 2 3 3 Queue 4 4 Queue 3 4 (medium priority) Queue 5 Queue 3 Queue 2 5 5 Queue 6 6 Queue 4 6 (high priority) Queue 7 7 7 Queue 8