QoS settings operate on two levels:
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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 queue 1 and 2 Low priority (1) 0 or 3 Normal priority (2) 4 and 5 Medium priority (3) 6 and 7 High priority (4) (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.)
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Configuring a priority for outbound packets and a service (priority) policy for use by downstream devices:
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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 six bits) in the ToS byte.
— Set a new 802.1p priority for the packet.
(Setting DSCP policies requires IPv4 inbound packets.)
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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 802.1p priority settings and outbound queue assignment, 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: ]](images/note.gif)
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: The switches covered in this guide allow up to 400 802.1p priority rules and/or DSCP policies in any combination.
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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
| QoS priority settings | Outbound port queue |
|---|---|
| 1, 2 | Low priority |
| 0, 3 | Normal priority |
| 4, 5 | Medium priority |
| 6, 7 | High priority |
If a packet is not in a VLAN-tagged port environment, then the QoS settings in QoS priority settings and operation 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 Mapping switch QoS priority settings to device queues). 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 | |||
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NOTE: The QoS queue configuration feature can change the number of outbound port queues in the switch from four (the default) to eight queues or two queues. For more information, see QoS queue configuration. |
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NOTE: Regarding using multiple criteria: HP recommends that you configure a minimum number of the available QoS classifiers for prioritizing any given packet type. Increasing the number of active classifier options for a packet type increases the complexity of the possible outcomes and consumes switch resources. |
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The switches covered in this guide provide six QoS classifiers (packet criteria) you can use to configure QoS priority.
Classifier search order and precedence
| Search order | Precedence | Global QoS classifier |
|---|---|---|
| 1 | 1 (highest) | UDP/TCP application type (port) |
| 2 | 2 | Device priority (destination or source IP address) |
| 3 | 3 | IP type of service (ToS): precedence and DSCP bit sets (IP packets only) |
| 4 | 4 | IP protocol (IP, IPX, ARP, AppleTalk, SNA, and NetBeui) |
| 5 | 5 | VLAN ID |
| 6 | 6 | Incoming source-port on the switch |
| Default | 7 (lowest) | The incoming 802.1p priority (present in tagged VLAN environments) is preserved if no global QoS classifier with a higher precedence matches. |
Where multiple classifier types are configured, a switch uses the highest-to-lowest search order shown in Classifier search order and precedence to identify the highest-precedence classifier to apply to any given packet. When a match between a packet and a classifier is found, the switch applies the QoS policy configured for that classifier and the packet is handled accordingly.
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NOTE: On the switches covered in this guide, if the switch is configured with multiple classifiers that address the same packet, the switch uses only the QoS configuration for the QoS classifier that has the highest precedence. In this case, the QoS configuration for another, lower-precedence classifier that may apply is ignored. For example, if QoS assigns high priority to packets belonging to VLAN 100, but normal priority to all IP protocol packets, since protocol priority (4) has precedence over VLAN priority (5), IP protocol packets on VLAN 100 will be set to normal priority. |
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