• Specify which traffic has higher or lower priority, regardless of current network bandwidth, or the relative
priority setting of the traffic when it is received on the switch.
• Change (upgrade or downgrade) the priority of outbound traffic.
• Override "illegal" packet priorities set by upstream devices or applications that use 802.1Q VLAN tagging
with 802.1p priority tags.
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:
Table 16: 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
170
Aruba 2530 Advanced Traffic Management Guide for
ArubaOS-Switch 16.09