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5-9
Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
You can configure a QoS priority of 0 through 3 for an outbound packet. When
the packet is then sent to a port, the QoS priority determines which outbound
queue the packet uses:
Table 5-2. QoS Priority Settings and Operation
If a packet is not in a VLAN-tagged port environment, then the QoS settings in
table 5-2 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 table 5-3). In either case, an IP packet can also
carry a priority policy to downstream devices by using DSCP-marking in the
ToS byte.
Table 5-3. Mapping Switch QoS Priority Settings to Device Queues
Note The number of outbound port queues in the switch is fixed at four.
QoS Priority Setting Outbound Port Queue
1-2 low priority (1,2)
0-3 normal priority (3,4)
4-5 medium priority (5,6)
6-7 high priority (7,8)
Priority Setting Outbound Port
Queues in the
Switch
802.1p Priority Setting Added to
Tagged VLAN Packets Leaving
the Switch
Queue
Assignment in
Downstream
Devices With:
4 Queues
1
Queue 1
1 (low priority)
Queue 1
22
0
Queue 2
0 (normal priority)
Queue 2
33
4
Queue 3
4 (medium priority)
Queue 3
55
6
Queue 4
6 (high priority)
Queue 4
77