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Configuring the Switch
Class of Service (CoS): Managing Bandwidth More Effectively
Configuring the Switch
Basic Operation
CoS settings operate on two levels:
■ Controlling the priority of outbound packets: Each switch port has
two outbound traffic queues; “normal” priority and “high” priority. (High-
priority packets leave the switch port first. Normal-priority packets leave
the switch port after the port’s high-priority queue is emptied.) With no
CoS control, all traffic (except IGMP traffic configured for high priority)
goes through the “normal” outbound port queues. However, with a CoS
configuration, you can determine the outbound priority queue to which a
packet is sent. (In an 802.1Q tagged VLAN environment, if CoS is not
configured on the switch, but is configured on an upstream device, high
priority traffic received by the switch is forwarded through high priority
queues.)
■ Configuring the 802.1p priority of outbound packets in a tagged
VLAN environment for use by downstream devices: If an outbound
packet is in an 802.1Q tagged VLAN environment (that is, if the packet is
assigned to a tagged VLAN on the outbound port), then the packet carries
an 802.1p priority setting that was configured in the switch. This priority
setting can range from 0 to 7, and can be used by downstream devices
having up to eight queues. Thus, while packets within the switch move
only at high or normal priority, they still can carry the 802.1p priority that
can be used by downstream devices having more than two priority levels.
Also, if the packet enters the switch with an 802.1p priority setting, CoS
can override this setting if configured to do so.
Note: If you are not using multiple tagged VLANs in your network, you
can still use the tagged VLAN feature by configuring the default VLAN as
a tagged VLAN.
You can configure a CoS priority of 0 through 7 for an outbound packet. When
the packet is then sent to a port, the CoS priority determines which outbound
queue the packet uses:
CoS Setting Outbound Port
Queue
Operation
0 — 3 normal priority Packets in this queue leave the port after the high-priority
queue is emptied.
4 — 7 high priority Packets in this queue leave the port first.