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5. Configuring Quality of Service
5.1. CoS
The CoS feature lets you give preferential treatment to certain types of traffic over others. To set up this
preferential treatment, you can configure the ingress ports, the egress ports, and individual queues on the
egress ports to provide customization that suits your environment.
The level of service is determined by the egress port queue to which the traffic is assigned. When traffic is
queued for transmission, the rate at which it is serviced depends on how the queue is configured and
possibly the amount of traffic present in other queues for that port. Some traffic is classified for service (i.e.,
packet marking) before it arrives at the switch. If you decide to use these classifications, you can map this
traffic to egress queues by setting up a CoS Mapping table.
Each ingress port on the switch has a default priority value (set by configuring VLAN Port Priority in the
Switching sub-menu) that determines the egress queue its traffic gets forwarded to. Packets that arrive
without a priority designation, or packets from ports you’ve identified as “untrusted,” get forwarded
according to this default.
5.1.1. Trusted and Untrusted Port Modes
Ports can be configured in trusted mode or untrusted mode with respect to ingress traffic.
Ports in Trusted Mode: When a port is configured in trusted mode, the system accepts at face value a
priority designation encoded within packets arriving on the port. You can configure ports to trust priority
designations based on one of the following fields in the packet header:
802.1 Priority: values 0–7
IP DSCP: values 0–63
A mapping table associates the designated field values in the incoming packet headers with a traffic class
priority (actually a CoS traffic queue).
Ports in Untrusted Mode: If you configure an ingress port in untrusted mode, the system ignores any
priority designations encoded in incoming packets, and instead sends the packets to a traffic queue based
on the ingress port’s default priority.
5.1.2. Traffic Shaping on Egress Traffic
For slot/port interfaces, you can specify a traffic shaping rate for the port (in Kbps) for egress traffic. The
traffic shaping rate specifies an upper limit of the transmission bandwidth used.
5.1.3. Defining Traffic Queues
For each queue, you can specify:
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