Cisco Catalyst 2928, Software Configuration Manual

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Catalyst 2928 Switch Software Configuration Guide
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Chapter 31 Configuring QoS
Understanding QoS
The QoS label is based on the CoS value in the packet and decides the queueing and scheduling actions
to perform on the packet. The label is mapped according to the trust setting and the packet type.
You specify which fields in the frame or packet that you want to use to classify incoming traffic. For
non-IP traffic, you have this classification option:
Trust the CoS value in the incoming frame (configure the port to trust CoS). Layer 2 ISL frame
headers carry the CoS value in the 3 least-significant bits of the 1-byte User field. Layer 2 IEEE
802.1Q frame headers carry the CoS value in the 3 most-significant bits of the Tag Control
Information field. CoS values range from 0 for low priority to 7 for high priority.
For configuration information on port trust states, see the “Configuring Classification Using Port Trust
States” section on page 31-9 .
After classification, the packet is sent to the ingress queueing and scheduling stages.
For information about the CoS ingress queue threshold maps, see the “Queueing on Ingress Queues”
section on page 31-4 . For information about the CoS egress queue threshold maps, see the “Queueing on
Egress Queues” section on page 31-5 .
Queueing Overview
The switch has queues at specific points to help prevent congestion, as shown in Figure 31-3 .
Figure 31-3 Ingress and Egress Queue Location
Because the total inbound bandwidth of all ports can exceed the bandwidth of the internal ring, ingress
queues are located after the packet is classified, policed, and marked and before packets are forwarded
into the switch fabric. Because multiple ingress ports can simultaneously send packets to an egress port
and cause congestion, outbound queues are located after the internal ring.
Weighted Tail Drop
Both the ingress and egress queues use an enhanced version of the tail-drop congestion-avoidance
mechanism called weighted tail drop (WTD). WTD is implemented on queues to manage the queue
lengths and to provide drop precedences for different traffic classifications.
As a frame is enqueued to a particular queue, WTD uses the assigned QoS label for the frame to subject
it to different thresholds. If the threshold is exceeded for that QoS label (the space available in the
destination queue is less than the size of the frame), the switch drops the frame.
For more information, see the “Mapping CoS Values to an Ingress Queue” section on page 31-12 and the
“Mapping CoS Values to an Egress Queue and to a Threshold ID” section on page 31-15 .
Queueing on Ingress Queues
Figure 31-4 shows the queueing and scheduling flowchart for ingress ports.
Figure 31-4 Queueing Flowchart for Ingress Ports
Note Shaped round robin (SRR) services the priority queue for its configured share before servicing the other
queue.