Packet Modification; Configuring Standard Qos - Cisco Catalyst 2928 Software Configuration Manual

Configuring Standard QoS

The buffer space is divided between the common pool and the reserved pool. The switch uses a buffer
allocation scheme to reserve a minimum amount of buffers for each egress queue, to prevent any queue
or port from consuming all the buffers and depriving other queues, and to control whether to grant buffer
space to a requesting queue. The switch detects whether the target queue has not consumed more buffers
than its reserved amount (under limit), whether it has consumed all of its maximum buffers (over limit),
and whether the common pool is empty (no free buffers) or not empty (free buffers). If the queue is not
over limit, the switch can allocate buffer space from the reserved pool or from the common pool (if it is
not empty). If there are no free buffers in the common pool or if the queue is over limit, the switch drops
the frame.
WTD Thresholds
You can assign each packet that flows through the switch to a queue and to a threshold. Specifically, you
map CoS values to an egress queue and map CoS values to a threshold ID. You use the mls qos srr-queue
output cos-map queue queue-id {cos1...cos8 | threshold threshold-id cos1...cos8} global configuration
command. You can display the CoS output queue threshold map by using the show mls qos maps
privileged EXEC command.
The queues use WTD to support distinct drop percentages for different traffic classes. Each queue has
three predefined default drop thresholds that are not changeable. For more information about how WTD
works, see the

Packet Modification

A packet is classified, policed, and queued to provide QoS. Packet modifications can occur during this
process. For IP and non-IP packets, classification involves assigning a QoS label to a packet based on
the CoS of the received packet. However, the packet is not modified at this stage; only an indication of
the assigned CoS value is carried along.
Configuring Standard QoS
Before configuring standard QoS, you must have a thorough understanding of these items:
•
•
•
•
These sections contain this configuration information:
•
•
•
•
•
Catalyst 2928 Switch Software Configuration Guide
31-6
"Weighted Tail Drop" section on page
The types of applications used and the traffic patterns on your network.
Traffic characteristics and needs of your network. Is the traffic bursty? Do you need to reserve
bandwidth for voice and video streams?
Bandwidth requirements and speed of the network.
Location of congestion points in the network.
Default Standard QoS Configuration, page 31-7
Enabling QoS Globally, page 31-9
Configuring Classification Using Port Trust States, page 31-9
Configuring Ingress Queue Characteristics, page 31-12
Configuring Egress Queue Characteristics, page 31-14
31-4.
(required)
(optional)
(optional)
Chapter 31 Configuring QoS
(required
OL-23389-01