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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter27 Configuring Quality of Service
Overview of QoS
Mapping Tables
During QoS processing, the switch represents the priority of all traffic (including non-IP traffic) with an
internal DSCP value:
•During classification, QoS uses configurable mapping tables to derive the internal DSCP (a 6-bit
value) from received CoS. These maps include the CoS-to-DSCP map.
•During policing, QoS can assign another DSCP value to an IP or non-IP packet ( if the packet is out
of profile and the policer specifies a marked down DSCP value). This configurable map is called the
policed-DSCP map.
•Before the traffic reaches the scheduling stage, QoS uses the internal DSCP to select one of the four
egress queues for output processing. The DSCP-to-egress queue mapping can be configured using
the qos map dscp to tx-queue command.
The CoS-to-DSCP and DSCP-to-CoS map have default values that might or might not be appropriate for
your network.
For configuration information, see the “Configuring DSCP Maps” section on page 27-51.
Queueing and Scheduling
Each physical port has four transmit queues (egress queues). Each packet that needs to be transmitted is
enqueued to one of the transmit queues. The transmit queues are then serviced based on the transmit
queue scheduling algorithm.
Once the final transmit DSCP is computed (including any markdown of DSCP), the transmit DSCP to
transmit queue mapping configuration determines the transmit queue. The packet is placed in the
transmit queue of the transmit port, determined from the transmit DSCP. Use the qos map dscp to
tx-queue command to configure the transmit DSCP to transmit queue mapping. The transmit DSCP is
the internal DSCP value if the packet is a non-IP packet as determined by the QoS policies and trust
configuration on the ingress and egress ports.
For configuration information, see the “Configuring Transmit Queues” section on page 27-48.

Active Queue Management

Active queue management (AQM) is the pro-active approach of informing you about congestion before
a buffer overflow occurs. AQM is done using Dynamic buffer limiting (DBL). DBL tracks the que ue
length for each traffic flow in the switch. When the queue length of a flow exceeds its limit, DBL will
drop packets or set the Explicit Congestion Notification (ECN) bits in the packet headers.
DBL classifies flows in two categories, adaptive and aggressive. Adaptive flows reduce the rate of packet
transmission once it receives congestion notification. Aggressive flows do not take any corrective action
in response to congestion notification. For every active flow the switch maintains two para meters,
“buffersUsed” and “credits”. All flows start with “max-credits”, a global parameter. When a flow with
credits less than “aggressive-credits” (another global parameter) it is considered an aggressive flow and
is given a small buffer limit called “aggressiveBufferLimit”.
Queue length is measured by the number of packets. The number of packets in the queue determines the
amount of buffer space that a flow is given. When a flow has a high queue length the computed value is
lowered. This allows new incoming flows to receive buffer space in the queue. This allows all flows to
get a proportional share of packets through the queue.