36-11
Cisco IE 3000 Switch Software Configuration Guide
OL-13018-03
Chapter 36 Configuring QoS
Understanding QoS
•
Before the traffic reaches the scheduling stage, QoS stores the packet in an ingress and an egress
queue according to the QoS label. The QoS label is based on the DSCP or the CoS value in the packet
and selects the queue through the DSCP input and output queue threshold maps or through the CoS
input and output queue threshold maps. In addition to an ingress or an egress queue, the QOS label
also identifies the WTD threshold value. You configure these maps by using the
mls qos srr-queue
{
input
|
output
}
dscp-map
and the
mls qos srr-queue
{
input
|
output
}
cos-map
global
configuration commands.
The CoS-to-DSCP, DSCP-to-CoS, and the IP-precedence-to-DSCP maps have default values that might
or might not be appropriate for your network.
The default DSCP-to-DSCP-mutation map and the default policed-DSCP map are null maps; they map
an incoming DSCP value to the same DSCP value. The DSCP-to-DSCP-mutation map is the only map
you apply to a specific port. All other maps apply to the entire switch.
For configuration information, see the
“Configuring DSCP Maps” section on page 36-50
.
For information about the DSCP and CoS input queue threshold maps, see the
“Queueing and
Scheduling on Ingress Queues” section on page 36-13
. For information about the DSCP and CoS output
queue threshold maps, see the
“Queueing and Scheduling on Egress Queues” section on page 36-15
.
Queueing and Scheduling Overview
The switch has queues at specific points to help prevent congestion as shown in
Figure 36-5
.
Figure 36-5
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.
M
a
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Ingre
ss
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s
Intern
a
l
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Egre
ss
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s
Policer
M
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Policer
Cl
ass
ify
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ffic
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