Auto-QoS 187
Auto-QoS
Auto-QoS eases the deployment of QoS on Catalyst switches by applying recommended
QoS configuration for typical networks, especially those deploying VoIP. For the Catalyst
2950 Family and Catalyst 3550 Family of switches, the Auto-QoS feature, as supported in
12.1.12c(EA1), aids in configuration of QoS for classification, egress scheduling, and
congestion management of VoIP traffic for Cisco IP telephony. Upcoming software
versions will offer additional Auto-QoS features.
Auto-QoS Classification
The Catalyst 2950 Family and 3550 Family of switches support two methods of classifi-
cation when Auto-QoS is enabled. The first method classifies traffic strictly based on
ingress CoS. Moreover, this configuration option just adds the trust CoS command to the
interface for classification purposes. This method is useful for classifying traffic for inter-
faces connecting other switches.
Example 6-32 User Verifying Configuration of Minimum Buffer Levels
Switch#show running-config
Building configuration...
!
(text deleted)
!
mls qos min-reserve 1 170
mls qos min-reserve 7 20
mls qos min-reserve 8 10
mls qos
!
!
interface FastEthernet0/4
no ip address
mls qos trust dscp
wrr-queue bandwidth 10 20 30 40
wrr-queue min-reserve 2 5
wrr-queue min-reserve 3 7
wrr-queue min-reserve 4 8
!
(text deleted)
!
end
Switch#show mls qos interface buffers
FastEthernet0/1
Minimum reserve buffer size:
170 100 100 100 100 100 20 10
Minimum reserve buffer level select:
1 2 3 4
188 Chapter 6: QoS Features Available on the Catalyst 2950 and 3550 Family of Switches
The second method uses the trust Cisco IP Phone classification method as discussed in the
“Trust Cisco IP Phone Device” section of this chapter. In brief, this method trusts ingress
CoS only when a Cisco IP Phone is connected to an interface. This configuration option just
adds the mls qos trust device cisco-phone and mls qos trust cos commands to the
interface for classification.
To configure an interface for Auto-QoS of classification based on trust, use the following
interface command:
aa
aa
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oo
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tt
To configure an interface for Auto-QoS of classification based on trust and whether a Cisco
IP phone is discovered on the interface, use the following interface command:
aa
aa
uu
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tt
tt
oo
oo
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qq
oo
oo
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vv
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oo
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cc
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--
--
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ee
Both of these classification options for Auto-QoS trust CoS for ingress frames. With these
configuration, the switch classifies all untagged frames with a CoS value of zero regardless
of the DSCP value. As a result, Auto-QoS is not the method of choice for applying classi-
fication based on DSCP or IP precedence.
Auto-QoS Congestion Management
Both the configuration options discussed in the preceding section also configure the
Catalyst 2950 Family and Catalyst 3550 Family of switches for congestion management.
Foremost, the Auto-QoS feature modifies the CoS-to-DSCP mapping resulting in the
nondefault internal DSCP values for classified frames. On the Catalyst 2950 Family of
switches, the CoS-to-DSCP mapping is a global configuration. On the Catalyst 3550 Family
of switches, the CoS-to-DSCP mapping is an interface configuration. The nondefault CoS-to-
DSCP mapping is a follows for CoS values 0 through 7, respectively: 0 8 16 26 32 46 48 56. In
addition, Auto-QoS configures both switches for mapping CoS 5 to queue 4.
In addition, the Auto-QoS feature modifies egress scheduling such that the switch only uses
three egress transmission queues. The Auto-QoS feature achieves this configuration by
assigning CoS value 5 to queue 4; CoS values 3, 6, and 7 to queue 3; and CoS values 0, 1,
and 2 to queue 1. The Auto-QoS feature assigns the CoS value of 5 to queue 4, the expedite
queue, because Cisco IP Phones mark voice traffic with CoS value 5. The Auto-QoS assigns
CoS value 3 along with the CoS value 6 and 7, which are used by routing protocols and
spanning-tree bridge protocol data units (BPDUs), because Cisco IP Phones mark control
traffic with a CoS value of 3.
Auto-QoS also assigns different weights for WRR to each queue. For the Catalyst 2950,
queuing is based on strict priority for queue 4. Therefore, queue 4 does not receive a
weighted value. Similarly, for the Catalyst 3550 Family of switches, Auto-QoS configures
queue 4 as the expedite queue. On both switches, queue 3 and queue 1 receive weights of
80 percent and 20 percent, respectively.
Auto-QoS 189
Finally, on the Catalyst 3550, Auto-QoS adjusts the buffer size of the Gigabit Ethernet and
Fast Ethernet interface queues. The higher-priority queues receive a smaller queue size,
whereas the lower-priority queues receive a larger queue size. This algorithm is used
because high-priority traffic is affected by jitter and queuing packets increases jitter.
Furthermore, data traffic occupying lower-priority queues is less affected by jitter; therefore,
storing more packets prevents excessive drops while maintaining low-priority service.
In summary, the Auto-QoS provides a macro for configuring classification and congestion
management based on trusting CoS for all ingress frames or only on interfaces connected
to Cisco IP Phones.
Auto-QoS on the Catalyst 2950 Family and Catalyst 3550 Family of switches adds the
following configuration commands as illustrated in Tables 6-8 to 6-12 in software version
12.1.12c(EA1):
The mls qos trust device cisco-phone
command is only added by Auto-QoS when the
interface is configured for the cisco-phone option.
Table 6-8 Catalyst 2950 Auto-QoS-Added Global Configuration Commands
wrr-queue bandwidth 20 1 80 0
wrr-queue bandwidth 20 1 80 0
wrr-queue cos-map 1 0 1 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
mls qos map cos-dscp 0 8 16 26 32 46 48 56
Table 6-9 Catalyst 2950 Auto-QoS-Added Interface Configuration Commands
mls qos trust cos
mls qos trust device cisco-phone
Table 6-10 Catalyst 3550 Auto-QoS-Added Global Configuration Commands
mls qos map cos-dscp 0 8 16 26 32 46 48 56
mls qos min-reserve 5 170
mls qos min-reserve 6 10
mls qos min-reserve 7 65
mls qos min-reserve 8 26
mls qos
190 Chapter 6: QoS Features Available on the Catalyst 2950 and 3550 Family of Switches
Auto-QoS supports a debug output that displays all commands applied globally and to each
interface upon configuration of Auto-QoS on an interface. Use the following debug
command to display this output:
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dd
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gg
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aa
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tt
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In addition, use the following command to display all configuration commands previously
added by Auto-QoS:
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aa
aa
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tt
tt
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Table 6-11 Catalyst 3550 Auto-QoS-Added Gigabit Interface Configuration Commands
mls qos trust cos
mls qos trust device cisco-phone
wrr-queue bandwidth 20 1 80 0
wrr-queue queue-limit 80 1 20 1
wrr-queue cos-map 1 0 1 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
priority-queue out
Table 6-12 Catalyst 3550 Auto-QoS-Added Fast Ethernet Interface Configuration Commands
mls qos trust cos
mls qos trust device cisco-phone
wrr-queue bandwidth 20 1 80 0
wrr-queue min-reserve 1 5
wrr-queue min-reserve 2 6
wrr-queue min-reserve 3 7
wrr-queue min-reserve 4 8
wrr-queue cos-map 1 0 1 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
priority-queue out
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