Cisco Catalyst QoS: Quality of Service in Campus Networks

Congestion Avoidance 113

Congestion Avoidance

The Catalyst 5000 Family of switches achieves congestion avoidance through the use of

queue management on certain line modules based on CoS values. No support for output

scheduling or queuing based on DSCP or IP precedence values exists for the Catalyst 5000

Family of switches. The two uplink modules and three line modules described in Table 4-2

represent the only line modules to support output scheduling. All other line modules only

utilize a single transmit queue, and the transmit queue tail drops all frames when exhausting

buffer queue space.

The uplink modules and line modules in Table 4-2 utilize a single transmit queue with four

conﬁgurable transmit queue drop thresholds, 1q4t, to achieve congestion avoidance. The

switch drops packets with speciﬁc CoS values when the transmit buffer reaches speciﬁc

thresholds. The CoS values assigned to each threshold are conﬁgurable. This method of

congestion avoidance delivers Weighted Random Early Detection (WRED) for a single

queue. Chapter 1, “Quality of Service: An Overview,” discussed WRED in more detail.

For example, Table 4-7 describes the default behavior of congestion management on the

Catalyst 5000 Family of switches.

Use the following command to conﬁgure the CoS values that map to a speciﬁc threshold

number:

set qos map

port_type q# threshold#

cos

cos_list

port_type is always 1q4t and q# is always 1 on the Catalyst 5000 Family of switches. The

threshold# represents one of four thresholds used for CoS value mapping. After making

conﬁguration changes to the transmit queue, use the following commands to verify the

conﬁguration changes:

show qos info <runtime | config> <

mod/port

show qos info config

port_type

Table 4-7 Default Output Scheduling Behavior of Ports Supporting 1q4t Transmit Queue

Threshold No. Threshold of Transmit Queue Buffer CoS Values

1 30% Full 0 and 1

2 50% Full 2 and 3

3 80% Full 4 and 5

4 100% Full 6 and 7

114 Chapter 4: QoS Support on the Catalyst 5000 Family of Switches

Example 4-14 illustrates a user conﬁguring and verifying the QoS threshold number to CoS

mapping. Only module 3 of this chassis supports QoS features; and therefore, the switch

warns that QoS is not supported on modules 1, 2, 4, and 15.

The threshold’s values are also conﬁgurable on a global basis. Use the following command

to conﬁgure the threshold percentage drop values:

set qos wred-threshold

port_type

tx queue

q# threshold_percentage_values

As with all Catalyst 5000 QoS commands, port_type is always 1q4t and q# is always 1.

threshold_percentage_values represents the four threshold drop values. Example 4-15

illustrates a user conﬁguring the threshold drop values.

To view the number of packets dropped per threshold number, use the following command:

show qos statistics

mod_num/port_num

Example 4-14 User Conﬁguring and Verifying QoS Threshold to CoS Mapping

Console> (enable) ss

QoS is not supported on module 1.

QoS is not supported on module 2.

QoS is not supported on module 4.

QoS is not supported on module 15.

Qos tx priority queue and threshold mapped to cos successfully.

Console> (enable)

QoS is not supported on module 1.

QoS is not supported on module 2.

QoS is not supported on module 4.

QoS is not supported on module 15.

Qos tx priority queue and threshold mapped to cos successfully.

Console> (enable)

QoS setting in NVRAM for 1q4t transmit:

QoS is enabled

Queue and Threshold Mapping:

Queue Threshold CoS

----- --------- ---------------

1 1 0 1 2

1 2 3 4

1 3 5

1 4 6 7

Queue # Thresholds - percentage

------- ------------------------------------------

1 20% 40% 50% 100%

Example 4-15 User Conﬁguring Drop Thresholds for Transmit Queues

Console> (enable) ss

WRED thresholds for queue 1 set to 20 and 30 and 40 and 95 on all WRED-capable 1q4t

ports.

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