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
tx
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
ss
ee
ee
tt
tt
qq
qq
oo
oo
ss
ss
mm
mm
aa
aa
pp
pp
11
11
qq
qq
44
44
tt
tt
11
11
11
11
cc
cc
oo
oo
ss
ss
00
00
--
--
22
22
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)
ss
ss
ee
ee
tt
tt
qq
qq
oo
oo
ss
ss
mm
mm
aa
aa
pp
pp
11
11
qq
qq
44
44
tt
tt
11
11
22
22
cc
cc
oo
oo
ss
ss
44
44
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)
ss
ss
hh
hh
oo
oo
ww
ww
qq
qq
oo
oo
ss
ss
ii
ii
nn
nn
ff
ff
oo
oo
cc
cc
oo
oo
nn
nn
ff
ff
ii
ii
gg
gg
11
11
qq
qq
44
44
tt
tt
tt
tt
xx
xx
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
ss
ee
ee
tt
tt
qq
qq
oo
oo
ss
ss
ww
ww
rr
rr
ee
ee
dd
dd
--
--
tt
tt
hh
hh
rr
rr
ee
ee
ss
ss
hh
hh
oo
oo
ll
ll
dd
dd
11
11
qq
qq
44
44
tt
tt
tt
tt
xx
xx
qq
qq
uu
uu
ee
ee
uu
uu
ee
ee
11
11
22
22
00
00
33
33
00
00
44
44
00
00
99
99
55
55
WRED thresholds for queue 1 set to 20 and 30 and 40 and 95 on all WRED-capable 1q4t
ports.

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