Cisco Catalyst 2000 Configuration Handbook page 265

244 Cisco LAN Switching Configuration Handbook
Tip The weight value for a queue specifies how many bytes are transmitted before mov-
ing to the next queue. A whole frame is always transmitted, even if you choose a weight
value that is smaller. Therefore, make sure you choose a value for weight1 (the lowest pri-
ority queue) that is at least as large as the MTU (the largest frame that can be sent). Then
scale the other weight values proportionately.
The larger the weights for higher priority queues, the more time elapses before lower-prior-
ity queues are serviced. This increases the latency for lesser-priority queues.
After setting the weights for a port, confirm that the hardware is using an appropriate
value. Use the show qos info runtime mod/port command and look at the transmit queue
ratio information. The ratio and number of bytes are shown.
10. (Optional) Map internal DSCP values to egress CoS values:
(global) mls qos map dscp-cos dscp-list to cos-value
The internal DSCP generates a final CoS value for each frame. The final CoS is writ-
ten into the CoS fields of egress trunks and is also used to control egress scheduling
and congestion avoidance.
DSCP values dscp-list can be a single value (0 to 63), a hyphenated range of values,
or multiple values and ranges separated by commas. The CoS value is given by cos-
value (0 to 7).
Table 13-7 shows the default DSCP-to-CoS map.
The IOS command can be repeated several times until all the CoS mappings are
defined.
11. (Optional) Tune egress scheduling and congestion avoidance.
a. (Optional) Set the congestion-avoidance thresholds.
Table 13-7 DSCP-to-CoS Map Default
DSCP 0–7
CoS 0
(Optional) Use standard tail-drop receive queues:
(interface) wrr-queue threshold queue-id threshold-percent-1 threshold-
percent-2
For 2q2t switch ports, standard tail-drop congestion avoidance can be used. For
each threshold, you can assign the percentage of the buffer that is available to
transmit frames. The threshold-percentage-n values (1 to 100 percent) are given
in sequential order. When the buffer rises above the threshold level, new out-
bound frames are dropped. By default, threshold 1 is set to 100 percent, and
threshold 2 is set to 60 percent.
8–15 16–23
1 2
24–31 32–39 40–47
3 4 5
48–55 56–63
6 7