Cisco 3560 Congestion Avoidance
The 3560 uses a different method for congestion avoidance, known as weighted tail drop, or WTD.
WTD creates three thresholds per queue into which traffic can be divided, based on CoS value, for
tail drop when the associated queue reaches a particular percentage. For example, you can
configure a queue so that it drops traffic with CoS values of 0–3 when the queue reaches 40 percent
full, then drops traffic with CoS 4 and 5 at 60 percent full, and finally drops CoS 6 and 7 traffic
only when the queue is 100 percent full. Figure 13-9 shows this behavior.
WTD Configuration in Graphical Form
CoS 6-7
100%
CoS 4-5
60%
CoS 0-3
40%
1000
600
400
0
NOTE Figure 13-9 is redrawn from Figure 31-6 in “Configuring QoS” at http://
www.ciscosystems.com/en/US/products/hw/switches/ps5528/
products_configuration_guide_chapter09186a00802113d0.html#wp1284809.
Because WTD is configurable separately for all six queues in the 3560 (two ingress, four egress),
a great deal of granularity is possible in 3560 configuration (maybe even too much!).