Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 473)

Tail drop
Tail drop queuing is the most basic form of congestion control. Frames are queued in FIFO order and
queue buildup can continue until all buffer memory is exhausted. This is the default behavior when no
additional QoS has been configured.
The basic tail drop algorithm does not have any knowledge of multiple priorities and per traffic class
drop thresholds can be associated with a queue to address this. When the queue depth breaches a
threshold, then any frame arriving with the associated priority value will be dropped. The figure below
illustrates how you can utilize this feature to ensure that lower-priority traffic cannot totally consume the
full buffer memory.
FIGURE 52 Queue depth
Thresholds can also be used to bound the maximum queuing delay for each traffic class. Additionally, if
the sum of the thresholds for a port is set below 100 percent of the buffer memory, then you can also
ensure that a single port does not monopolize the entire shared memory pool allocated to the port. The
tail drop algorithm can be extended to support per-priority drop thresholds. When the ingress port CoS
queue depth breaches a threshold, then any frame arriving with the associated priority value will be
dropped.
CoS thresholds
Every port has associated with it a total of 9 CoS thresholds, one for the port tail-drop threshold and the
other eight are thresholds for per priority. To give a fair allocation of buffers for the traffic from all
priorities, the port buffers are allocated among different priorities. That is achieved through per-priority
tail-drop thresholds. The port tail-drop threshold represents the amount of buffers given to the port, and
the per-priority tail-drop threshold (called the CoS tail-drop threshold from here on) represents the
buffers allocated to each CoS.
Whenever the buffers allocated to a priority are fully exhausted, all the traffic coming in on that priority is
dropped. In the absence of per-priority tail-drop thresholds (and with only port tail-drop thresholds), the
buffers would be consumed on a first-come, first-served basis, resulting in an unfair share of buffers
among all the priorities. If you know which priority traffic is most seen, then providing a sufficient number
of buffers for those priorities results in fewer packet drops for those priorities.
Tail drop
Network OS Administrator’s Guide 473
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