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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter36 Configuring QoS Understanding QoS
Queueing and Scheduling Overview
The switch has queues at specific points to help prevent congestion as shown in Figure 36-6.
Figure36-6 Ingress and Egress Queue Location
Because the total inbound bandwidth of all ports can exceed the bandwid th of the stack or internal ring,
ingress queues are located after the packet is classified, policed, and marked and before packets are
forwarded into the switch fabric. Because multiple ingress ports can simultaneously send packets to an
egress port and cause congestion, outbound queues are located af t er the sta ck o r int erna l ri ng .

Weighted Tail Drop

Both the ingress and egress queues use an enhanced version of the tail-drop congestion-avoidance
mechanism called weighted tail drop (WTD). WTD is im ple ment ed o n q ue ues to mana ge th e queue
lengths and to provide drop precedences for different traffic classifications.
As a frame is enqueued to a particular queue, WTD uses th e frame’s assigned QoS label to subject it to
different thresholds. If the threshold is exceeded for that QoS labe l (the space a vail able in the des tination
queue is less than the size of the frame), the switch drops the frame.
Each queue has three threshold values. The QOS label is determines which of the three threshold values
is subjected to the frame. Of the three thresholds, two are configurable (e xplicit) an d one is not (imp licit).
Figure 36-7 shows an example of WTD operating on a queue whose size is 10 00 fr am es. Thr ee dro p
percentages are configured: 40 percent (400 frames), 60 p er cent (600 fra me s) , and 1 00 perc en t (1 000
frames). These percentages mean that up to 400 frames can b e qu eued at the 40-percent threshold, up to
600 frames at the 60-percent threshold, and up to 1000 fr ame s a t the 1 00- perc en t t hresh old.
In this example, CoS values 6 and 7 have a greater importance than the other CoS values, and they are
assigned to the 100-percent drop threshold (queue-full state) . CoS values 4 and 5 a re a ssign ed to th e
60-percent threshold, and CoS values 0 to 3 are assigned to the 40-pe rce nt t hr eshol d.
Suppose the queue is already filled with 600 frames, and a new frame arrives. It contains CoS values 4
and 5 and is subjected to the 60-percent threshold. If this frame is added to th e queue, the threshold will
be exceeded, so the switch drops it.
MarkerPolicer
MarkerPolicer
Marker
Ingress
queues
Stack ring Egress
queues
Policer
MarkerPolicer
Classify
T
raffic SRRSRR
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