Controlling Packet Flow On Layer 2 Queues - Cisco 10000 Series Configuration Manual

Chapter 11 Managing Packet Queue Congestion

Controlling Packet Flow on Layer 2 Queues

The ATM VC queue depth feature controls the depth of Layer 2 ATM cell queues and is used primarily
for flow control. You specify the VC queue depth when you create the ATM VC on the line card.
To be fully effective, you must set the segmentation and reassembly (SAR) line card queue depth for
each VC interface queue above the weight for VC weighting by using the queue-depth command. This
command enables you to set the high watermark and low watermark, which define the depth of the VC
interface queue. The optimum threshold values for these watermarks are a function of a number of
variables, such as the following:
•
•
•
•
•
Because so many variables influence watermark threshold values, you might need to experiment with
different values to determine the optimum high and low watermark values for your configuration. In
general, the following guidelines apply:
•
•
For more information about the high and low watermarks, see the
Default Values" section on page 15-25
Table 11-8
and unspecified bit rate (UBR) virtual circuits.
Table 11-8
Type of ATM
Virtual Circuit
Variable Bit Rate (VBR)
Unspecified Bit Rate (UBR)
When changing the minimum and maximum threshold values, consider the following guidelines:
•
•
•
•
OL-7433-09
Number of queues on a VC
Priority queue latency requirements
Bandwidth of the VC
Accuracy of VC bandwidth utilization
Per-queue bandwidth accuracy
Set the low watermark equal to the VC weight. If the low watermark is less than the VC weight, a
full weight worth of cells might not be enqueued in the SAR mechanism when the scheduler
round-robin gets to the VC. As a result, the VC might not get its fair share.
Set the high watermark equal to the low watermark plus 2.
lists the default minimum and maximum threshold values for ATM variable bit rate (VBR)
ATM Default Threshold Values
To enhance virtual circuit utilization accuracy, increase the minimum threshold and possibly the
maximum threshold.
To enhance per-queue accuracy, increase the spread between the thresholds. For example, if the
minimum threshold is 66 and the maximum threshold is 70, increase the maximum threshold to 72
to increase the spread.
To reduce latency, decrease the maximum threshold.
To increase the number of queues, increase the maximum threshold.
in Chapter 15, "Oversubscribing Physical and Virtual Links."
Virtual Circuit Rate
(in bps)
0 to 18999
19000 to 40999
41000 to 99999
100000 to 622000
Not Applicable
Cisco 10000 Series Router Quality of Service Configuration Guide
Controlling Packet Flow on Layer 2 Queues
"High Watermark and Low Watermark
Minimum Maximum
Threshold Threshold
48 56
64 72
128 144
224 240
224 240
11-39