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Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 15 Oversubscribing Physical and Virtual Links
How the Router Determines VC Weights
weight Command History
Usage Guidelines for the weight Command
On the PRE2, for the
weight
command to be fully effective, use the
queue-depth
command to set the
segmentation and reassembly (SAR) line card queue depth for each VC interface queue above the VC
weight.
High Watermark and Low Watermark Default Values
On ATM line cards, the SAR mechanism has a queue for each PVC. Each PVC queue has two thresholds
associated with it called the high watermark and low watermark. These watermarks define the number
of cells the queue can hold.
The high and low watermark settings define the depth of the PVC interface queue on the line card.
Table 15-3
lists the default high and low watermark values for ATM variable bit rate (VBR) VCs, based
on the ATM VC rate.
Table 15-4
lists the default high and low watermark values for ATM unspecified bit rate (UBR) VCs,
based on the ATM VC rate.
The behavior of the high watermark threshold and the low watermark threshold depends on whether the
atm pxf queuing mode or the no atm pxf queuing mode is configured. In atm pxf queuing
mode, the high
watermark is a threshold that triggers a flow-off signal and the low watermark is a threshold that triggers
a flow-on signal. The high watermark is the maximum number of cells that can be in the PVC queue for
the ATM SAR mechanism before a flow-off signal is sent to the PXF. With the exception of priority
Cisco IOS Release
Description
Release 12.3(7)XI1
The
weight
command was introduced on the PRE2.
Release 12.2(28)SB
This command was integrated in Cisco IOS Release 12.2(28)SB for the
PRE2.
Release 12.2(31)SB2
This command was introduced on the PRE3.
Table 15-3
Default Watermark Values for ATM VBR VCs
VC Rate (bps)
Low Watermark
High Watermark
0 to 18,999
48
56
19,000 to 40,999
64
72
41,000 to 99,999
128
144
100,000 to 622,000
224
240
Table 15-4
Default Watermark Values for ATM UBR VCs
VC Rate (bps)
Low Watermark
High Watermark
0 to 18,999
224
240
