Guidelines For Changing Watermark Values - Cisco 10000 Series Configuration Manual

How the Router Determines VC Weights
queue packets and system queue packets, the flow-off signal stops packets for this PVC from being sent
to the ATM SAR mechanism. Priority queue packets and system queue packets continue to be sent to the
line card. Packets for other classes are queued at the PXF.
If packets are being queued at the PXF because the flow-off signal has been sent, when the number of
cells in the PVC queue reaches the low watermark threshold, a flow-on signal is sent to the PXF. The
flow-on signal restarts the packet flow (for packets of all classes) from this PVC to the ATM SAR
mechanism.
For example, after router startup the flow signal defaults to on. If the number of cells in the PVC queue
for the ATM SAR mechanism is 50, the high watermark is 56, and the low watermark is 48, and packets
of all classes flow from the PVC to the ATM SAR mechanism. If the number of cells increases to 56, the
high watermark triggers the flow-off signal, which stops packet flow for all packets, except the priority
queue and system queue packets. Packet flow for all other packets is not restarted until the number of
cells decreases to 48 (for example, when the low watermark triggers the flow-on signal).
In no atm pxf queuing mode, the high watermark is the maximum number of cells that can be in the PVC
queue for the ATM SAR mechanism before packet drops occur. The low watermark does not apply in no
atm pxf queuing mode.
While the high watermark threshold and low watermark threshold are defined by the numbers of cells,
Note
the crossing of the high watermark is evaluated on a packet boundary. This means that only whole
packets are placed in the PVC queue for the ATM SAR mechanism.
When you want to better control priority queuing latency or for better TCP performance, modify the
watermark values for each ATM variable bit rate (VBR) VC by using the queue-depth command. For
more information, see the

Guidelines for Changing Watermark Values

If you need to change the watermark values, follow these guidelines:
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The high watermark can be calculated based on the worst-case latency, VC traffic speed, IP packet size,
and ATM encapsulation type used (PPPoE, PPPoA, RBE). The low watermark is typically 80 percent of
the high watermark. The following formula is used to calculate the high watermark:
where:
High_water is the high watermark.
max latency is the maximum latency in milliseconds (ms).
max_pktsize is the maximum packets size.
VC_speed is the VC rate.
The maximum packet size is calculated using the following formula:
Cisco 10000 Series Router Quality of Service Configuration Guide
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"Configuring VC Queue Depth" section on page
For better VC utilization and accuracy, raise the low watermark value (and possibly the high
watermark value).
For better per-queue accuracy, increase the spread between the watermark values.
For better latency, decrease the high watermark value.
For a higher number of cells in the queue or for better TCP performance, increase the high
watermark value.
High_water = ((max latency (ms) – (max_pktsize / VC_speed) ms) * VC_speed) / (53 * 8)
Chapter 15 Oversubscribing Physical and Virtual Links
15-38.
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