Calculating The Default Weight Of A Vc—Pre2; Calculating The Default Weight Of A Vc—Pre3 - Cisco 10000 Series Configuration Manual
Quality of service configuration guide
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Chapter 15
Oversubscribing Physical and Virtual Links
Calculating the Default Weight of a VC—PRE2
For the PRE2, the router calculates the default weight of a VC programmed in the segmentation and
reassembly (SAR) mechanism by using the following formulas:
Tunnel Speed
VC Speed
5 MB or less
Greater than 1937 kbps
All other VC speeds
Greater than 5 MB
Less than 256 kbps
Less than or equal to
4010 kbps
Greater than 4010 kbps
Calculating the Default Weight of a VC—PRE3
For the PRE3, the weight in the SAR mechanism is always programmed to the value 5. The weight of
the VC used in the packet scheduler is based on the committed information rate (CIR) of the VC.
If no weight is assigned to an ATM VC, a default weight is assigned based on the sustained cell rate
(SCR) rate of the VC. The algorithm first checks the VCs with no weight and determines the maximum
and minimum SCR. It then distributes a weight to each VC that is proportional to the VC's SCR. For
example, a VC with an SCR of 50 kbps has a weight that is twice that of a VC with an SCR of 25 kbps.
Because it is not practical to configure weights on some ATM VCs and not on others, either all of the
Note
VCs have VC weights configured or none of them do.
A default weight of 1 is given to non-ATM subinterfaces with no weight assigned to them.
OL-7433-09
Weight Formula
Weight = 255
Weight = 0.1316 * VC speed
The router limits the weight to a maximum of 255. If the calculation
results in a weight that is greater than 255, the router rounds down to
ensure a valid weight.
Weight = 0.0459 * VC speed + 3.25
The weight must be between 5 and 255. If the calculation results in a
weight that is not between 5 and 255, the router rounds up or down to
ensure a valid weight.
This formula is also used to calculate weight for VBR and CBR VCs.
The value of the SCR (VBR) or the value of the PCR (CBR) is the VC
speed used in the calculation.
Weight = 0.0586 * VC speed
Weight = (20 / (Tunnel PCR – 4010)) * VC speed + ((235 * Tunnel PCR
– 4010) / (Tunnel PCR – 4010))
The router limits the weight to a maximum of 255. If the calculation
results in a weight that is greater than 255, the router rounds down to
ensure a valid weight.
Cisco 10000 Series Router Quality of Service Configuration Guide
How the Router Determines VC Weights
15-23
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