Cisco 10000 Series User Manual

15-23
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
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:
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.
Note Because it is not practical to configure weights on some ATM VCs and not on others, either all of the
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.
Tunnel Speed VC Speed Weight Formula
5 MB or less Greater than 1937 kbps Weight = 255
All other VC speeds 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.
Greater than 5 MB Less than 256 kbps 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.
Less than or equal to
4010
kbps
Weight = 0.0586 * VC speed
Greater than 4010 kbps 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 Specifications

General

Brand	Cisco
Model	10000 Series
Category	Network Router
Language	English