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cable load-balance group (global configuration)
In a system with balanced loads, the interfaces will contain the same number of cable modems only when
Tip
the interfaces are configured with the same modulation parameters.
Service Flow Method
The service-flows method of load balancing uses the number of active service flow IDs (SFIDs) on an
interface to determine the current load. This is a form of distribution-based load balancing, in which the
absolute numbers of service flows are used to determine whether interfaces are load balanced.
This method does not take into account the amount of traffic flowing on each SFID, but the system does
take into account the relative bandwidth of the channels being used, so that channels with higher
bandwidths are allocated higher numbers of SFIDs. This means that when interfaces are using different
channel widths or modulation profiles, the system can assign different numbers of SFIDs to the
interfaces to achieve a balanced load. For example:
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When both the channel width and different modulation profile are set differently on two interfaces, the
system calculates a "weight" value to use as a guide to determine the relative bandwidths of the
interfaces.
In a system with balanced loads, the interfaces will contain the same number of SFIDs only when the
Tip
interfaces are configured with the same modulation parameters.
Utilization Method
The utilization method uses an interface's current percentage of utilization to determine the current
load. This method uses the amount of traffic being sent over an interface, in the form of the percentage
of total bandwidth being used. (To avoid unnecessary movement of cable modems, the utilization
method does not perform load balancing until an interface is at least 25 percent of utilization.)
When using the utilization method, the system takes into account the relative throughput and bandwidth
(as determined by the modulation profiles and channel widths) of each interface when evaluating the load
on those interfaces. For example, if two upstreams are being load-balanced using the utilization method,
and the first upstream has twice the bandwidth of the second upstream, the two upstreams are considered
Cisco Broadband Cable Command Reference Guide
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Channel widths— For example, if two upstreams are being load balanced, and one upstream is
configured with a channel width of 1.6 MHz and the other upstream is configured for a channel
width of 3.2 MHz, the Cisco CMTS allocates twice as many SFIDs to the second upstream, because
its channel width is twice as large as the first upstream's channel width.
Modulation profiles— For example, if one downstream is configured for 64-QAM and the other
downstream is configured for 256-QAM, the Cisco CMTS allocates a proportionately larger number
of SFIDs to the second downstream so as to achieve a balanced load.
Do not use the utilization method of load balancing on cable interfaces that have a small
Note
number of cable modems and where a single modem is responsible for the majority of the
interface load. In this condition, the Cisco CMTS could end up continually moving cable
modems from one interface to another in an endless attempt to load balance the interfaces.
To avoid this, configure the utilization threshold to a value that is higher than what can be
caused by any single cable modem.
Chapter 2
Cisco CMTS Configuration Commands
OL-1581-08
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