Installation guide

5-4
Cisco uBR7100 Series Universal Broadband Router Software Configuration Guide
OL-2238-03
Chapter 5 Troubleshooting the System
Managing Cable Modems on the HFC Network
Tip Be sure you enter the correct slot and port number when you enter the cable interface configuration
mode.
Note If the Cisco uBR7100 series router is reloaded or the ARP table is cleared, all hosts on the network will
be forced to release and renew their IP addresses. Some systems might require restarting if the
IP protocol stack is unable to renew using a broadcast IP address.
Configuring Dynamic Contention Algorithms (Cable Insertion Interval, Range,
and Data Backoffs)
The Cisco uBR7100 series software includes:
• Algorithm that dynamically controls the rate of upstream contention slots —initial ranging and
bandwidth requests.
• Algorithm that varies backoff parameters CMs use within each of the initial ranging and bandwidth
request upstream contention subchannels.
These algorithms control the capacity of the contention subchannel and how efficiently a given
contention subchannel capacity is utilized.
In high contention mode, the Cisco uBR7100 series MAC scheduler uses collision statistics and sustains
a high frequency of initial ranging slots until it detects a steady ranging state. The CMTS dynamically
varies the frequency of initial ranging slots using the data grant utilization on the upstream channels. The
CMTS trades upstream bandwidth between data grants and initial ranging slots. The CMTS autodetects
a high collision state and switches to low insertion interval mode after a steady state is achieved where
few collisions occur.
The CMTS is careful when monitoring the ranging channel health to revert to a steady state. In steady
state mode, data grants—grant utilization—receive preference over initial ranging slots.
Although the binary exponential backoff algorithm operates in a distributed fashion at different CMs,
the CMTS provides centralized control for the backoff algorithm. To achieve this, it remotely monitors
traffic load—the backlog developing on the contention channel—and then varies the backoff start and
end specified in the MAPs for that upstream channel. This ensures colliding CMs are properly
randomized in time.
The following cable interface commands are available to configure the dynamic contention algorithms:
[no] cable insertion-interval [automatic [Imin [Imax]]] | [msecs]
[no] cable upstream port num range-backoff [automatic] | [start end]
[no] cable upstream port num data-backoff [automatic] | [start end]
Tip System defaults are to have dynamic ranging interval enabled, dynamic ranging backoff enabled, and
fixed data backoffs for each upstream of a cable interface.
The default
automatic insertion interval setting enables the Cisco automatic initial ranging period
algorithm where lower and upper default values of 50 msecs and 2 secs are used. The default
automatic
range-backoff
enables the dynamic backoff algorithm.