Installation guide

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Cisco uBR7100 Series Universal Broadband Router Software Configuration Guide
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Chapter 5 Troubleshooting the System
Troubleshooting Cable Flap Lists
Note Although this is a Cisco proprietary CMTS feature, it is compatible with all DOCSIS-compliant cable
modems. Unlike SNMP, the flap list uses zero bandwidth.
The flap list collects the following station maintenance statistics:
• Detects interface up/down flap; the reinsertion counter counts the number of times a cable interface
comes up and inserts into the network. This helps identify potential problems in the downstream
because improperly provisioned cable modems tend to try to reestablish a link repeatedly.
• Detects intermittent upstream; the keepalive hits versus misses is the number of times cable modems
do not respond or do not respond to the MAC layer keepalive messages. If there are a number of
misses, this points to a potential problem in the upstream.
• Lists cable interface MAC addresses sorted by flap rate or most recent flap time.
• Shows power adjustment statistics during station maintenance polling; this represents the number of
times the CMTS tells a cable interface to adjust the transmit power more than 3 dB. If constant
power adjustments are detected, this usually indicates a problem with an amplifier. By looking at
the cable modems in front and behind various amplifiers, you can find the source of failure.
The cable system administrator typically:
• Sets up a script to periodically poll the flap list, for example, every 15 minutes
• Uses the resulting data to perform trend analysis to identify the cable modems that are consistently
in the flap list
• Queries the billing and administrative database for cable interface MAC address-to-street address
translation and generates a report
These reports can be given to the Customer Service Department or the cable plant’s Operations and
Maintenance Department. Using these reports, maintenance personnel can quickly discern how
characteristic patterns of flapping cable modems, street addresses, and flap statistics can indicate which
amplifier or feeder lines are faulty. The reports also help you quickly discern whether problems exist in
your downstream or upstream path, and whether the problem is ingress noise or equipment related.
Default values for the following flap-list configuration commands are:
• cable flap-list miss-threshold—6 seconds
• cable flap-list power-adjust—2 dB
• cable flap-list insertion-time—180 seconds
Note Since the cable flap list was originally developed, polling mechanisms have been enhanced to have an
increased rate of 1/sec when polls are missed. Cable modems go offline faster than the frequency hop
period. This can cause the frequency to stay fixed while cable modems go offline. To compensate for
this, as appropriate, you can reduce the hop period to 10 seconds.
Tip The system supports automatic power adjustments. The show cable flap-list and show cable modem
commands indicate when the Cisco uBR7100 series CMTS has detected an unstable return path for a
particular modem, and has compensated with a power adjustment. An asterisk (*) appears in the power
adjustment field for a modem when a power adjustment has been made; an exclamation point (!) appears
when the modem has reached its maximum power transmit level and cannot increase its power level any
further.