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Cisco uBR7200 Series Universal Broadband Router Software Configuration Guide
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Chapter4 Troubleshooting the System Troubleshooting Using Cable Flap Lists
listkeeps track of the cable modemMAC address, up and down transitions, registration events, missed
periodic ranging packets, upstream power adjustments and the physical interface on the
Cisco uBR7200 series.
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:
•Detectsinterface up/down flap; the reinsertion counter counts the number of times a cable modem
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
modemsdon’t respond or don’t 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 modem MAC addresses sorted by flap rate or most recent flap time.
•Showspower adjustment statistics during station maintenance polling; this represents the number
of times the CMTS tells a cable modem 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
•Usesthe 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 cablemodem 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
characteristicpatterns of flapping cable modems, street addresses, and flap statistics can indicate which
amplifieror 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 Sincethe cable flap list was originally developed, polling mechanisms havebeen enhanced
since Cisco IOS Release 11.3.7 NA 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.