Configuring Headend Broadband Access Router Features
Troubleshooting Using Cable Flap Lists
MC-603
Cisco IOS Multiservice Applications Configuration Guide
Troubleshooting Using Cable Flap Lists
A cable flap list is a table maintained by the Cisco uBR7200 series for every cable modem (active or not
active) that is having difficulties communicating with the headend. Flapping refers to the rapid
disconnecting and reconnecting of a cable modem. The flap list contains cable modem MAC addresses
and logs the time of the most recent activity. You can configure the size, recording duration, and entry
thresholds for the flap list.
The following tips and scenarios allow you to use the flap list in the most effective way:
• If a subscriber cable modem shows substantial flap list activity, it is having communication
problems.
• If a subscriber’s cable modem shows little or no flap list activity, it is communicating reliably; the
problem is probably in the subscriber computer equipment or in the connection to the cable modem.
• The top 10 percent most active cable modems in the flap list are most likely to have difficulties
communicating with the headend.
• Cable modems with more than 50 power adjustments per day have a suspect upstream path.
• Cable modems with approximately the same number of hits and misses and with many insertions
have a suspect downstream path (for example, low level into the cable modem).
• All cable modems incrementing the insertion at the same time indicates a provisioning server
failure.
• Cable modems with a high number of CRC errors have unstable upstream paths or in-home wiring
problems.
• Correlating cable modems on the same physical upstream port with similar flap list statistics can
quickly isolate outside plant problems to a particular node or geography.
• Monitoring the flap list cannot affect cable modem communications.
• The flap list should be “dumped” to a database computer and cleared at least once a day.
• Important upstream performance data can be obtained by tracking flap list trend data.
• Important installation quality control and performance data is directly available from the flap list.
Setting Cable Flap List Aging
You can specify the number of days to record and retain flapping activity on cable modems currently in
the flap list table. This value is known as the age of the flap list. The valid range is from 1 to 60 days.
To set the age of the flap list, use the following command in global configuration mode:
Command Purpose
CMTS01(config)# cable flap-list aging days
Specifies the number of days to record and retain flapping
activity for the cable modems connected to this Cisco uBR7200
series router.
