Restrictions for Flap List Troubleshooting
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The Flap List Troubleshooting feature can be used only with two-way cable modems. The flap-list does
not support telco-return cable modems or set-top boxes.
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 can go offline faster than the frequency hop
period, which can cause the frequency to stay fixed while cable modems go offline. To compensate for
this, reduce the hop period to 10 seconds.
Note
Information About Flap List Troubleshooting
This section describes the following information about the Flap List Troubleshooting feature:
Feature Overview
The Flap List Troubleshooting is a patented tool that is incorporated in the Cisco IOS software for the Cisco
Cable Modem Termination System (CMTS) routers. The flap list tracks “flapping” cable modems, which are
cable modems that have intermittent connectivity problems. A flapping cable modem can indicate either a
problem with that particular cable modem, or it could indicate an RF noise problem with the upstream or
downstream portion of the cable plant.
The flap-list feature supports any cable modem that conforms to the Data-over-Cable Service Interface
Specifications (DOCSIS) because it does use any special messaging to poll cable modems or to request any
special information from them. Instead, this feature monitors the normal registration and station maintenance
activity that is already performed over a DOCSIS cable network.
This allows the Cisco CMTS to collect the flap-list data without generating additional packet overhead and
without impacting network throughput and performance. It also means that although the Flap List
Troubleshooting feature is a proprietary feature for Cisco CMTS routers, it is compatible with all
DOCSIS-compliant cable modems. In addition, unlike other monitoring methods that use the Simple Network
Management Protocol (SNMP), the flap list uses zero bandwidth.
Information in the Flap List
The Flap List Troubleshooting feature tracks the following situations:
• Reinsertions—A reinsertion occurs when the cable modem re-registers more frequently than the
user-specified insertion time. A pattern of reinsertions can indicate either potential problems in the
downstream or that the cable modem is being improperly provisioned.
• Hits and Misses—A hit occurs when a cable modem successfully responds to the station maintenance
messages (MAC-layer “keepalive” messages) that the Cisco CMTS sends out to conform to the DOCSIS
standard. A miss occurs when the cable modem does not respond to the request within the user-specified
Cisco cBR Series Converged Broadband Routers Troubleshooting and Network Management Configuration Guide
for Cisco IOS XE Fuji 16.8.x
197
Flap List Troubleshooting
Restrictions for Flap List Troubleshooting
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