Troubleshooting guide

7-25
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 7 Troubleshooting the CMTS
Troubleshooting the Cisco uBR7246VXR
Monitoring the Cisco uBR7246VXR Flap List
The Cisco uBR7200 series maintains a database of flapping cable modems to assist in locating cable
plant problems. The flapping cable modem detector tracks the upstream and downstream performance
of all cable modems on the network, without impacting throughput and performance, or creating
additional packet overhead on the broadband network. The cable modem flap list keeps track of the cable
modems MAC address, up and down transitions, registration events, missed periodic ranging packets,
upstream power adjustments and the physical interface on the Cisco uBR7200 series.
The flap list feature 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 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 modems don'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.
Shows power adjustment statistics during station maintenance polling
This represents the number of times the Cisco uBR7246VXR 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
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 modem MAC address-to-street address
translation and generates a report
Using these reports, you 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.