Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 7 Troubleshooting the CMTS
Troubleshooting the Cisco uBR7246VXR
Troubleshooting with the Flap List
This section includes suggestions on how to interpret different network conditions based on the flap list
statistics:
• Condition 1: Low miss/hit ratio (< 10% for MC11 card, < 2% for MC16 card), low insertion, low
P-adj, low flap counter and old timestamp.
Analysis: This exhibits an optimal network situation.
• Condition 2: High ratio of misses over hits (> 10%)
Analysis: Hit/miss analysis should be done after the Ins count stops incrementing. In general, if the
hit and miss counts are about the same order of magnitude, then the upstream may be experiencing
noise. If the miss count is greater, then the cable modem is probably dropping out frequently and not
completing registration. The upstream or downstream is perhaps not stable enough for reliable link
establishment. Very low hits and miss counters and high insertion counters indicate provisioning
problems.
Power
Adjustments
(P-Adj)
The station maintenance poll in the Cisco uBR7246VXR constantly adjusts the
cable modem transmit power, frequency, and timing. The P-Adj column
indicates the number of times the router instructs the cable modem to adjust
transmit power more than 3dB. The power adjustment threshold may be set using
the cable flap power threshold command with a value range of 0 to 10 dB and a
default value of 2 dB. Tuning this threshold is recommended to decrease
irrelevant entries in the flap list. Power Adjustment values of 2 dB and below will
continuously increment the P-Adj counter. the cable modem transmitter step size
is 1.5 dB, whereas the headend may command 0.25 dB step sizes. Power
adjustment flap strongly suggests upstream plant problems such as:
• Amplifier degradation
• Poor connections
• Thermal sensitivity
• Attenuation problem
Flap The Flap counter indicates the number of times the cable modem has flapped.
This counter is increased when one of the following events is detected:
Unusual modem insertion or re-registration attempts—The Flap and the Ins
counters are increased when the cable modem tries to reestablish the RF link with
the Cisco uBR7246VXR within a period of time that is less than the
user-configured insertion interval value.
Abnormal Miss/Hit ratio—The Flap counter is increased when N consecutive
Misses are detected after a Hit here N can be configured with a default value of 6.
Unusual power adjustment—The Flap and P-adj counters are increased when
the cable modem's upstream power is adjusted beyond a configured power level.
Time Time is the timestamp indicating the last time the cable modem dropped the
connection (flapped). The value is based on the clock configured on the local
Cisco uBR7200 series. If no time is configured, this value is based on the current
uptime of the Cisco uBR7200 series. When a cable modem meets one of the three
flap list criteria, the Flap counter is increased and Time is set to the current time.
Table 7-2 Flap List Statistics Description (continued)
Statistic Description