Installation guide

5-36
Cisco uBR7100 Series Universal Broadband Router Software Configuration Guide
OL-2238-03
Chapter 5 Troubleshooting the System
Troubleshooting Cable Flap Lists
Hit and Miss The HIT and MISS columns are keepalive polling statistics between the
Cisco uBR7100 series and the cable modem. The station maintenance
process occurs for every modem approximately every 25 seconds. When the
CMTS receives a response from the modem, the event is counted as a Hit.
If the CMTS does not receive a response from the cable modem, the event
is counted as a Miss. A cable modem will fail to respond either because of
noise or if it is down. Modems which only log Misses and zero Hits are
assumed to be powered off.
Misses are not desirable since this is usually an indication of a return path
problem; however, having a small number of misses is normal. The flap
count is incremented if there are M consecutive misses where M is
configured in the cable flap miss-threshold parameter. The parameter value
ranges from 1 to 12 with a default of 6.
Ideally, the HIT count should be much greater than the Miss counts. If a
modem has a HIT count much less than its MISS count, then registration is
failing. Noisy links cause the MISS/HIT ratio to deviate from a nominal
1% or less. High Miss counts can indicate:
• Intermittent upstream possibly due to noise
• Laser clipping
• Common-path distortion
• Ingress or interference
• Too much or too little upstream attenuation
Cyclical Redundancy
Check (CRC)
This statistic tracks the CRC error counter per modem. CRC errors are
usually an indication of noise on a plant. A low count can be always be
expected but a high CRC number calls for some the plant troubleshooting.
The CRC counter indicates:
• Intermittent upstream
• Laser clipping
• Common-path distortion
• Impulsive noise or interference
Table 5-8 Flap List Statistics Description (continued)
Statistic Description