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
Hit and Miss The HIT and MISS columns are keepalive polling statistics between the Cisco
uBR7200 series and the cable modem. The station maintenance process occurs
for every modem approximately every 25 seconds. When the router receives a
response from the cable modem, the event is counted as a Hit. If the router 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
increased 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 cable
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 7-2 Flap List Statistics Description (continued)
Statistic Description