Specifications

The maximum rate that frequency hopping will occur (in
seconds)
Corr FEC
Errors The number of correctable forward error corrections (FEC)
errors on this upstream port. FECs measure noise.
Uncorr FEC
Errors The number of uncorrectable FEC errors on this upstream
port
You can use the show cable hop command to check for correctable and uncorrectable FEC errors on a
particular interface. Counters should have a low value. High or rapidly increasing uncorrectable errors
typically indicates a problem with noise within the RF plant. If this is the case, check the received upstream
SNR level.
Finally use the ping docsis command to verify the layer two connectivity to the cable modem:
VXR#ping docsis ?
A.B.C.D Modem IP address
H.H.H Modem MAC address
Note: Use this command to ping either the modem IP or MAC address:
VXR#ping docsis 10.1.61.3
Queueing 5 MAC−layer station maintenance intervals, timeout is 25 msec:
!!!!!
Success rate is 100 percent (5/5)
VXR#
Using the Flap List for Diagnosing RF Problems
One of the most powerful tools on the CMTS for diagnosing RF problems on cable networks is the show
cable flap−list command. To assist in locating cable plant problems the CMTS maintains a database of
flapping cable modems . In this document we highlight the most important things that are used by this feature
from a practical point of view. For very detailed information about the flap list feature, please read Flap−List
Troubleshooting for the Cisco Cable Modem Termination System, which contains all you want to know about
this feature.
Below is a sample output from the show cable flap−list command. Note that an asterisk appears in the power
adjustment field when an unstable return path for a particular modem has been detected and a power
adjustment has been made. An exclamation point appears when so many power adjustments have been made
that the modem has reached its maximum power transmit level and cannot increase it any further. Both of
these indicate a problem in the RF plant.
VXR#sh cable flap−list
MAC Address Upstream Ins Hit Miss CRC P−Adj Flap Time
0001.64ff.e47d Cable6/1/U0 0 20000 1 0 *30504 30504 Oct 25 08:35:32
0001.9659.47bf Cable6/1/U0 0 30687 3 0 *34350 34350 Oct 25 08:35:34
0004.27ca.0e9b Cable6/1/U0 0 28659 0 0 !2519 2519 Oct 23 16:21:18
0020.4086.2704 Cable6/1/U0 0 28637 4 0 2468 2468 Oct 23 16:20:47
0002.fdfa.0a63 Cable6/1/U0 0 28648 5 0 2453 2453 Oct 23 16:21:20
*
Means that a power adjustment has been made
! Indicates that a cable modem increased its power level to the maximum
level. For Cisco cable modems that equals 61 dBmV.
Cisco − Determining RF or Configuration Issues On the CMTS