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
Caution Running debug commands on a uBR (Universal Broadband Router) with more than a handful of
modems may cause the Cisco uBR7246VXR to halt the system in order to keep up with the
debugging. In this case, all the cable modems may lose sync and debugging will be useless.
If no packets are seen through debug messages, check the configuration of the cable helper-address
statement on the cable interface to which this modem is attached. If this is configured correctly and a
packet trace of the DHCP server subnet also reveals no DHCP packets from the cable modem, then a
good place to look is the output errors of the cable modem's cable interface or the input errors of the
cable interface of the Cisco uBR7246VXR. It might be a good idea to boost the transmitter power of the
MTA a bit more with more attenuation.
If packets are seen to be transmitted onto the DHCP server subnet, it would be a good idea to double
check the cable modem debug messages to see if there are parameter request or assignment errors. This
would be the stage of troubleshooting where you should investigate the routing between the MTA and
the DHCP server. It would also be advisable to double-check the DHCP server configuration and the
DHCP logs.
Below is a sample debug taken at the MTA by running the debug cable-modem mac log verbose
command:
1w3d: 865015.920 CMAC_LOG_RANGING_SUCCESS
1w3d: 865015.920 CMAC_LOG_STATE_CHANGE dhcp_state
1w3d: 865053.580 CMAC_LOG_RNG_REQ_TRANSMITTED
1w3d: 865053.584 CMAC_LOG_RNG_RSP_MSG_RCVD
1w3d: 865055.924 CMAC_LOG_WATCHDOG_TIMER
131.CABLEMODEM.CISCO: 1w3d: %UBR900-3-RESET_DHCP_WATCHDOG_EXPIRED:
Cable Interface Reset due to DHCP watchdog timer expiration
1w3d: 865055.924 CMAC_LOG_RESET_DHCP_WATCHDOG_EXPIRED
1w3d: 865055.924 CMAC_LOG_STATE_CHANGE reset_interface_state
1w3d: 865055.924 CMAC_LOG_DHCP_PROCESS_KILLED
1w3d: 865055.924 CMAC_LOG_STATE_CHANGE reset_hardware_state
As you can see, the above the DHCP process failed and the cable modem was reset.
DHCP - init(i) state
Once a reply to the DHCP request has been received and an IP address assigned to the cable modem the
next show cable modem command gives its state as init(i):
#show cable modem
In the output above, the MTA never gets beyond state init(i). Repetitive show cable modem displays will
usually show the cable modem cycling between init(r1), init(r2), init(rc), init(d) and init(i) indefinitely.
Following is a list of the more common reasons for a cable modem not getting further than init(i):
• Incorrect or invalid DOCSIS file specified in the DHCP server
• TFTP server issues, for example: incorrect IP address, TFTP server unreachable
• Problems getting TOD or Timing Offset
• Incorrect Router setting in the DHCP configuration
Interface Prim
Sid
Online
State
Timing
Offset
Rec
Power QoS CPE IP address MAC address
Cable2/0/U0 7 init(i) 2815 -0.25 2 0 10.1.1.20 0030.96f9.65d9