Troubleshooting guide

Troubleshooting the CMTS
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5. If the cable interface is receiving a DHCP discover message from the CM,
determine if the cable interface is forwarding DHCP discover messages to the
DHCP server by checking if the DHCP sever is receiving DHCP discover
packets and if it is sending DHCP offer packets to the CM.
a.
If the DHCP server is receiving DHCP discover messages from the
CM, the DHCP server can reply by sending a DHCP offer. However, if
the DHCP server is not receiving DHCP discover messages from the
CM through the cable interface, the cable helper IP address may not be
configured properly. Refer to Step 6 for more information.
b. If the DHCP server is receiving DHCP discover messages from the
CM, but is not sending DHCP offer messages, the DHCP server is
misconfigured. Make sure that the DHCP is configured to service the
subnet associated with the CM.
c. If the CM receives a DHCP offer, the CM sends a DHCP request
message to the DHCP server so that it can use the information received
in the offer message. When the DHCP server acknowledges the
request, it provisions the CM with an IP address so that the CM can use
TFTP to continue the registration process. If the CM is able to obtain
an IP address, but is unable to complete the registration process, make
sure that the TFTP server IP address and CM configuration file name,
including the full path from the TFTP root directory, are properly
configured.
6. If there is no communication between the DHCP server and the cable
interface, the CM does not have an IP address. Determine if DHCP parameters
are set correctly on the cable interface. The ip helper-address command is
used to disassemble a cable modem’s DHCP broadcast packet, and reassemble
the DHCP broadcast packet into a unicast packet so that the packet can
traverse through the router and communicate with the DHCP server.
To configure the helper IP address for the cable modems (CMs) to forward
only UDP broadcasts, use the ip helper-address cable-modem command in
Interface Configuration mode, as shown in the following example:
RDN(config-if)#ip helper-address <ip-address> cable-modem
where:
ip-address is the IP address of the destination DHCP server