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2-3
Cisco uBR924 Software Configuration Guide
OL-0337-05 (8/2002)
Chapter 2 DOCSIS-Bridging Configuration
DOCSIS Configuration File
After making a successful DHCP request, the cable modem contacts the ToD server to get the current
date and time. It also begins the TFTP download of the DOCSIS configuration file, which is described
in the next section, “DOCSIS Configuration File” section on page 2-3.
Note At this point in the registration process, the DHCP server provides an IP address only for the cable
modem, not for the CPE devices it is connecting to the network. The same DHCP server can provide the
IP addresses for the CPE devices after the cable modem goes online, or the cable modem itself can be
configured as a DHCP server (see “Routing with DHCP Server” section on page 3-4).
DOCSIS Configuration File
The DOCSIS specification requires that a DOCSIS-compliant cable modem download a DOCSIS
configuration file during its power-on or reset sequence. This file must be in the format described in the
SP-RFI-IO5-991105 specification (or later revision) and must contain the information shown in
Table 2-2.
Note The parameters shown in Table 2-2 are organized according to the categories used in the Cisco DOCSIS
Cable Modem Configuration tool, which is available on CCO at
http://www.cisco.com/support/toolkit/CableModem. (You must have an account on CCO to access this
tool.)
IP address for the DHCP relay agent A DHCP relay agent is required if the DHCP server is located on a different
network than the IP address assigned to the cable modem’s cable interface. The
DHCP relay agent is also used if the DHCP server is providing IP addresses to
the CPE devices connected to the cable modem and the CPE devices are on a
different subnet than the cable modem.
Complete filename for the DOCSIS
configuration file
This is the filename for the DOCSIS configuration file that the cable modem
should download from the TFTP server.
IP address for one or more time of day
(ToD) servers
The cable modem uses the ToD server to get the current date and time so that it
can accurately timestamp its SNMP messages and error log entries.
One or more IP addresses for the routers
that will forward IP traffic from the cable
modem
Typically, the CMTS acts as the default gateway for the cable modem.
Note Typically, the DHCP server sets the default gateway for DOCSIS cable
modems. When this is done on Cisco routers, the default gateway does
not appear in the Cisco IOS configuration file, to indicate that the
gateway is being set dynamically by the DHCP server and should not be
saved after a reset of the router. To display the default gateway, use the
show ip default-gateway command.
One or more IP addresses for System Log
(SYSLOG) servers
The cable modem can send its error log messages to the SYSLOG servers, which
are optional and typically located at the headend.
Table 2-1 DHCP Server Parameters (continued)
Parameter Description