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Cisco uBR924 Software Configuration Guide
OL-0337-05 (8/2002)
Chapter 2 DOCSIS-Bridging Configuration
DHCP Server Configuration
Caution Before attempting to reconfigure the Cisco uBR924 cable access router at a subscriber site, contact your
provisioning or billing system administrator to ensure remote configuration is allowed. If remote
configuration is disabled, settings you make and save at the local site will not remain in effect after the
Cisco uBR924 router is powered off and on. Instead, the router’s settings will return to the previous
configuration.
DHCP Server Configuration
The DOCSIS specification (SP-RFI-IO5-991105 or later revision) requires that a DOCSIS-compliant
cable modem connect to a DHCP server at power-on or reset to establish temporary IP connectivity with
the cable network. This enables the cable modem to download the additional configuration information
needed to establish a permanent connection with the headend and cable network.
The DHCP server can be a CMTS with DHCP server capabilities (such as a Cisco uBR7200 series
universal broadband router), or it can be a dedicated server located at the headend. The server can be
configured manually for each cable modem or it can be part of an automated provisioning system such
as Cisco Network Registrar (CNR).
Note The DOCSIS specification requires that every DOCSIS cable modem obtain its IP address from an
authorized DHCP server during the reset or power-on provisioning process. Any IP address specified in
an IOS configuration file is overwritten by the one assigned by the DHCP server. The only way to assign
a static IP address to a cable modem is to configure the DHCP server so that it assigns the desired IP
address on the basis of the cable modem's MAC address. However, service providers should warn
subscribers that changes in the cable network's topology—due to traffic levels, growth, or changes to the
cable plant and other hardware—might still require changing the subnets and IP addresses assigned to a
particular cable modem.
The DHCP server provides the information shown in Table 2-1 to each cable modem.
Table 2-1 DHCP Server Parameters
Parameter Description
IP address for the cable modem’s cable
interface
This IP address typically is assigned dynamically but the service provider can
also statically assign IP addresses on the basis of each modem’s MAC address.
Note When the router is in DOCSIS-bridging mode, it automatically assigns
this IP address to both the cable and Ethernet interfaces. When the router
is in routing mode, it assigns this IP address only to the cable interface;
the IP address for the Ethernet interface must be configured separately.
IP subnet mask for the cable modem’s
cable interface
This subnet mask typically is used for all cable modems using the same
downstream, but this depends on the setup of the CMTS network as well as
subscribers’ needs.
IP address for the TFTP server This TFTP server provides the DOCSIS configuration file to the cable modem
and is typically a dedicated server located at the headend.