Feature Overview
8Cisco IOS Release 12.0(7)T
The Cisco uBR900 series complies with the DOCSIS standards for interoperable cable access
routers; it supports full transparent bridging as well as DOCSIS-compliant transparent bridging.
Note If the attached CPE devices and the coaxial cable interface are in different IP subnets, the
cable interface must have a secondary address.
Figure 2 Cisco uBR900 Series Cable Access Router in a Bridging Configuration
DOCSIS-compliant transparent bridging is the factory default configuration of the Cisco uBR900
series cable access router. If your cable service provider is using a DHCP server, all you need to do
is connect the cables and power on the cable access router; your service provider’s configuration
program will automatically configure both the coaxial cable interface and the bridging functionality.
You do not need to set up IP addresses for the attached PCs or enter any Command Line Interface
(CLI) configuration commands. This type of operation is called plug-and-play bridging.
In DOCSIS-compliant bridging mode, the cable access router is able to locate a downstream and
upstream channel; find the TOD, TFTP, and DHCP server(s); obtain an IP address; download a
DOCSIS configuration file; and obtain DHCP parameters to work in a bridging mode.
You can configure a customized bridging application on the CiscouBR900 series using a
downloadable configuration file or the CLI. See the sections “Configuring Bridging” on page41 and
“Customizing the Cable Access Router Interface” on page44 for details.
Routing Applications
The Cisco uBR900 series cable access router can be configured to act as a router to preserve IP
address space and limit broadcasts that can impact the performance of the network. A typical use
would be if you are connecting the cable access router to an internal Ethernet hub that is connected
to an existing PC network. The Cisco uBR900 series supports Routing Information Protocol
Version2 (RIP V2) for this application.
When configured in routing mode, the Cisco uBR900 series is automatically configured to use the
headend’s IP address as its IP default gateway. This allows the cable access router to send packets
not intended for the Ethernet interface to the headend when IP host-routing is configured.
RIP V2 routing is useful for small internetworks in that it enables optimization of Network Interface
Center (NIC)-assigned IP addresses by defining variable-length subnet masks (VLSMs) for network
addresses, and it allows classless interdomain routing (CIDR) addressing schema.
Cisco uBR7200 series
CMTS
CATV
coaxial cable
HFC network
Cisco uBR900 series
cable access router
Ethernet
13305
Ethernet
Ethernet
Ethernet
PC
PC
PC or hub
PC