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Configuring Subscriber-End Broadband Access Router Features
Subscriber-End Broadband Access Router Data Operations
MC-614
Cisco IOS Multiservice Applications Configuration Guide
Partitioning the upstream plant into smaller segments significantly reduces the number of potential
ingress sources and failure points. The CMTS divides the cable plant into downstream channels and
upstream segments or clusters of nodes.
Downstream and Upstream Data Transfer
When operating normally, the Cisco uBR900 series cable access router receives data addressed to it from
the CMTS. The router reads the address in the header of the message, filters the message, and forwards
it to the appropriate device at the subscriber site.
Note Bandwidth at the subscriber site is shared by the active data users connected to the network
segment.
For upstream data transfer, the Cisco cable access router uses a request/grant mechanism to obtain
upstream bandwidth. The CMTS configures, via MAC messages, upstream parameters associated with
transmissions from all Cisco cable access routers on the system. Service class registration is granted
based on class assignment and load provisioning. Upstream channels are time-slotted and divided into
basic scheduling time units.
The CMTS informs the Cisco cable access router of minislot structures on the upstream channel. Some
minislots are marked as contention-based—shared by routers to make bandwidth (timeslot) requests
with the CMTS. Others are grouped into unicast grants for specific routers to send their data bursts. Yet
others are grouped into maintenance slots for keepalive messages from routers to the CMTS.
Bridging Applications
In bridging applications, the Cisco uBR900 series cable access router acts as a transparent bridge for up
to 254 devices, depending on the version of Cisco IOS software you are using. Older versions of software
allow a maximum of three CPE devices to be bridged. The cable access router is connected to the Internet
through the coaxial cable interface. All four 10BaseT Ethernet ports are treated as one Ethernet interface
by the Cisco IOS software. The IP addresses for the CPE devices and the coaxial cable interface are
typically in the same subnet, although this is not a requirement.
Figure 111 shows the Cisco uBR900 series cable access router in a typical bridging environment. The
Cisco uBR900 series complies with the DOCSIS standards for interoperable cable access routers; it
supports full transparent bridging and 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.