Qos Policy Propagation On Border Gateway Protocol; Overview Of Docsis-Compliant Downstream Signals - Cisco uBR10012 Software Configuration Manual

Appendix A DOCSIS and CMTS Architectural Overview

QoS Policy Propagation on Border Gateway Protocol

BGP typically operates between the cable operator's regional network and external networks, providing
routing information exchange between different networks. The Open Shortest Path First (OSPF) protocol
is used in regional networks usually. For additional explanation of BGP in the context of DOCSIS NTSP
cable plants, refer to "Overview of DOCSIS NTSC Cable Plants" section on page A-2.
The Policy Propagation feature is a packet classification feature that provides a powerful, scalable means
of utilizing BGP attributes to propagate destination-based packet classification policy throughout a large
network via BGP routing updates.
IP precedence classes or QoS group IDs are associated with BGP community values, and in turn
customers' prefixes are tagged with appropriate community values based on the class of service they
have purchased from the network operator.
Normal BGP protocol operation then performs path selection, and the community value is mapped to the
associated IP precedence class and installed in the express forwarding table along with the associated routing
prefixes. Subsequent packets express forwarded to the selected destination prefixes are then tagged with the
appropriate IP precedence value. Thus, packet classification policy can be propagated by scale via BGP
without writing and deploying complex access lists at each of a large number of routers, which in turn ensures
that return traffic to premium customers is handled as premium traffic by the network.

Overview of DOCSIS-Compliant Downstream Signals

Downstream signals are modulated using QAM-64 or QAM-256 quadrature amplitude modulation,
based on the cable interface card used, your cable plant, and the significance of the data. DOCSIS
defines the messages and data types for CMTS to CM (or CM in an STB) communications. All CMs
listen to all frames transmitted on the downstream channel on which they are registered and accept those
where the destinations match the units themselves or the devices that each CM supports.
The Cisco uBR10000 series CMTS supports multicast groups using standard protocols such as Protocol
Independent Multicast (PIM), Distance Vector Multicast Routing Protocol (DVMRP), and Internet
Group Management Protocol (IGMP) to determine if multicast streams are to be forwarded to a
prescribed downstream CM or STB, or to a multicast routing peer.
The Cisco uBR10000 series software periodically sends MAC allocation and management
messages—known as MAPs—to all CMs on the network, defining the transmission availability of
channels for specific periods of time. The MAP rate is fixed—every 2 milliseconds.
Different transmission intervals are defined that associate an interval with a service identifier (SID).
SIDs define the devices allowed to transmit, and provide device identification and class of service
management. Software defines what type of transmission is allowed during the interval.
The CMTS system administrator typically assigns one or more SIDs to each CM, corresponding to the
classes of service the CM requires. Each MAP is associated with a particular upstream channel. The SID
concept supports multiple data flows and use of protocols that allow IP backbone QoS features to be
extended to the CMTS. The CMTS schedules the times granted for sending and receiving packets, and
if defined, manipulates the type of service (ToS) field in the IP packet header to accommodate QoS.
Note Cisco IOS Release 12.2XF software supports extensions to DOCSIS 1.0 to operate with
DOCSIS 1.0-based CMs or cable RF CPE devices (such as Cisco uBR924 cable access routers or
Cisco uBR910 cable data service units) that also support DOCSIS 1.0 extensions.
DOCSIS 1.0 extensions build intelligence into the MAP file, which the CMTS sends to voice-enabled
CMs to address jitter and delay. The extensions support unsolicited grants that are used to create a
constant bit-rate-like stream between the CMTS and the CM. This is in contrast to typical data
applications where CMs request grants from the CMTS before they can transmit upstream.
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Cisco uBR10012 Universal Broadband Router Software Configuration Guide
DOCSIS Specification Summary
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