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Chapter 2
Cisco CMTS Configuration Commands
cable default-phy-burst
To specify a value for the upstream Maximum Traffic Burst parameter for CMs that do not specify their
own value, use the cable default-phy-burst command in cable interface configuration mode. To reset
the maximum burst size to its default, use the no form of this command.
Syntax Description
burst-size
Defaults
2000 bytes
Command Modes
Interface configuration (cable interface only)
Command History
Release
12.2(8)BC1
12.2(15)BC2
Usage Guidelines
The DOCSIS 1.0 specification allows CMs to register without specifying a maximum upstream burst
size, or to register with a value of 0, which means an unlimited burst size. This behavior can interfere
with DOCSIS 1.1 networks because excessively large bursts on an upstream will generate unpredictable
jitter and delay in voice calls. DOCSIS 1.1 CMs can also cause this problem if they register without
enabling fragmentation of packets at the DOCSIS MAC layer.
This command allows you to specify a default burst size for CMs that register without specifying a burst
size or that register with a burst size of 0. It also specifies the maximum size of long data grants if a CM
specifies a size of 0 (unlimited) in the Upstream Channel Descriptor (UCD) packet. If a CM specifies a
long data grant that would exceed the maximum burst size, and the CM is not using DOCSIS
concatenation, the DOCSIS scheduler drops the bandwidth request.
To prevent CMs from registering without defining a burst profile, use this command with a burst-size of
zero. CMs that do not define a burst profile will not be allowed to register and come online.
OL-1581-08
cable default-phy-burst burst-size
no cable default-phy-burst
Modification
This command was introduced for the Cisco uBR7100 series,
Cisco uBR7200 series, and Cisco uBR10012 routers.
This command is superceded by the
command.
Specifies the maximum number of bytes that are allowed in a burst.
The valid range is 0 to 4096 bytes, with a default of 2000 bytes. A
value of 0 specifies that CMs cannot register unless they specify a
valid burst size.
Do not set burst-size to less than 1522 bytes because the
Note
DOCSIS 1.1 specification requires an absolute minimum
value of 1522 bytes for this parameter.
Cisco Broadband Cable Command Reference Guide
cable default-phy-burst
cable upstream fragment-force
2-37
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