Cisco Broadband Cable Command Reference Guide

cable default-phy-burst
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Cisco Broadband Cable Command Reference Guide
Chapter 2 Cisco Cable Modem Termination System Commands
The default PHY burst parameter interacts with two other parameters that are configured in the DOCSIS
configuration file:
DOCSIS 1.1 configuration files can also specify a value for the maximum concatenation burst,
which has a default of 1522 bytes. If this parameter is greater than the default PHY burst size, the
CM can override the default PHY burst when it is using concatenation.
DOCSIS 1.0 and 1.1 configuration files can also specify a maximum transmit burst size. In DOCSIS
1.1 configurations, this parameter defaults to 3044 bytes, with a minimum of 1522 bytes. DOCSIS
1.0 configuration files could set this parameter to a value lower than 1522 bytes, depending on what
version of the DOCSIS 1.0 specification they support. The CMTS will enforce the lower limit
between the default phy burst and the maximum transmit burst, so you should ensure that all
configuration files in your network specify a minimum of 1522 bytes for the maximum transmit
burst size.
Examples The following example shows the default maximum burst size to 1522 bytes, which is the minimum size
required by the DOCSIS 1.1 specification:
Router(config)# interface cable 5/1
Router(config-if)# cable default-phy-burst 1522
Router(config-if)#
The following example shows the default maximum burst size to 0, which means that a CM must specify
a valid burst profile before the Cisco CMTS allows it to register and come online:
Router(config)# interface cable 5/1
Router(config-if)# cable default-phy-burst 0
Router(config-if)#
The following example shows the default maximum burst size being reset to its default of 2000 bytes:
Router(config)# interface cable 5/1
Router(config-if)# no cable default-phy-burst
Router(config-if)#
Related Commands Command Description
show cable modem Displays information for the registered and unregistered
CMs, including the QoS configuration for individual CMs.
show controllers cable Displays the downstream MPEG framing format (Annex A or
Annex B) setting.
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