Cisco Systems OL-7396-01 Reducing ITT Fragmentation, System and Startup ITT Fragmentation, Solution: Minimum block-size per-VPI, Using the minblock Command to Specify a Minimum Block Size

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ATM Switch Router Software Configuration Guide
OL-7396-01
Chapter7 Configuring Virtual Connections
Input Translation Table Management
•Capability to display ITT allocation
•Capability to autoshrink ITT blocks
Reducing ITT Fragmentation
It is important to make adjustments to the VC configuration processing, both at initial boot-up and in
response to interface flaps. Optimal-size ITT blocks will be allocated on the first pass, and eliminate
fragmentation due to sequentially growing the ITT blocks.
System and Startup ITT Fragmentation
Two sources of ITT fragmentation are the way that configured connections are inst alled in hardware
upon startup and the way they are installed when an interface comes up.
When a startup configuration file is created (e.g. entering the write terminal command), the PVC
cross-connect definitions are specified in the file in ascending order by interface, first addressing VPI s,
and then VCIs (choosing one interface of a PVC as the source). This is the order in which they are
processed when the system reads the file at startup. If the interface is considered up when the startup
configuration is read, the VCI values in a VPI are allocated starting with the low values and proceeding
to the high values; this can result in a series of steps that contribute to the growth of the ITT block used
by the VPI.
Whether or not interfaces are up at startup, the startup configuration software creates data structures
representing the PVCs specified in the startup configuration file.
Following a similar procedure, these data structures also order the PVCs by VPI, then VCI, and
allocations start with the low values and proceed to the high values.
Whenever an interface comes up, connection management software evaluates each of the connections
defined (in data structures) as residing on the interface, to see whether the connecti on can be brought up.
This evaluation also proceeds by VPI, then VCI, and can result in fragmentation due to growth of the
ITT blocks.
Solution: Minimum block-size per-VPI
The remedy proposed is to provide hints in configuration for the minimum ITT block size to allocate
when allocating a block for a VPI on an interface.
Using the minblock Command to Specify a Minimum Block Size
Use the minblock command to specify the minimum block size for each VPI on an interface. Use the
force keyword to specify a minimum ITT block size if autominblock mode is not enabled, or to ensure
that the block size is not overridden by the autominblock mode. The minblock command is an interface
configuration mode command.
Command Purpose
Step1 Switch(config-if)# interface slot/subslot/port Selects the interface to be configured.
Step2 Switch(config-if)# atm input-xlate-table
minblock vpi vpi-value block-size force
Specifies the minimum block size (as a power of
2) for a VPI. Use the force keyword.