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Cisco 6400 Software Setup Guide

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AppendixC Optimizing the Number of Virtual Connections on the Cisco 6400

An Overview of the ITT and Virtual Connection Limitations

How VCI Values Limit the Number of Virtual Connections

Each ATM interface supports VPIs as large as 8 bits (0 to 255) and VCIs as large as 14 bits (0to16,383).

While these ranges provide a broad selection of VPI/VCI combinations per interface (up to 4,194,304),

the method that you use to select these combinations can affect how many virtual connections you can

configure on the Cisco 6400.

The ITT allocates resources in blocks of adjacent entries where each block size, in bits, must be a power

of 2. Each VPI and ATM port combination requires a dedicated ITT block, and the block size must be

greater than the largest VCI. As a result, using unnecessarily large VCI values can dramatically reduce

the number of supported virtual connections.

FigureC-1 Example of Two PVCs Using Extremely High VCI Values

In FigureC-1, two PVCs are configured between four ATM ports. In this example, all VCIs are close to

the maximum allowed VCI value of 16383. Because the ITT block size must be a power of 2, each of

the four ATM port/VPI/VCI combinations require 16K of allocated ITT resources. As a result, these two

PVCs exhaust all possible ITT resources, and additional ATM port and VPI combinations cannot be

configured.

How ITT Fragmentation Limits the Number of Virtual Connections

Each VPI and ATM port combination requires a dedicated ITT block, and the block size must be greater

than the largest VCI. If you configure a VCI greater than the current size of an existing ITT block, the

block must expand to the next power of 2 block size that can accommodate the new VCI. The method

of ITT block expansion, however, often results in many small and unusable fragments, and further limits

the number of virtual connections configurable on the Cisco 6400.

For an existing ITT block to expand in size:

1. The ITT allocates a new block within the same bank. The block size is determined by the largest

VCI value, rounded up to the next power of 2.

2. The ITT copies the entries from the original block to the new block.

3. The ITT frees the original block from allocation.

As the ITT allocates, expands, and frees its blocks, the total memory breaks into fragments of used

memory and free memory. The total free memory can be larger than the size of any single block, but the

fragments might be too small to use.

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