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ATM Switch Router Software Configuration Guide
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Chapter7 Configuring Virtual Connections
Input Translation Table Management
2. For each direction of a transit VP or VC installed in the hardware, there is an entry in the ITT.
3. If the VPI is valid, the entry in the look-up table maps to either a single ITT entry, in the case of
transit VP, or to a block of ITT, in the case of a VPI that consists of transit VCs.
For the Catalyst 8510 MSR, the LightStream 1010, and the 6400NSP1, the ITT is implemented as
two banks of 32,000 entries each.
The ITT is a hardware data structure designed to handle incoming cells. The ITT consists of entries
that, for Virtual Circuit (VC) switching, are allocated in contiguous blocks, and each block is
dedicated to a Virtual Path Identifier (VPI) on an interface. ITT functionality is us ed only when both
interfaces through which the VC transits are up.
VC Block Allocation
Interfaces must be up in order for connections to be installed in ha rdware. No connections are installed
for interfaces that are down (either as a result of an administrative shutdown or because the physical
interface is down). Only cross-connects are installed in hardware (PVC/PVP legs that are not
cross-connected are not installed), and the installation only occurs in both interfaces participating in the
cross-connect are up.
No ITT space is allocated for connections that are not installed in hardware; shutting down an interface
releases all ITT blocks allocated for input from that interface.
Freeing an ITT Block
When an ITT block is freed, an attempt is made to combine it with a same-size ITT block already in the
free-pool, thereby resulting in a block of a size qualifying for the next-largest category on the free -chain
list. This process (attempting to combine blocks) is continued up the list until a match is no longer found;
however, blocks are not merged across the 16K VP support line.
Growing an ITT Block
When a request occurs for a new VC in a VPI, and the VCI exceeds the size of the current ITT block, it
is possible to expand the size of the ITT block, without significant service interruption. To do this,
software allocates a new block of the desired size, copies the entries found in the small block to the large
block, modifies the LUT to point to the new block, and frees the smal l block.
On LightStream 1010 platforms, the process of combining ITT blocks i s restricted to same-bank blocks;
the new block must reside in the same bank as the old block (similar to th e way that other hardware data
structures are “banked”).
ITT Fragmentation
ITT memory can become fragmented as blocks are allocated, grow, and are freed; blocks then consist of
numerous used and free memory sections, of varying sizes. Under such circumstances, the aggregate
amount of free memory can be significantly larger than the capaci ty of the largest single block.
Benefits
The primary benefits of the ITT management feature are:
•Reduced fragmentation in ITT blocks