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This appendix describes how to optimize the number of supported virtual connections on the Cisco 6400 carrier-class broadband aggregator by:
For general information on configuring virtual connections, see the "Configuring Virtual Connections" chapter of the ATM Switch Router Software Configuration Guide.
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Note The method of assigning VPIs and VCIs can affect how you configure virtual connections on network devices that are connected to the Cisco 6400. |
This appendix includes the following sections:
The Cisco 6400 supports a maximum of 32K virtual connections, a limit determined by a hardware data structure in the node switch processor (NSP) called the ITT. The ITT consists of two banks (Bank 0 and Bank 1), each of which can hold a maximum of 32K entries. Each configured virtual connection occupies two ITT entries (one for each direction of cell flow). Unidirectional connections (such as point-to-multipoint connections) occupy only one ITT entry.
ITT entries are not maintained for:
Each ATM interface supports VPIs as large as 8 bits (0 to 255) and VCIs as large as 14 bits (0 to 16,383). 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.
In Figure C-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.
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.
Use the following methods to maximize the number of virtual connections on the Cisco 6400:
For each ATM port and VPI combination, assign VCIs using the following guidelines:
Step 2 Incrementally assign VCI values for additional virtual connections for the ATM port and VPI combination. Avoid skipping any numbers. This incremental assignment prevents the ITT from allocating a larger block than is necessary for the virtual connections.
Suppose you want to configure five virtual connections between ATM 1/0/1 (VPI = 0) and ATM 8/0/0 (VPI = 5). Configure the virtual connections in the order shown in Table C-1. Notice that the VCI values increase without skipping any numbers.
Table C-1 Virtual Connections Between ATM 1/0/1 (VPI = 0) and ATM 8/0/0 (VPI = 5)
| VPI/VCI Values on ATM 1/0/1 |
VPI/VCI Values on ATM 8/0/0 |
|---|---|
To verify that you optimally configured virtual connections on a particular ATM interface, use the show atm vc interface atm slot/subslot/port EXEC command. Check that for each VPI value, the VCI values start at 32 and do not skip any numbers.
If you know the maximum VCI that will be used for a particular ATM port and VPI combination, you can use the highest VCI to determine the minimum ITT block size for that ATM port and VPI combination. Specifying the minimum block size reduces fragmentation by avoiding block expansion as virtual connections are created.
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Note This functionality is available in Cisco IOS Release 12.1(4)DB and later releases. |
To specify the minimum ITT block size for an ATM port and VPI combination, complete the following steps beginning in global configuration mode:
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Note If the system cannot accommodate the specified minimum ITT block size, the system tries to allocate a block large enough to accommodate the VCI of the virtual connection being created. |
In the following example, the minimum ITT block sizes are specified for virtual connections on ATM 1/0/0 with VPI values 0, 1, and 4:
To verify successful configuration of the minimum ITT block size, use the more system:running-config EXEC command. Make sure that the atm input-xlate-table minblock command appears for the correct interfaces and VPI values.
To verify that the minimum block size was allocated, use the show atm input-xlate-table inuse EXEC command. Check the Size field for the ATM port and VPI combinations that you configured.
The NSP can automatically track the size of the required ITT block as virtual connections are created and deleted. The required block size is stored in the running configuration and is used to optimally allocate ITT resources after an interface flap. When you enter the copy system:running-config nvram:startup-config EXEC command, the required block size is also saved to the startup configuration, so that optimal ITT allocation occurs at the next reload.
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Note This functionality is available in Cisco IOS Release 12.1(4)DB and later releases. |
To enable automatic determination of the minimum ITT block size, use the following command in global configuration mode:
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Note The no version of the atm input-xlate-table autominblock command deletes all manually or automatically configured minblock keyword entries, except the entries that include the force keyword. |
In the following example, the system determines the optimal ITT block size for all ATM port and VPI combinations in the configuration. The system automatically inserts the minblock keyword version of the command for every ATM port and VPI combination, except when manually entered with the force keyword.
To verify that you successfully enabled automatic determination of the minimum ITT block size, use the more system:running-config EXEC command. Make sure that the atm input-xlate-table autominblock command appears in the running configuration.
To verify that the minimum block size is allocated globally, use the show atm input-xlate-table inuse EXEC command. Check the Size field for every ATM port and VPI on which PVCs or Soft PVC source legs are configured.
Once an ITT block expands, it does not automatically shrink if the block becomes larger than necessary for the ATM port and VPI combination. The unused portion of the block, especially when adjacent to an unused fragment of ITT memory, can instead be used to create another ITT block.
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Note This functionality is available in Cisco IOS Release 12.1(4)DB and later releases. |
To enable ITT blocks to shrink in place when they are larger than necessary for the current virtual connections, use the following command in global configuration mode:
| Command | Purpose |
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Enables ITT blocks to shrink in place when they are larger than necessary to accommodate the configured virtual connections. |
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Note Enable block shrinking only when you actively remove virtual connections or high VCI values. Block shrinking significantly increases processor and memory resource usage, and can affect system performance. |
In the following example, block shrinking is enabled while PVCs with high VCIs are deleted. Then block shrinking is disabled to prevent draining processor and memory resources.
To verify ITT block shrinking, use the show atm input-xlate-table inuse EXEC command in combination with the show atm vc interface atm slot/subslot/port EXEC command. The displayed size of the blocks in use should be just large enough to accommodate the displayed VCIs.
To display ITT allocation, use the following command in EXEC mode:
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Note This functionality is available in Cisco IOS Release 12.1(4)DB and later releases. |
In the following example, the inuse keyword is used to display all allocated ITT blocks and their associated ATM interfaces and VPIs:
In the following example, the inuse keyword is excluded to display both the used and free ITT blocks:
Posted: Mon Jun 23 14:27:50 PDT 2003
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