IBM SG24-4817-00 manual How Point-to-Multipoint Connections Are Supported

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•The VPI value is always 0 on the UNI and SSI links. On the NNI links, the VPI value is as defined in the SET LOGICAL_LINK command for that NNI link.

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The VCI values 0 through 31 are always reserved for ITU and ATM Forum.

The 127 odd-numbered VCIs between 32 and 286 (that is VCI 33, 35, 37, etc., through 285) are reserved for point-to-multipoint connections.

•The even-numbered VCIs between 32 and 286 (32, 34, 36, etc., through 286) and all the VCIs from 287 up to and including 1023 can be used by the SVCs.

•The VPI/VCI allocation algorithm is to increment the highest previously allocated VCI value and verify that this value is not in use by a VC or a VP connection. When incrementing the VCI value, the following considerations apply:

−If the connection is a point-to-point connection and the currently highest allocated VCI is less than 286, the VCI value is incremented by two.

−If the connection is a point-to-point connection and the currently highest allocated VCI is 286 or higher, the VCI value is incremented by one.

−If the connection is a point-to-multipoint connection, the VCI value is incremented by one.

−If the currently allocated VCI value for the point-to-point connections is 1023, the next VCI allocated will be the first free VCI value starting from 32.

−If the currently allocated VCI value for the point-to-multipoint connections is 285, the next VCI allocated will be the first free VCI value starting from 33.

For PVCs, as described in 5.1.5, ªHow to Configure PVCsº on page83, you can either specify the VPI/VCI values allocated to the PVC at the two ports, which are the endpoints of the PVC, or you may leave it to the 8285 to select the VPI/VCIs which are allocated. If you choose the latter, the 8285 will use the algorithm described for SVCs to allocate the VPI/VCI values. For the intermediate links on a PVC, it is always up to the 8285 to allocate the VPI/VCI values using the previous algorithm.

For PVPs, as described in 5.1.7, ªHow to Define PVPsº on page 85, you can either specify the VPI value allocated to the PVP at the two ports, which are the endpoints of the PVP or you may leave it to the 8285 to select the VPI/VCIs which are allocated. If you choose the latter, the 8285 will increment the highest previously allocated VPI and check to see if this value is not already in use. When the VPI value reaches the upper bound of VPI (15), the next VPI value wraps to 0. For intermediate links on a PVP, it is always up to the 8285 to allocate the VPI values using the previous algorithm.

5.1.9 How Point-to-Multipoint Connections Are Supported

To support point-to-multipoint connections, one cell destined for multiple ports occupies only one cell location in the shared switch memory. However, multiple output queues (one per media module) point to that one cell location. When the multicast cell arrives at the top of the output queue, it is sent to the output module. Within the output module, if the point-to-multipoint connection spans over multiple ports, then the multicast cell is replicated as required. The switch keeps track of when the last output port has transmitted the cell, thereby allowing its memory locations to be freed. This technique minimizes the amount of memory space required for multicast messages.

Chapter 5. 8285 ATM Network Specifications 87