IBM RS/6000 SP Handbook page 317

When using the SP Switch-8 (F/C 4008), the tall frame can only mount one to
eight nodes. A minimum configuration is formed by one Tall model frame
equipped with an SP Switch-8 and a single PCI thin node, one wide node, one
MCA high node (a PCI high node is not supported), or one pair of MCA thin
nodes.
Similar to Short frames, no imbedded empty drawer is allowed and, if there is a
single PCI thin node in a drawer, it must be placed as the last node in a frame.
Figure 10-4 on page 293 shows examples of SP Switch-8 Tall frame
configurations.
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T h in
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Figure 10-4 Examples on SP Switch-8 Tall frame configurations
Frame configuration - rule 5:
The SP Switch (16 ports) allows a maximum of four switched Tall expansion
frames to attach to the Tall model frame, without an Intermediate Switch
Board (ISB).
The SP Switch (16 ports) (F/C 4011 and F/C 4012) allows a maximum of four
switched Tall expansion frames to attach to the Tall model frame, without ISB.
Therefore, 1 to 80 processor nodes are supported in this configuration. If your SP
system has no more than 16 nodes, a single SP Switch is enough to fulfill the
system requirements. In this circumstance, non-switched expansion frames may
be added, depending on the number of nodes and node locations. Figure 10-5
on page 294 and Figure 10-6 on page 295 show several examples of single
stage SP Switch (16 ports) configurations with no more than 16 nodes.
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Chapter 10. Clustering configurations
293