Using F/C 1550 Switched Expansion Frames - IBM RS/6000 SP Handbook

Note: If the switch used in this configuration is an SP Switch-8, the nodes
must be placed sequentially in this system configuration. Empty node drawers
are not allowed. If the switch is a 16-port SP Switch, nodes can be placed in
any order and empty drawers are permissible; however, node-to-node
Ethernet BNC jumper cables might be too short if a drawer is left empty.
Frames containing processor nodes only
Non-switched expansion frames (configured with processor nodes only) can be
added to the Model 550 SP frame to take advantage of unused node slots. In
these cases, node-to-node data transfers are completed over the SP LAN.

4.5.2 Using F/C 1550 switched expansion frames

A switched expansion frame is defined as a base offering expansion frame
equipped with processor nodes and a switch. These frames are added to SP
systems with switch-configured Model 550 frames. Configuration rules permit
you to attach up to four switched expansion frames to these model frames. In
some system configurations, you can have unused switch ports in either the
model frame or the switched expansion frames. Those unused switch ports can
be used with non-switched expansion frames to complete your system.
If your SP system uses single-stage switching, you can scale your SP system
into a system containing up to 80 nodes.
If your SP system uses two-stage switching, you can scale your SP system
even larger. In these systems, 128 nodes (or more) are supported.
152 RS/6000 SP and Clustered IBM ^ pSeries Systems Handbook