Model 555 Standalone Switch For Clustered Nodes - IBM RS/6000 SP Handbook

Note: Alternate two-stage switch configurations mounting fewer than 65
nodes or more than 128 nodes are available. Two-stage configurations using
less than 65 nodes are simpler to scale up when you add more switches than
are single-stage switch configurations.

4.4 Model 555 standalone switch for clustered nodes

The Model 555 is a tall frame with an integral SEPBU power supply, one SP
Switch (F/C 4011), but no processor nodes. This frame provides node
interconnectivity for Clustered Enterprise Server systems for configurations of 2
to 16 servers.
For configurations of 17 to 32 servers, add another SP Switch (F/C 4011) in the
9076 model 555. The configurator then adds an SP Expansion frame (F/C 1550),
which contains an SP Switch. The SP Switch ports in these frames connect to
the SP System Attachment adapters (RS/6000 F/C 8396) in the CES servers.
Figure 4-4 on page 149 shows a 9076-555 model 555 with two SP-attached
servers and a CWS.
148 RS/6000 SP and Clustered IBM ^ pSeries Systems Handbook