When sizing over two hops, consider that all of the ISLs going to the switch where the Flash System 9200 and 9100 is connected will also handle the traffic coming from the switches on  the edges, as shown in Figure 2-1. Figure 2-1 ISL data flow GLYPH<SM590000> If possible, use SAN directors to avoid many ISL connections. Problems that are related to  oversubscription or congestion are much less likely to occur within SAN director fabrics. GLYPH<SM590000> When interconnecting SAN directors through ISL, spread the ISL cables across different  directors blades. In a situation where an entire blade fails, the ISL will still be redundant  through the links connected to other blades. GLYPH<SM590000> Plan for the peak load, not for the average load. GLYPH<SM590000> Single switch fabric GLYPH<SM590000> Core-edge fabric GLYPH<SM590000> Edge-core-edge GLYPH<SM590000> Full mesh This section covers the following topologies: Some preferred practices, as mentioned in 2.1, “SAN topology general guidelines” on  page 12, apply to all SANs. However, there are specific preferred practices requirements to  each SAN topology available. The following topic shows the difference between the different  kinds of topology and highlights the specific considerations for each of them.  2.2 SAN topology-specific guidelines 14  IBM FlashSystem 9200 and 9100 Best Practices and Performance Guidelines