content_image/999439.jpg
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. 

<img src='content_image/999439.jpg'>

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. 

## 2.2 SAN topology-specific guidelines 

GLYPH<SM590000> Plan for the peak load, not for the average load. 

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. 

This section covers the following topologies: 

GLYPH<SM590000> Single switch fabric 

GLYPH<SM590000> Core-edge fabric 

GLYPH<SM590000> Edge-core-edge 

GLYPH<SM590000> Full mesh 

