Lowest cost metric refers to the speed of the links in the routes. As the speed of the link increases, the cost of the route decreases. FSPF refers to the number of ISLs or “hops” between the host and its storage. EMC strongly recommends that a fabric be constructed so that it has multiple equal, lowest-cost, shortest-path routes between any combination of host and storage. Routes that are not the shortest, lowest-cost path will not be used at all - until there is an event in the fabric that causes them to become the shortest, lowest-cost path. This is true even if currently active routes are close to peak utilization.Routes are assigned to devices for each direction of the communication. The route one way may differ from the return route. Routes are assigned in a round-robin fashion after the device is logged into the fabric. These routes are static for as long as the device is logged in. Routing tables on each switch are updated during events that change the status of links in the system. The calculation of routes, and the switch’s ability to perform this function in a timely fashion, is important for fabric stability. For this reason, as well as the fact that every ISL effectively removes two ports that would otherwise be available for connecting storage or hosts, EMC recommends using reasonable limits on the number of ISLs in a fabric. For a reliable estimate of required ISLs, ISL utilization should be periodically monitored, and the level of actual protection from link failures critically examined.