HP Surestore 64 FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availabil - Page 93
through at least one ISL; if minimal traffic is expected, the switches are
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Planning Considerations for Fibre Channel Topologies When using low port-count fabric elements, mesh fabrics are best used when the fabric is not expected to grow beyond four or five switches. The cost of ISLs becomes prohibitive for larger mesh fabrics. In addition, full-mesh fabrics do not scale easily because the addition of a switch requires that at least one additional ISL be added from every existing switch in the fabric. If less than four fabric elements are used in a full-mesh fabric: ■ A two-switch full mesh fabric is identical to a two-switch cascaded fabric. ■ A three-switch full mesh fabric is identical to a three-switch ring fabric. A modified or partial-mesh fabric is similar to a full-mesh fabric, but each switch does not have to be directly connected to every other switch in the fabric. The fabric is still resilient to failure, but does not carry a cost premium for unused or redundant ISLs. In addition, partial-mesh fabrics scale easier than full-mesh fabrics. Partial-mesh fabrics are useful when designing a SAN backbone for which traffic patterns between SAN islands connected to the backbone are well known. If heavy traffic is expected between a pair of switches, the switches are connected through at least one ISL; if minimal traffic is expected, the switches are not connected. In general, mesh fabrics can be difficult to scale without downtime. The addition of directors or switches usually involves disconnecting fabric devices, and may involve disconnecting in-place ISLs. As a result, full or partial-mesh fabrics are recommended for networks that change infrequently or have well-established traffic patterns. SAN High Availability Planning Guide 93