HP StorageWorks 64 - SAN Director Switch Planning Manual page 93

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:
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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
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.
Planning Considerations for Fibre Channel Topologies
93