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Planning Considerations for Fibre Channel Topologies
3
Redundant Fabrics
McDATA Products in a SAN Environment - Planning Manual
3-38
Fibre Channel fabrics are classified by four levels of resiliency and
redundancy. From least available to most available, the classification
levels are:
•
Nonresilient single fabric - Directors and switches are connected
to form a single fabric that contains at least one single point of
failure (fabric element or ISL). Such a failure causes the fabric to
fail and segment into two or more smaller fabrics.
•
Resilient single fabric - Directors and switches are connected to
form a single fabric, but no single point of failure can cause the
fabric to fail and segment into two or more smaller fabrics.
•
Nonresilient dual fabric - Half of the directors and switches are
connected to form one fabric, and the remaining half are
connected to form an identical but separate fabric. Servers and
storage devices are connected to both fabrics. Each fabric contains
at least one single point of failure (fabric element or ISL). All
applications remain available, even if an entire fabric fails.
•
Resilient dual fabric - Half of the directors and switches are
connected to form one fabric, and the remaining half are
connected to form an identical but separate fabric. Servers and
storage devices are connected to both fabrics. No single point of
failure can cause either fabric to fail and segment. All applications
remain available, even if an entire fabric fails and elements in the
second fabric fail.
A dual-fabric resilient topology is generally the best design to
meet high-availability requirements. Another benefit of the
design is the ability to proactively take one fabric offline for
maintenance or upgrade without disrupting SAN operations.
If high availability is important enough to require dual-connected
servers and storage, a dual-fabric solution is generally preferable to a
dual-connected single fabric. Dual fabrics maintain simplicity and
reduce (by 50%) the size of fabric routing tables, name server tables,
updates, and Class F management traffic. In addition, smaller fabrics
are easier to analyze for performance, fault isolate, and maintain.
Figure 3-16
illustrates simple redundant fabrics. Fabric "A" and
fabric "B" are symmetrical, each containing one core director and
four edge switches. All servers and storage devices are connected to
both fabrics.
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