SAN Design: March 29, 2001 3:18 pm
53-0001575-01 BROCADE Technical Note Page: 11 of 31
2.6 HIGHLY AVAILABLE SMALL FABRIC CONFIGURATION
FIGURE 6. HIghly Available Small Fabric
SW SW
•In this design, any switch can fail and there will still be an alternative
path to the host and storage devices. [Assumes hosts/storage have
intelligence to fail over.] Any single switch could be powered off for
servicing and replaced in the fabric without loosing device connectivity.
•This simple switch topology allows for multiple routes through the
fabric ensuring that any single inter-switch link failure (GBIC or cable) will
not result in loss of connectivity. BROCADE switches do automatic
failover to alternate link and will recompute routing tables for all N-port
devices.
•The typical path through the fabric for a host requires only one hop to
get to the storage. Should path 1 fail, traffic will route via 5 and 4 to get to
disk. If the switch fails, the alternate HBA in the host can be used to still
allow a single hop to the storage using path 6. Should paths 1 and 6 fail,
there is a still a three hop path to the storage device (2-3-4). This provides
for considerable resiliency and flexibility to fabric reconfigurations
•There are a number of host and storage suppliers that can provide for
detection of failed HBAs, failed paths, and failed ports on dual ported
storage devices and that will use host based software to initiate a failover
scenario. At this time failover is managed by hosts or by intelligent RAID
systems that can detect the failure, it is not done by switch software
•Storage in a fabric is globally available to all host elements (assuming
a shared storage file system is in place). Where this is not the case, and
storage is typically associated with a specific host, the storage is best
placed on the same switch as the host to minimize inter-switch link traffic.
•This design is appropriate when the fabric itself needs to be highly
available; a single switch can fail and/or be taken off-line for maintenance
and the fabric will still support all connected devices (devices do require
one redundant entry point to the fabric)
SW SW
8 port switches -- 20 N-port devices
supported with max of 2 hops. Inter switch
Link failure always has alternate link
available at hop count penalty
16 port switches -- 52 N-port connections
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Numbers above are labels for the links, see text