Brocade Fabric OS Administrators Guide (Supporting Fabric OS v7.3.0) - Manual (Page 101)

FSPF guarantees a routing loop-free topology at all times. It is essential for a fabric to include many
physical loops because, without loops, there would not be multiple paths between switches, and
consequently no redundancy. Without redundancy, if a link goes down, part of the fabric is isolated.
FSPF ensures both that the topology is loop-free and that a frame is never forwarded over the same ISL
more than once.
FSPF calculates paths based on the destination domain ID. The fabric protocol must complete domain
ID assignments before routing can begin. ISLs provide the physical pathway when the Source ID (SID)
address has a frame destined to a port on a remote switch Destination ID (DID). When an ISL is
attached or removed from a switch, FSPF updates the route tables to reflect the addition or deletion of
the new routes.
As each host transmits a frame to the switch, the switch reads the SID and DID in the frame header. If
the domain ID of the destination address is the same as the switch (intra-switch communications), the
frame buffer is copied to the destination port and a credit R_RDY message is sent to the host. The
switch only needs to read word zero and word one of the Fibre Channel frame to perform what is known
as cut-through routing. A frame may begin to emerge from the output port before it has been entirely
received by the input port. The entire frame does not need to be buffered in the switch.
If the destination domain ID is different from the source domain ID, then the switch consults the FSPF
route table to identify which local E_Port provides Fabric Shortest Path First (FSPF) to the remote
domain.
Fibre Channel NAT
Within an edge fabric or across a backbone fabric, the standard Fibre Channel FSPF protocol
determines how frames are routed from the source Fibre Channel (FC) device to the destination FC
device. The source or destination device can be a proxy device.
Fibre Channel fabrics require that all ports be identified by a unique port identifier (PID). In a single
fabric, FC protocol guarantees that domain IDs are unique, and so a PID formed by a domain ID and
area ID is unique within a fabric. However, the domain IDs and PIDs in one fabric may be duplicated
within another fabric, just as IP addresses that are unique to one private network are likely to be
duplicated within another private network.
In an IP network, a network router can maintain network address translation (NAT) tables to replace
private network addresses with public addresses when a packet is routed out of the private network,
and to replace public addresses with private addresses when a packet is routed from the public network
to the private network. The Fibre Channel routing equivalent to this IP-NAT is Fibre Channel network
address translation (FC-NAT). Using FC-NAT, the proxy devices in a fabric can have PIDs that are
different from the real devices they represent, allowing the proxy devices to have appropriate PIDs for
the address space of their corresponding fabric.
Inter-switch links
An inter-switch link (ISL) is a link between two switches, E_Port-to-E_Port. The ports of the two
switches automatically come online as E_Ports once the login process finishes successfully. For more
information on the login process, refer to
Understanding Fibre Channel Services on page 25.
You can expand your fabric by connecting new switches to existing switches. Figure 6 shows a new
switch being added into an existing fabric. The thick red line is the newly formed ISL.
Fibre Channel NAT
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