220 Fabric OS Administrator’s Guide
53-1002446-01
Logical fabric overview
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By default, the physical ISL path is favored over the logical path (over the XISL) because the
physical path has a lower cost. This behavior can be changed by configuring the cost of the
dedicated physical ISL to match the cost of the logical ISL.
If you disable a base switch, all of the logical ISLs are broken and the logical switches cannot
communicate with each other unless they are connected by a physical ISL.
Base fabric
Base switch ports on different chassis can be connected together to form a fabric, called a base
fabric. Similar to other logical switches, the base switches must have the same FID to be
connected. If the base switches have different FIDs, the link between the switches is disabled.
The base fabric follows normal routing policies. As long as physical connectivity is available, the
base fabric maintains connectivity for the logical fabrics.
Logical ports
As shown in Figure 27, logical ISLs are formed to connect logical switches. A logical port represents
the ports at each end of a logical ISL. A logical port is a software construct only and does not
correspond to any physical port.
Most port commands are not supported on logical ports. For example, you cannot change the state
or configuration of a logical port.
The World Wide Name (WWN) for logical ports is in NAA=5 format, using the following syntax:
5n:nn:nn:nz:zz:zz:zx:xx
The NAA=5 syntax uses the following variables:
• nnnnnn is the Brocade Organizationally Unique Identifier (OUI).
• zzzzzz is the logical fabric serial number.
• xxx is the logical port number, in the range 0 through FFF.
Logical fabric formation
Fabric formation is not based on connectivity, but on the FIDs of the logical switches. The basic
order of fabric formation is as follows:
1. Base fabric forms.
2. Logical fabrics form when the base fabric is stable.
3. Traffic is initiated between the logical switches.
4. Devices begin recognizing one another.
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