Brocade Fabric OS Administrators Guide (Supporting Fabric OS v7.3.0) - Manual (Page 276)

Base switch and extended ISLs
One way to connect logical switches is to use extended ISLs and base switches.
When you divide a chassis into logical switches, you can designate one of the switches to be a base
switch. A base switch is a special logical switch that is used for interconnecting the physical chassis.
A base switch has the following properties:
• ISLs connected through the base switch can be used for communication among the other logical
switches.
• Base switches do not support direct device connectivity. A base switch can have only E_Ports,
VE_Ports, EX_Ports, or VEX_Ports, but no F_Ports.
• The base switch provides a common address space for communication between different logical
fabrics.
• A base switch can be configured for the preferred domain ID just like a non-Virtual Fabrics switch.
• You can have only one base switch in a physical chassis.
A base switch can be connected to other base switches through a special ISL, called a shared ISL or
extended ISL (XISL). An extended ISL connects base switches. The XISL is used to share traffic
among different logical fabrics.
Fabric formation across an XISL is based on the FIDs of the logical switches.
Figure 26 shows two physical chassis divided into logical switches. Each chassis has one base switch.
An ISL connects the two base switches. This ISL is an extended ISL (XISL) because it connects base
switches.
FIGURE 26 Base switches connected by an XISL
Traffic between the logical switches can now flow across this XISL. The traffic can flow only between
logical switches with the same fabric ID. For example, traffic can flow between logical switch 2 in
chassis 1 and logical switch 6 in chassis 2, because they both have FID 1. Traffic cannot flow between
logical switch 2 and logical switch 7, because they have different fabric IDs (and are thus in different
fabrics).
Base switch and extended ISLs
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