Fabric OS Administrator's Guide v7.0.0 (53-1002148-02, June 2011)

Fabric OS Administrator’s Guide 11
53-1002148-02
Device login
1
Principal switch
In a fabric with multiple switches, and one inter-switch link (ISL) exists between any two switches, a
principal switch is automatically elected. The principal switch provides the following capabilities:
• Maintains time for the entire fabric. Subordinate switches synchronize their time with the
principal switch. Changes to the clock server value on the principal switch are propagated to all
switches in the fabric.
• Manages domain ID assignment within the fabric. If a switch requests a domain ID that has
been used before, the principal switch grants the same domain ID unless it is in use by another
switch.
E_Port login
An E_Port does not use a FLOGI to log in to another switch. Instead, the new switch exchanges
frames with the principal switch to establish that the new switch is an E_Port and that it has
information to exchange. If everything is acceptable to the principal switch, it replies to the new
switch with an SW_ACC (accept) frame. The initializing frame is an Exchange Link Parameters (ELP)
frame that allows an exchange of parameters between two ports, such as flow control,
buffer-to-buffer credits, RA_TOV, and ED_TOV. This is not a negotiation. If one or the other port’s link
parameters do not match, a link does not occur. Once an SW_ACC frame is received from the
principal switch, the new switch sends an Exchange Switch Capabilities (ESC) frame. The two
switches exchange routing protocols and agree on a common routing protocol. An SW_ACC frame is
received from the principal switch and the new switch sends an Exchange Fabric Parameters (EFP)
frame to the principal switch, requesting principal switch priority and the domain ID list.
Buffer-to-buffer credits for the device and switch ports are exchanged in the SW_ACC command
sent to the device in response to the FLOGI.
Fabric login
A device performs a fabric login (FLOGI) to determine if a fabric is present. If a fabric is detected
then it exchanges service parameters with the fabric controller. A successful FLOGI sends back the
24-bit address for the device in the fabric. The device must issue and successfully complete a
FLOGI command before communicating with other devices in the fabric.
Because the device does not know its 24-bit address until after the FLOGI, the source ID (SID) in
the frame header making the FLOGI request are zeros (0x000000).
Port login process
The steps in the port initialization process represent a protocol used to discover the type of device
connected and establish the port type and negotiate port speed.
The possible port types are as follows:
• U_Port — A universal FC port is the base Fibre Channel port type, and all unidentified or
uninitiated ports are listed as U_Ports.
• L_/FL_Port — A loop or fabric loop port connects loop devices. L_Ports are associated with
private loop devices and FL_Ports are associated with public loop devices.
• G_Port — A generic port acts as a transition port for non-loop fabric-capable devices.
• E_Port — An expansion port is assigned to ISL links to expand your fabric by connecting it to
other switches.