This Tip discusses SAN - differentiating Fibre Channel logins.
There are three different types of login for Fibre Channel. These are:
- Fabric login
- Port login
- Process login
After the fabric capable Fibre Channel device is attached to a fabric switch, it will carry out a fabric login (FLOGI). Similar to port login, FLOGI is an extended link service command that sets up a session between two participants. With FLOGI a session is created between an N_Port or NL_Port and the switch. An N_Port will send a FLOGI frame that contains its Node Name, its N_Port Name, and service parameters to a well-known address of 0xFFFFFE.
A public loop NL_Port first opens the destination AL_PA 0x00 before issuing the FLOGI request. In both cases the switch accepts the login and returns an accept (ACC) frame to the sender. If some of the service parameters requested by the N_Port or NL_Port are not supported, the switch will set the appropriate bits in the ACC frame to indicate this.
When the N_Port logs in it uses a 24-bit port address of 0x000000. Because of this the fabric is allowed to assign the appropriate port address to that device, based on the Domain-Area-Port address format. The newly assigned address is contained in the ACC response frame.
When the NL_Port logs in a similar process starts, except that the least significant byte is used to assign AL_PA and the upper two bytes constitute a fabric loop identifier. Before an NL_Port logs in it will go through the LIP on the loop, which is started by the FL_Port, and from this process it has already derived an AL_PA. The switch then decides if it will accept this AL_PA for this device or not. If not a new AL_PA is assigned to the NL_Port, which then causes the start of another LIP. This ensures that the switch assigned AL_PA does not conflict with any previously selected AL_PAs on the loop.
After the N_Port or public NL_Port gets its fabric address from FLOGI, it needs to register with the SNS. This is done with port login (PLOGI) at the address 0xFFFFFC. The device may register values for all or just some database objects, but the most useful are its 24-bit port address, 64-bit Port Name (WWPN), 64-bit Node Name (WWN), class of service parameters, FC-4 protocols supported, and port type, such as N_Port or NL_Port.
Port login is also known as PLOGI. Port login is used to establish a session between two N_Ports (devices) and is necessary before any upper level commands or operations can be performed. During the port login, two N_Ports (devices) swap service parameters and make themselves known to each other.
Process login is also known as PRLI. Process login is used to set up the environment between related processes on an originating N_Port and a responding N_Port. A group of related processes is collectively known as an image pair. The processes involved can be system processes, system images, such as mainframe logical partitions, control unit images, and FC-4 processes. Use of process login is optional from the perspective of Fibre Channel FC-2 layer, but may be required by a specific upper-level protocol as in the case of SCSI-FCP mapping.
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