Global Knowledge
Fabric Login (FLOGI).
Fabric login (FLOGI request) will be sent to your storage or server when you connect it. This FLOGI request serves two purposes.
Fibre Channel ID is the WWPN connection from the FLOGI server to this port.
With the switch, you can also get exchange buffer credits
It is easy to understand the first function if you rewrite it as “request an address for a MAC from the DHCP server”.
Yes, it’s true, an FCID in the FC realm is almost identical to an IP address in a networking world. An IP address can have two main IDs. The network ID is unique for devices within the same Layer 2 domain. Each host has a unique host ID.
FCID consists of two main parts
The Domain ID is unique for all devices connected to the same switch.
Each device connected to the switch has a unique Port ID.
What is the first thing your laptop will do when it connects to your network? It will ask for an IP address to allow it to communicate with other devices. It will send a DHCP BC frame containing the MAC address to do this. WWPN is the FC equivalent of MAC, and the FC equivalent of a DHCP Server is the LOGIN service (a service that runs inside the FC switch).
LOGIN server is a service that runs inside every storage switch. It provides a unique FCID for each device connected to your storage switch, similar to how DHCP provides each host with an IP address.
FLOGI is functional because it provides the buffer credit exchange from server to switch.
The FC is completely lossless. It works on a link by link basis. This means that two devices on the same line will exchange buffer credits. But why?
The above figure will show that the TX (Server), now knows that the maximum number RX (Switch), can hold in the buffer, is 4. It will then send a second variable, BB_Credit_CNT. It will set its value to 0 at login. Every frame that the TX sends to the RX will cause the value to increase by 1. TX can then send frames to RX up until the value of the BB_Credit_CNT equals the BB_Credit. An RX layer will send the TX an RDY frame if a buffer space has been offloaded on the RX side. The TX will receive the frame and decrease the value of the BB_Credit_CNT one time. TX can then send one more frame.
This is what makes FC loseless. Without a switch to or server to switch, no frame will be sent between devices.
Port Login (PLOGI).
Each device will now have its own FCID after they have completed the FLOGI phase. They don’t know how they can reach other devices that are connected to the same switch, or connected to different switches in the fabric. Each device must perform PLOGI in order to communicate with the other devices.
The switch also has a Name server service. Any device that registers with this server will send its WWPN map and FCID to the server. The Name server will expose the device and all register requests it receives, so they can communicate with one another.
Process Login (PRLI).
Communication between the devices is now possible because they can communicate using the FC network SCSI. SCSI can perform two operations: SCSI read and SCSI write.
Learn more about FLOGI and PLOGI in our Networking and Wireless training courses.
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