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Storage Mastery: FC Login Types and Topologies

Editor’s note: This is the fifth post in a blog series by CBT Nuggets trainer Anthony Sequeira that will cover IT storage technology.

mastering_storage_series_EMAIL5The last post in my Storage Mastery blog series focused on the Fibre Channel architecture. Let’s build on that knowledge here and cover the oft-misunderstood login types. We will wrap this short post up with a look at two common FC topologies found in use today.

Fibre Channel Login Types

There are three login types that are defined in a Fibre Channel network:

Fabric Login (FLOGI): This is performed between an N-port and an F-port. In this login type, a device sends a FLOGI frame with the World Wide Node Name (WWNN) and the World Wide Port Name (WWPN) parameters to the login service at the well-known FC address off FFFFFE on the FC switch. The switch accepts the login and sends an Accept (ACC) frame with the signed FC address for the device. Following the FLOGI process, the node will then register itself with the name server on the switch. The information registered is the WWNN, the WWPN, and the FC address of the node.

Port Login (PLOGI): This is performed between an N_port and another N_port in order to establish a session. With this login, there is a simple exchange of PLOGI and ACC frames between the nodes. Following this exchange, the devices exchange service parameters relevant to the sessions.

Process Login (PRLI): This process is also performed between an N_port and another N_port. This login relates to the Upper Layer Protocols (Layer 4) in the Fibre Channel stack. An example would be SCSI. In this login type, the devices exchange SCSI-3 related service parameters. Information is exchanged regarding the Layer 4 protocol in use, the SCSI initiator or the target.

Note: You should be aware of an option known as Fan-Out in the Fibre Channel network. This refers to multiple server ports communicating with a single server port. Fan-In refers to the ability of a host port accessing multiple storage ports on multiple arrays. Architectural limits restrict the limits on the numbers of Fan-In and Fan-Out ports.
Fibre Channel Topologies

There are two main Fibre Channel topologies, with the Core-Edge Fabric being the most popular:

Core-Edge Fabric: In this topology, there are two types of switch tiers: A core and an edge. The edge tier is all about switches that connect hosts to the fabric in the least expensive manner possible. The core tier consists of enterprise level directors that ensure high fabric availability and high levels of performance.

Mesh Topology: In the mesh, each switch connects directly to other switches using Inter Switch Links (ISLs). This topology defines both a full mesh and a partial mesh.

Until next time, I hope you found this blog post informative, and I’d like to thank you for reading!


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