The main function of a switch is to relay frames from one data link to another. These frames are sent and received using Fibre Channel interfaces/ports.
Each Fibre Channel interface can be used as a downlink (connected to a server) or as an uplink (connected to the data center SAN). Each physical Fibre Channel interface in a switch may operate in one of several port modes, such as E port, F port, TE port, SD port, and ST port. SD and ST ports will be discussed in Chapter 11, “Describe Software Management and Infrastructure Monitoring.” The use of NP, TF, and TNP modes will be more clear after we discuss the NPV and NPIV modes of fabric switches later in this chapter. Besides these modes, each interface may be configured in auto or Fx port modes. These two modes determine the port type during interface initialization. Interfaces are created in VSAN 1 by default. We discuss VSAN later in this chapter; for now let’s just compare VSANs in the storage world with VLANs in the Ethernet world.
Each interface has an associated administrative configuration and an operational status. The administrative configuration does not change unless you modify it. Administrative mode has various attributes that you can configure. The operational status represents the current status of a specified attribute, such as the interface speed. This status cannot be changed and is read-only. Some values, for example, operational speed, may not be valid when the interface is down.
Figure 8-11 shows various Fibre Channel port types.
Figure 8-11 Fibre Channel Port Types
E Port
In expansion port (E port) mode, an interface functions as a fabric expansion port. This port can be connected to another E port to create an inter-switch link between two switches. E ports carry frames between switches for configuration and fabric management. They serve as a conduit between switches for frames destined for remote N ports. An E port connected to another switch can also be configured to form a port channel.
F Port
In fabric port (F port) mode, an interface functions as a fabric port. This port can be connected to a peripheral device (host or disk) operating as an N port. An F port can be attached to only one N port.
NP Ports
An NP port is a port on a device that is in NPV mode and connected to the core switch via an F port. NP ports function like N ports, except that in addition to providing N port operations, they also function as proxies for multiple physical N ports.
TE Port
In trunking E port (TE port) mode, an interface functions as a trunking expansion port. It can be connected to another TE port to create an extended ISL (EISL) between two switches. TE ports expand the functionality of E ports to support the following:
VSAN trunking
Transport quality of service (QoS) parameters
Fibre Channel traceroute (fctrace) feature
In TE port mode, all the frames are transmitted in EISL frame format, which contains VSAN information. Interconnected switches use the VSAN ID to multiplex traffic from one or more VSANs across the same physical link.
TF Port
In trunking F port (TF port) mode, an interface functions as a trunking expansion port. It can be connected to another trunked N port (TN port) or trunked NP port (TNP port) to create a link between a core switch and an NPV switch or a host bus adapter (HBA) in order to carry tagged frames. TF ports expand the functionality of F ports to support VSAN trunking.
In TF port mode, all the frames are transmitted in EISL frame format, which contains VSAN information. Interconnected switches use the VSAN ID to multiplex traffic from one or more VSANs across the same physical link.
TNP Port
In trunking NP port (TNP port) mode, an interface functions as a trunking expansion port. It can be connected to a trunked F port (TF port) to create a link to a core NPIV switch from an NPV switch in order to carry tagged frames.
Fx Port
Interfaces configured as Fx ports can operate in either F port mode or FL port mode. FL port mode is used in arbitrated loop topology, which we do not discuss in this book. The Fx port mode is determined during interface initialization depending on the attached N port. Fx port mode configuration disallows interfaces to operate in any other mode—for example, preventing an interface to connect to another switch.
Auto Mode
Interfaces configured in auto mode can operate in F port, E port, TE port, or TF port mode. The port mode is determined during interface initialization. For example, if the interface is connected to a node (host or disk), it operates in F port mode depending on the N port mode. If the interface is attached to a third-party switch, it operates in E port mode. If the interface is attached to another FC switch—for example, Cisco MDS 9000 Series multilayer switches, it may become operational in TE port mode.