By default, Cisco Nexus 5000 Series switches operate in fabric mode. In this mode, the switch provides standard Fibre Channel switching capability and features.
In fabric mode, each switch that joins a SAN is assigned a domain ID. Each SAN (or VSAN) supports a maximum of 239 domain IDs, so the SAN has a limit of 239 switches. In a SAN topology with a large number of edge switches, the SAN may need to grow beyond this limit. NPV alleviates the domain ID limit by sharing the domain ID of the core switch among multiple edge switches.
In NPV mode, the edge switch relays all traffic from server-side ports to the core switch. The core switch provides F port functionality (such as login and port security) and all the Fibre Channel switching capabilities.
The edge switch appears as a Fibre Channel host to the core switch and as a regular Fibre Channel switch to its connected devices.
In NPV mode, the edge switch relays all traffic to the core switch, which provides the Fibre Channel switching capabilities. The edge switch shares the domain ID of the core switch.
To convert a switch into NPV mode, you set the NPV feature to enabled. This configuration command automatically triggers a switch reboot. You cannot configure NPV mode on a per-interface basis. NPV mode applies to the entire switch.
In NPV mode, a subset of fabric mode CLI commands and functionality is supported. For example, commands related to fabric login and name server registration are not required on the edge switch, because these functions are provided in the core switch. To display the fabric login and name server registration databases, you must enter the show flogi database and show fcns database commands on the core switch.
Server interfaces are F ports on the edge switch that connect to the servers. A server interface may support multiple end devices by enabling the N port identifier virtualization (NPIV) feature. NPIV provides a means to assign multiple FC IDs to a single N port, which allows the server to assign unique FC IDs to different applications.
Note To use NPIV, enable the NPIV feature and reinitialize the server interfaces that will support multiple devices. For additional information about NPIV, see the “About N Port Identifier Virtualization” section.
Server interfaces are automatically distributed among the NP uplinks to the core switch. All of the end devices connected to a server interface are mapped to the same NP uplink.
In Cisco Nexus 5000 Series switches, server interfaces can be physical or virtual Fibre Channel interfaces.
NP Uplinks (External Interfaces)
All interfaces from the edge switch to the core switch are configured as proxy N ports (NP ports).
An NP uplink is a connection from an NP port on the edge switch to an F port on the core switch. When an NP uplink is established, the edge switch sends a fabric login message (FLOGI) to the core switch, and then (if the FLOGI is successful) it registers itself with the name server on the core switch. Subsequent FLOGIs from end devices connected to this NP uplink are converted to fabric discovery messages (FDISCs). For additional information about fabric login, see the “Information About Fabric Login” section.
Cisco Nexus 5000 Series NX-OS Software Configuration Guide
Troubleshooting NPort Virtualization
Switch Fabric- Troubleshooting tips