What is the difference between the Up, No Link and Down states of an interface?

Short answer

These states describe both the administrative and the physical condition of the interface (Interface and IP statuses). Up (green): the interface is on and there is a physical connection (carrier); traffic can flow. No link (yellow/amber): the interface is on but there is no signal from the other side; the cable is not plugged in, the port on the other side is off or, in a virtual environment, the card is not connected. Down (gray): the interface has been shut down administratively. An interface that is in the configuration but not present in the system is shown as Absent in system (red), and when the peer device cannot be reached in a cluster, the column of the peer device is shown as Host is down (gray). These states appear on the Network → Interfaces screen and in the status strip on the Interfaces tab of the Left Monitor Panel. In a cluster, if VIP Transition Method is TR7 link check or TR7 link and GW check, the interface not being Up is factored into the VIP transition decision.

Where It Is Used in TR7

Where	Field / label	What for
Network → Interfaces	Status indicator; in a cluster, two half circles (left this device, right the peer device) (Interface and IP statuses)	The instant condition of the interface; IP addresses inherit the status of the interface they are bound to
Right-click on the interface	Down / Up	Turning the interface off and on (on both devices at once in a cluster)
Interface preview → Details tab	Status row and the Up / Down buttons per device column	Seeing the status per device; turning the interface off and on on a single device only
Left Monitor Panel → Interfaces	Status strip, Status filter	Counting and filtering the interfaces by status
Dashboard screen of the interface	Network Status panel (Interface Up, Carrier series)	The change of the administrative status and the carrier over time
WEB-CLI	sys-cmd ip show address (ip show address)	The UP (administratively on) and LOWER_UP (carrier present) flags in square brackets

States

Status	Color	Administrative state	Physical connection (carrier)	Possible cause
Up	Green	On	Present	Normal operation
No Link	Yellow/amber	On	None	Cable unplugged, port on the other side off, virtual card not connected
Down	Gray	Off	-	The interface has been shut down
Absent in System	Red	-	-	The interface in the configuration is not present in the system
Host Is Down	Gray	-	-	The network service of the device (of the peer device in a cluster) cannot be reached; the status cannot be read

Common Misconceptions

• "No Link shows that the interface has been shut down." The interface is on; the problem is in the physical connection. In a cluster, Up appearing on one device and No Link on the other points to the cable or the switch port of that device.
• "An interface taken Down stays off permanently." Down is not a configuration but an instant operation; each time a network configuration is applied, TR7 turns the interfaces it finds off back on (Up/Down).
• "Every interface can be shut down." The interfaces that carry a management access IP and the ones that are the cluster sync interface are protected; a shutdown attempt is rejected with Interface '…' is protected.

How it is done in the documentation

• Interfaces List · Interface and IP Statuses
• Edit Interface · Up/Down
• Left Monitor Panel · Interfaces · The Status Strip
• Interfaces Dashboard · Network Status

Related questions

• What is an interface; what is the difference between a physical (ETH) interface and a virtual interface?
• What is Gateway Monitoring; how does it affect VIP failover?
• What is the Left Monitor Panel; which objects does it let me monitor?