This article is intended to explain why a network interface has Link state "Up" on any Juniper Networks switching platform, even when the same is not configured nor being used for traffic forwarding within the network.

The interface is showing Link state "up", but the same is not known to be forwarding traffic or actively used in the network for handling business services. The customer states that it does not make sense as configuration has not been input for this interface and they deleted all default configuration on the Juniper Networks switch.

{master:0}
user@switch> show interfaces terse | match 0/0/29
ge-0/0/29:0       up  up

Whenever there is a device connected to the interface, and the connectivity at physical layer is ok, the interface will acquire Link state "Up", and the Admin state will always be "Up" as long as the interface is not intentionally disabled by network administration actions such as disabling the interface. The Link state does not depend on the configuration of the interface, it would come up even if it was not configured.