This notification-only value is the MIB-II interface index on the managed entity that received the authentication request, or zero if the interface is unknown. Admins use this to determine which physical or logical interface an authentication attempt came in on, useful when correlating with out-of-band versus in-band management access. For example, this object reporting the ifIndex of the out-of-band management port would tell an admin the authentication attempt came through that dedicated management interface rather than a data port.