This is the number of frames with authentication failures recognized by this Intermediate System. Unlike the type-mismatch counter, this increments when the authentication type matches but the actual credential fails verification, most commonly pointing to a mistyped or recently rotated key that was not updated consistently across all neighbors on that circuit. For example, a key rollover on one router that is not mirrored on its neighbor at the same moment would cause this counter to climb on both sides for the duration of the mismatch, until the keys are realigned.