This is the number of frames with authentication type mismatches recognized by this Intermediate System. It increments whenever the router receives a PDU whose advertised authentication type does not match what this router expects, a classic symptom of a neighbor being reconfigured with a different authentication scheme without both sides being updated together. For example, after an admin changes this router's IS-IS authentication from cleartext to MD5 but forgets to update a neighboring router to match, this counter would begin climbing on both sides as each rejects the other's now-mismatched PDUs.