This counter records TACACS+ packets received on the authentication port whose type field the switch doesn't recognize at all, distinct from a malformed packet since the framing is intact but the packet type itself is unexpected. A nonzero value here is unusual in a healthy deployment and often indicates a TACACS+ server running a newer protocol extension the switch's firmware predates, or traffic from an entirely different protocol accidentally landing on the TACACS+ port. For example, this could increment if another appliance is misconfigured to send its own proprietary traffic to TCP port 49.