This read-write octet string, up to 255 bytes and defaulting to 16 nulls, holds an authentication code that arriving CDP packets on a port must contain to be accepted, applied globally across the entire managed entity. An administrator sets a shared secret here to create isolated CDP domains, preventing devices without the matching code from being recognized as legitimate CDP neighbors. For example, to prevent an unmanaged third-party device from being falsely discovered as a trusted neighbor, the admin configures a custom authentication code that only trusted internal devices are provisioned with.