This value selects the BFD authentication type used for this session, valid only when the authentication-present flag is set, and per the description read-create access on this and related auth objects supports managing only one key ID at a time (key rotation isn't handled, and key updates must be done atomically via a single set of all affected objects). Admins set it to match the authentication method configured on the remote BFD peer. The default is noAuthentication, but an admin securing the session would set this to a keyed authentication type matching what's configured on the peer.