fallback mode if for the radius part; client limit is for multiple authent on one port (ie phone + pc) From doc :
aaa port-access authenticator <port-list> client-limit <1-32>
Used after executing aaa port-access authenticator <port-list> to convert authentication from port-based to user-based. Specifies user-based 802.1X authentication and the maximum number of 802.1X-authenticated client sessions allowed on each of the ports in <port-list>. If a port currently has no authenticated client sessions, the next authenticated client session the port accepts determines the untagged VLAN membership to which the port is assigned during the session. If another client session begins later on the same port while an earlier session is active, the later session will be on the same untagged VLAN membership as the earlier session.