i) Monitor mode
--> This mode allows to implement 802.1x in the network without affecting users in the network.
--> User with failed authentication will also get network access.
--> With this mode, network administrators can get the information which users are getting successful or failure authentications in the network.
--> Failure authentications can be solved without affecting end user access to the network.
ii) Low Impact Mode
--> Low Impact mode is also same as monitor mode with prebuilt acl on the switch port.
--> Pre built acl provides to provide access what ever required before authentication and after failed authentication ( Like DHCP, DNS, Internet Access).
--> Once the device authenticates then the switch will authorize the port with data vlan, downloadable acls and security group tags.
iii) Closed Mode
--> In closed mode only EAPOL traffic is allowed before authentication.
--> This is the default behavior on cisco switches when you configure 802.1x authentication.
--> This is the most secure mode in the 802.1x