For anyone facing the same issue; after a long search, we reviewed the interface configuration with command: 'show running-config interface gigabitethernet 1/0/1 all' and noticed a the following config: 'dot1x timeout guest-vlan-period 90'.    After reviewing the CLI manual this was causing the 90 second delay mentioned in my post. Dell's description: "Use the dot1x timeout guest-vlan-period command in InterfaceConfiguration mode to set the number of seconds that the switch waitsbefore authorizing the client if the client is an 802.1X unaware client. Use theno form of the command to return the timeout to the default value."   After decreasing this to 10 seconds, we noticed 802.1x unaware clients authenticated using MAB after failing 802.1x within 10 seconds. Because this was a default config parameter, it was not shown in the running config unless issuing the '.. all' command. Shame no one at Dell could tell me this!.. 
I've setup a N3048P (Access) switch to do Dot1x and Mac Authentication Bypass (MAB) using ClearPass as a RADIUS server. This works all fine as I see authentication requests getting allowed / denied in ClearPass. On the interface config, i've setup to first try dot1x, and if that fails, try MAB for authentication. The only problem i'm facing is that the timeout between a Dot1x failure and MAB authentication is taking too long (1.45 minutes). I've tried to configure the failover timeout using: dot1x timeout tx-period, dot1x max-req and dot1x max-reauth-req. This resulted in the same outcome: the client authenticates using MAB only after 1.45 minutes, every time.  Im using the following global config:  aaa accounting dot1x default start-stop radiusaaa accounting update newinfoauthentication enabledot1x system-auth-controlaaa authentication dot1x default radiusaaa authorization network default radiusdot1x dynamic-vlan enableaaa server radius dynamic-authorclient server-key 7 " " auth-type anyexitradius server attribute 4 radius server attribute 6 on-for-login-authradius server source-ip radius server key 7 " " radius server auth primaryname "CLEARPASS-RADIUS-VIP1"exitradius server auth name "CLEARPASS-RADIUS-VIP2"exitradius server acct name "CLEARPASS-RADIUS-VIP1"exit And the following interface configuration: N3048P(config)#show running-config interface gigabitethernet 1/0/24 spanning-tree portfastswitchport access vlan dot1x port-control mac-baseddot1x reauthenticationdot1x timeout tx-period 5dot1x max-req 3dot1x max-reauth-req 3mabauthentication order dot1x mabauthentication priority dot1x mab   My question: What configuration is required to decrease the timeout of MAB authentication, after trying dot1x first? 
N3048P - Decrease MAB failover timeout after Dot1x