FWM means Forwarding Manager. It is a mechanism to count the number of MAC-move-backs and weigh them based on the number of times the MAC address moves.It determines the total MAC-move-backs count (switch-wide across all VLANs, MACs, and interfaces), declares the message in the log %FWM-2-STM_LOOP_DETECT, and disables learning to protect FWM in loopy conditions.
eg log messages
2014 Jan 16 02:24:23 Nexus-5000 %FWM-2-STM_LOOP_DETECT:Loops detected in the network among ports Eth119/1/13 and Po90 vlan 218 -Disabling dynamic learn notifications for 180 seconds
2014 Jan 16 02:27:23 Nexus-5000 %FWM-2-STM_LEARNING_RE_ENABLE:Re enabling dynamic learning on all interfaces