STP should be used carefully in MLAG environments. The following rules apply for STP with MLAG (from the concepts guide):
* STP cannot be enabled on MLAG ports.
* STP should not be enabled on the ports present in the remote node which connects to the MLAG ports.
* You should ensure that the ISC port is never blocked by STP.
So, in short if STP is enabled on the ports of the switch that link up to the EXOS switches it needs to be disabled.