As original poster did not respond, I will add comment about
swhite65 system:
First off: for system of this size, did you get approval from your local RA solution architect before buying components? You should. If you did not, then this is your mistake.
Next, based on this thread you started in November
http://www.plctalk.net/qanda/showpost.php?p=243886&postcount=3
you had L63 controllers, why now you changed it to L64? If because "program does not fit", then this was mistake.What is your scan time? What SOTS set for?
Next, count connections like TW recomended.
I am not sure how you layed out your I/O, but you can be easily at the limit. In any case with this amount of I/O and comms and unconnected buffers at defaut it will take some time to establish all I/O connections. Did you try to increase unconnected buffers?
Now about error 16#0204:
if all above looks OK, did you try to do traffic cature at the switch port AENT connected to? You should see controller requests to establish connections.
If you don't know how to do it, then you have no business to set Ethernet system of this size. I am suspecting that connection open request not routed by switches correctly. IGMP snooping has nothing to do with this, error 204 indicates that connection was never open (it is done via TCP and IGMP snooping is for multicast). But you may have VLAN set incorrectly and it can block "forward open" requests.