After enabling LACP load balancing in an Active/Active mode on multiple NICs, the Symantec Endpoint Protection (SEP) firewall blocks all traffic.

No errors visible but no network connectivity will be available.

LACP uses a custom ethertype which is not included in the default SEP firewall rules and as such is blocked.

Create a firewall rule to allow the ethertypes needed for your LACP configuration; these ethertypes can added in the Service field. The specific requirements may vary between vendor/implementation, please consult the documentation or vendor for the proper firewall rules that are needed for that implementation.