To reduce the amount of traffic sent over the ISL between Switch-1 and Switch-2, enable Equal Cost Multi Path (ECMP) on both Switch-1 and Switch-2 is not a correct explanation of how to control how core-to-access traffic is forwarded. Switch-1, Switch-2, and the router run OSPF on LAG 100, which is a Layer 3 LAG. ECMP is a feature that allows a router to load balance traffic destined to some network that is reachable through multiple equal cost route nexthops. Enabling ECMP on Switch-1 and Switch-2 would not reduce the amount of traffic sent over the ISL, but rather increase it by sending traffic over both links instead of one. A better way to reduce the amount of traffic sent over the ISL would be to enable active forwarding on LAG 100 on both Switch-1 and Switch-2, which would make one link active and one link standby for each direction of traffic.