Found the answer in the documentation : webMethods Integration Server Clustering Guide page no 11.
Load Balancing in a Cluster
Load balancing is an optimizing feature you use with clustered webMethods Integration
Servers. Load balancing controls how requests are distributed to the servers in the cluster.
You must use a third-party load balancer to perform load balancing. A third-party load
balancer can be useful if you need load balancing for multiple types of servers, for
example, web servers and application servers, in addition to webMethods Integration
Servers. Third-party load balancers also offer virtual IP support, but they are not “out of
the box.” Most third-party load balancers perform load balancing in a round-robin
manner or based on network level metrics such as TCP connections and network
response time.
So basically need to use third party software/hardware to perform the load balancing before it goes to webMethods cluster servers.
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