This article describes how to base HTML5 Gateway session affinity to a cookie instead of using source addressing.
Detailed setup for HAProxy described here: load balancing, affinity, persistence, sticky sessions: what you need to know
Examine the Using application session cookie for persistence section from the article above for details related to session affinity based on cookie.
The HTML5 Gateway uses a cookie with ID ASP.NET_SessionId
The load balancer must act in round robin mode
Assuming that IP addresses of the RAS Gateways are 10.125.1.10 and 10.125.1.20 respectively, The final configuration should be as follows:
NOTE: Please contact your load balancing solution vendor for detailed configuration.