With the release of cheaper load balancers (which are Microsoft certified as well), most of the companies tend to use a hardware load balancer for load balancing Exchange 2010 traffic. There are three options that work for routing the traffic. They are:
Source NAT (SNAT) : The packet from the client hits the load balancer, which passes the traffic onto one of the CAS servers after changing the source IP of the packet (from the client’s IP to the LB IP). CAS responds back to the load balancer, which sends the packet back to the client after changing the source IP.
This is the Microsoft recommended option for routing. All traffic (in & out) flows through the load balancer.
Direct Server Return (DSR) : The packet from the client hits the load balancer, which passes the traffic onto one of the CAS servers, without making any change to the source IP and the CAS server responds directly to the client. This means that the CAS server needs to have the load balancer’s IP configured as an additional one.
Load Balancer Default Gateway (LBDG) : In this method, the CAS servers have the LB IP as the default gateway.
The below screenshot (taken from TechEd video) explains the three options.