Flow-based load balancing is used mostly in layer 2 networks, although in Layer 3 routing, packets can be load balanced per packets or per flow, flow-based load balancing is commonly used with the Local area network, datacenter and datacenter interconnect technologies.
There are two important load balancing mechanisms in layer 2. Vlan-based load balancing and Flow-based load balancing. Understanding the differences of these two is important for network engineers, thus please read Vlan-based load balancing post from here as well.
Load-balancing is probably a wrong term though and load-sharing should be used for stateless devices and I explained the differences between load-balancing and load-sharing here.
Let’s look at below figure to understand flow-based load balancing.
Figure -1 Flow based load balancing with basic switch topology