Depending on the fields used in the hash-key algorithm and your network requirements, you can fine-tune the way traffic is load-balanced across your network. For example, if your network supports a large number of uses on routers running Network Address Translation (NAT) or Port Address Translation (PAT), the flows will be similar at Layer 3, so adding both Layer 3 and Layer 4 to the hash key can provide better load balancing. However, if a core router in your network is supporting tens of thousands of unrelated flows that vary significantly in source or destination addresses and incoming interfaces, including only Layer 3 in the hash key would probably result in a good distribution of traffic. With some exceptions, the more fields included in the hash-key algorithm, the greater the chance that traffic is unique, resulting in an optimal balance of traffic.
The following network examples illustrate various ways of using the hash key to load-balance traffic in different types of networks: