How Load Balancers works

At Layer 4, a load balancer has visibility on network information such as application ports and protocol (TCP/UDP). The load balancer delivers traffic by combining this limited network information with a load balancing algorithm, such as round-robin, and by calculating the best destination server based on least connections or server response times.

At Layer 7, a load balancer has application awareness and can use this additional application information to make more complex and informed load balancing decisions. With a protocol such as HTTP, a load balancer can uniquely identify client sessions based on cookies and use this information to deliver all a client’s requests to the same server. This server persistence using cookies can be based on the server’s cookie or by active cookie injection where a load balancer cookie is inserted into the connection.