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When a website or application is hosted on multiple servers, a load balancer helps distribute incoming requests so that no single server gets overwhelmed. The F5 BIG-IP is a popular load balancer device that sits between the users (clients) and a group of servers. It acts as a traffic proxy: clients send requests to the BIG-IP’s address, and the BIG-IP forwards each request to one of the backend servers in a way that spreads out the load. This ensures the application can handle more users and stay reliable even during peak traffic.

Key Components

  • Client (User): The end-user’s device or browser that sends a request for a website or application. In the diagram, this is represented by a user icon on the left.
  • BIG-IP Load Balancer: The F5 device that receives incoming client requests and redirects them to one of the servers in the backend pool. It has a special listening point called a Virtual Server.
  • Virtual Server (frontend IP): This is not an actual physical server, but a virtual IP address/port on the BIG-IP that clients connect to. It represents the multiple real servers as a single point of contact​​. For example, clients might connect to 203.0.113.10 (the virtual IP) which is hosted on the BIG-IP. The BIG-IP then knows the pool of actual servers behind it.
  • Pool (Backend Servers): The group of real servers that host the application (in the diagram, Server 1, Server 2, Server 3). The BIG-IP maintains a pool of these servers and can send incoming traffic to any server in the pool​. Each server has the same content, so it doesn’t matter which one handles a given request.
  • Health Monitors: These are automatic check-ups or “pings” that the BIG-IP continuously sends to each server to ensure it’s healthy (alive and responding). The purpose of health monitors is to check server health and performance. If a server is not responding or is down, the BIG-IP marks it unavailable and will stop sending traffic to it​. This way, users won’t be sent to a failed server.

How the Traffic Flows

  1. Client connects to the Virtual Server: The client (user) initiates a request (for example, loading a webpage) to the application’s public IP address. This IP is actually the Virtual Server on the F5 BIG-IP. From the client’s perspective, they are just contacting the website’s address – they do not know a load balancer is handling the traffic.
  2. BIG-IP receives the request: The BIG-IP device receives the incoming request on its Virtual Server (the front-end IP). At this point, the BIG-IP decides which backend server should handle the request. It has various algorithms (like round-robin, least connections, etc.), but the basic idea is to choose a server that will distribute the load evenly​. For example, if Server 1 is busy, the BIG-IP might pick Server 2 for this new request.
  3. Request is forwarded to a server in the pool: The load balancer forwards the client’s request to the chosen server on the back end. In our diagram, suppose the BIG-IP forwards the request to Server 2. The BIG-IP acts as an intermediary – from the outside, the client is still communicating with the BIG-IP’s IP, but the actual content is coming from Server 2 on the backend.
  4. Server processes request and responds: Server 2 processes the request (for example, it generates the webpage data) and sends the response back to the BIG-IP load balancer. The BIG-IP then returns the response to the client. This all happens quickly and transparently. (The return path is not explicitly drawn in the diagram, but the BIG-IP does relay the server’s response back to the client over the established connection.) The client sees a successful response from the website without knowing which server produced it.
  5. Health monitoring (continuous): All the while, the BIG-IP is running its health monitors in the background. It might be pinging each server or sending test requests at intervals to verify they respond correctly​. If, for example, Server 3 fails to respond to a health check (meaning it might be down), the BIG-IP will mark Server 3 as unhealthy and stop forwarding any new client requests to it. This ensures that users are only sent to Server 1 or Server 2 (the healthy servers) until Server 3 recovers. This health check mechanism prevents users from being routed to a broken or overloaded server.

Figure: How F5 BIG-IP Load Balancer Works to Distribute Traffic Across Servers. In this diagram, the client’s request goes to the BIG-IP (Virtual Server on the load balancer), and the BIG-IP forwards it to one of the available backend servers. Solid arrows show the flow of user traffic, and the dashed arrows labeled “Health Check” show the BIG-IP monitoring each server’s status. The BIG-IP will only send traffic to servers that are responding and healthy.

In summary, the F5 BIG-IP load balancer sits in front of the servers and acts as an “invisible facilitator” between the user and the server group​. It ensures all servers are used more or less equally and can take servers out of rotation if they are faulty. This way, no single machine becomes a bottleneck and the application remains available even if one server fails, resulting in better reliability and user experience.

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Last modified: March 27, 2025

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