SLB (Server Load Balancing) allows you to distribute load and traffic to specific groups of servers or to a specific server. The FortiBalancer appliance supports server load balancing in Layers 2-7 of the OSI network model. Layer 2 SLB is based on network interfaces. Layer 3 SLB works on IP addresses. Layer 4 SLB is mostly concerned with port based load balancing. Layer 7 is used when you want to perform load balancing based on URLs, HTTP headers or Cookies. The basic steps for setting up SLB are:
- Define the real servers.
- Define a group load balancing method.
- Add real servers to the group.
- Define a Virtual IP to listen for requests.
- Bind the group balancing method to the Virtual IP.
The real server, the VIP and the virtual service are the fundamental components of SLB deployment.
- The real server is an application server hosting varied applications or services. It processes the requests from the client side.
- The VIP in general is a public IP address that can be accessed from the external clients. As an entrance, it receives and forwards external requests, and sends the processed results from the real servers back to the client side.
- For the Layer 4 and Layer 7 SLB, the virtual service is commonly represented with a VIP/port pair and can be accessed by the external clients to get their target network resources. For example, if a client wants to access some Web resources by a predefined VIP or a Web site name (with DNS), all the requests from this client will go through the VIP and be sent out to different real servers by the FortiBalancer appliance hosting the VIP and real servers. With the virtual service, the internal network architecture and backend real servers are hidden from the external clients by only exposing the VIP address.
The remainder of this chapter will cover these steps and cover some examples of Layer 2, Layer 3, Layer 4 and Layer 7 load balancing strategies.
This following figure is a logical overview of load balancing using the FortiBalancer appliance.
Figure 6-1 SLB Architecture
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