Extreme Networks Px Series - Manual (Page 13)

Px Series Application Switch Installation and Configuration Guide 1-3
Load Balancing Modes
In this document, the Internet users are referred to as clients, because they are clients of
the application switch. The website, which is actually an address inside the application
switch, is also called a virtual IP address, or VIP. Because the Px series application switch
uses the unique combination of IP address and source port, the VIP is referred to as a
virtual service.
Load Balancing Modes
The Px series application switch can perform packet redirection for load balancing in
two different ways:
• Layer 4 load balancing
• Layer 7 load balancing
Layer 4 Load Balancing
In layer 4 mode, the application switch decides which server should receive a given
user request using server selection policies. It selects a server without looking at the
content of the request. The following server selection policies are supported by the Px
series application switch:
• Round robin
• Weighted round robin
• Least connections
• Weighted least connections
For more information on policies, see Chapter 6.
The application switch can balance almost any traffic using network address translation
(NAT) at layer 4. The application switch rewrites the destination IP address of the
request to point to the real server selected to handle the request, and sets the source IP
address of the request to point to one of the internal IP addresses of the Px series
application switch. When the server responds to the request, the application switch
rewrites the response so that it appears to originate from its VIP address, and forwards
the response to the client.
Figure 1-2 illustrates a single client-server transaction using layer 4.