Virtual Servers, Service Groups, and Load Balancing
Virtual servers
A virtual server (VIP) is the client-facing object: an IP address, a port, and a protocol. Port 0, or an any-address VIP, is a wildcard that is powerful but easy to mis-scope. The VIP is normally bound to a service group and optionally to persistence, source NAT, and SSL templates. Without a service group, a VIP has nowhere to send traffic.
Service groups
A service group is an ordered set of real servers, each with an IP, port, and weight, plus a load-balancing method. Common methods include round robin, weighted round robin, least connection, weighted least connection, shortest response, and source-IP hash. A weight of zero effectively removes a member; it is not a low-priority setting.
Traps
Mismatched ports are the classic error: a VIP on 443 pointing at servers listening on 80 with no translation. Another is assuming the configured method applies when persistence pins a client to one server. Health state overrides everything: a member marked down receives no new traffic regardless of weight. Finally, if real servers use the ADC as their gateway, source NAT may be required for return traffic.
Worked example
Publish an HTTPS application: create a service group with two real servers on 8443, attach an HTTPS health monitor, choose weighted least connection, then create the VIP on 443, bind the service group, and apply source NAT so replies return through the ADC. Use the service-group and VIP show commands to confirm both members are up before handoff.