Hardware load balancers solve many of the problems faced by the round robin software solution through virtual IP addresses. The load balancer shows a single (virtual) IP address to the outside world, which maps to the addresses of each machine in the cluster. So, in a way, the load balancer exposes the IP address of the entire cluster to the world.
When a request comes to the load balancer, it rewrites the request's header to point to other machines in the cluster. If a machine is removed from the cluster, the request doesn't run the risk of hitting a dead server, since all of the machines in the cluster appear to have the same IP address. This address remains the same even if a node in the cluster is down. Moreover, cached DNS entries around the Internet aren't a problem. When a response is returned, the client sees it coming from the hardware load balancer machine. In other words, the client is dealing with a single machine, the hardware load balancer.
Advantages :
High Availability Through Failover. Failover happens when one node in a cluster cannot process a request and redirects it to another. There are two types of failover:
1) Request Level Failover. When one node in a cluster cannot process a request (often because it's down), it passes it along to another node.
Transparent Session Failover. When an invocation fails, it's transparently routed to another node in the cluster to complete the execution.
2) Metrics. Since all requests to a Web application must pass through the load-balancing system, the system can determine the number of active sessions, the number of active sessions connected in any instance, response times, peak load times, the number of sessions during peak load, the number of sessions during minimum load, and more. All this audit information is used to fine tune the entire system for optimal performance
Disadvantages :
The drawbacks to the hardware route are the costs, the complexity of setting up, and the vulnerability to a single point of failure. Since all requests pass through a single hardware load balancer, the failure of that piece of hardware sinks the entire site.
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