For each service (CSS term for server) you can configure a whole host of different keepalive types. You can use the default of ICMP, a TCP socket connection, or a HTTP 'get', ect:
http://www.cisco.com/univercd/cc/td/doc/product/webscale/css/css_500/bsccfggd/services.htm#xtocid2118550
Once the keepalive maxfailure value has been reached (3 failures by default) the CSS will pull that particular service out of the farm so new requests are not sent there. It will continue to send the keepalives to that service and will pull it back 'in service' once keepalives stop failing.
It is important to configure keepalive types that most closely match the type of service the server is poroviding. Other wise, for example, if you have a web server with a keepalive type of ICMP, then in the event that your web server dies out and the box is still pingable, the CSS will keep it 'in service' and load balance subsequent requests to it. In this example, if you had a keepalive type of HTTP, the CSS would then know if the server is no longer able to handle web requests.
Hope that helps.
Cheers,
Perry.
