Web browsing clients must specify one IP address as their proxy, so I suppose this box must be available for browsing to work. If there was a second and more responsive Squid box available, most but not all of the time, how might the two Squid caches be related so that the more responsive box was the preferred cache, and the specified IP box was the stand by, without requiring reconfiguration of browsers when the preferred box was unavailable? With regard to network traffic: would browsers be redirected to the preferred box when it was available, or would the specified IP box retrieve objects from the preferred box and deliver them to the browsers?
