For traffic to be filtered, all network traffic needs to pass through a filter device. On smaller networks, the cache server can do the filtering (as it does in the above example network), but many people are now opting for secondary filter machines. These filter machines can be routers, Unix machines or even so-called layer four switches. These filtering machines allow for automatic failover (in case of cache failure) and load balancing. At the same time, the CPU load on the cache machine is vastly reduced: the CPU doesn't have to examine every passing packet and do caching.
Sometimes, data is load-balanced across multiple Internet lines. You must ensure that all outgoing data is routed through the cache machine: the ougoing packets have to pass through the filter server, so if you are load-balancing outgoing traffic across more than one line, you may have to restructure your network so that packets pass through the filter server before they reach the outside world.