54 CHAPTER 1: WEB CACHING CONCEPTS AND DEPLOYMENTYou can configure a child Webcache to forward requests to up to fourparent Webcaches. If one of the parent Webcaches does not have therequested content or is unavailable, the child Webcache willautomatically try the next specified parent Webcache. If none of theparent Webcaches are available, the child Webcache will forward itsrequest to the origin server.If you do not want all cache misses to be forwarded to the parentWebaches, you can configure an Exclusion List on the child Webcache.You can specify IP addresses, IP address ranges and domain names thatthe child Webcache should request directly from the origin server insteadof from the parent Webcaches.ExampleYou may want to exclude cache misses to a LAN server in your network. Ifyou choose not to exclude the server, the cache misses will be forwardedacross the network to the parent Webcaches and then back again, ratherthan being retrieved directly from the local server.All client machines and Web sites that you specify in the Cache Bypassscreen will not be sent to the parent Webcaches; for further information,see “Cache Bypass” on page 186.How does ParentCaching Work?Parent Caching operates in the following way:1 A URL is entered into a Web browser by a user on a client machine inyour network.2 The child Webcache receives the request for the URL from the clientmachine and checks its cache for the requested content.3 If the content is not in the cache or the content is expired (see“Currentand Expired Content” on page 32), the child Webcache forwards therequest to the parent Webcache.4 If the content is in the parent Webcache it is simultaneously served to theclient machine and stored in the child Webcache, as shown in Figure 14.Subsequent requests for that content will be served directly from the childWebcache.dua1611-5aaa04.book Page 54 Friday, November 29, 2002 8:56 PM
