WCCP helps in reducing traffic for congested WAN links in networks with many hosts. WCCP works by coordinating the work of edge routers and content engines (also referred to as cache engines).
Content engines collect data which are frequently accessed such as HTTP traffic, locally, so that when hosts access the same pages, the content can be delivered from the cache rather than crossing the WAN. WCCP works by allowing edge routers to communicate with content engines to make each aware of the other’s presence and to permit the router to redirect traffic to the content engine as appropriate.
Figure: WCCP Operations Between a Router and a Content Engine
Figure shows the followings steps, with the main decision point on the content engine coming at Step 4:
Step 1. The client sends a HTTP Get request with a destination address of the web server, as normal.
Step 2. The router’s WCCP function notices the HTTP Get request and redirects the packet to the content engine.
Step 3. The content engine looks at its disk storage cache to discover whether the requests object is cached
Step 4A. If the object is cached, the content engine sends a HTTP response, which includes the object, back to the client.
Step 4B, If the objects is not cached, the content engine sends the original HTTP Get request on to the original server.
Step 5. If the Step 4B was taken, the server replies to the client, with no knowledge that the [packet was ever redirected to a content engine.
Using WCCP, which uses UDP port 2048, a router an a content engine, or a pool of content engines( known as a cluster) become aware of each other.
WCCP Configuration Example
! First we enable WCCP globally on the router,
! specifying a service (web caching), a multicast address for
! the WCCP communication, and an MD5 password:
Ip wccp web-cache group-address 220.127.116.11 password cisco
! Next we configure an interface to redirect WCCP web-cache
! traffic outbound to a content engine
Ip wccp web-cache redirect out
! Finally, inbound traffic on interface fa0/1 is excluded from redirection:
Ip wccp redirect exclude in
WCCP can also make use of access list to filter traffic only for certain clients or for excluding WCCP for certain clients using ip wccp web-cache redirect-list access-list global command. WCCP can use the ACL as well to determine which types of redirected traffic the router should accept from content engines, using the global command ip wccp web-cache group-list access-list.