Content Routing | Campus Network Design Fundamentals

A content router can be added to a CDN to redirect users' requests to the closest content engine that contains the desired content.

The closest content engine is the one that has the shortest delay to the user. To determine this, the list of candidate content engines is configured on the content router, and a boomerang protocol is used between the content router and each content engine to determine the delay between the two devices.

This delay is then used in a Domain Name System (DNS) race process. When a content router receives a request for content that is serviced by multiple content engines, the content router forwards that request to a selection of the appropriate content engines, delaying each request by the delay determined by the boomerang protocol. Thus, each content engine should receive the request at the same time. The content engines then all respond to the request; the first response that is received by the client or the client's local DNS server is the winner of the race and is therefore the best content engine from which that particular client should receive the desired content. The client then requests the desired content from the winning content engine.

The content router can be used in either of two modes^[3]direct mode or WCCP modeas described in the following sections.

Direct Mode

Key Point

When used in direct mode, the content router acts as the authoritative DNS server for all domains for which it is configured. DNS address requests are sent directly from a DNS server that is local to the client to the content router.

As an example of a content router operating in direct mode, assume that the content router is to handle requests to http://www.cisco.com. The DNS server is thus configured to point to the content router as the name server for http://www.cisco.com, and all requests for content from this site are sent to the content router.

Figure 8-4 illustrates how a direct-mode content router interacts with other devices in the network.

Figure 8-4. A Content Router in Direct Mode Acts as a DNS Server

The steps involved when a content router is operating in direct mode are as follows:

Step 1.	In Figure 8-4, the user at the workstation in Toronto requests a web page from a server. The user's workstation (the client) sends a DNS query for the IP address of the content that it is looking for. This request goes to the client's local DNS server.
Step 2.	The local DNS server sends the query to the content router (which is in San Jose, in this example).
Step 3.	The content router forwards the request to a selection of the appropriate content engines (assuming that multiple content engines service the requested content). In this example, the request is forwarded to the content engines in San Jose and New York.
Step 4.	The content engines receive the request and then reply to the local DNS server. The first response is from the best content engine for this client, and this response is passed to the client.
Step 5.	The client communicates with the best content engine (which is in New York, in this example) and retrieves the requested web page for the user.

WCCP Mode

Key Point

When a content router is used in WCCP mode, users' requests are intercepted by a WCCP-enabled router and forwarded to the content router. (This is different from when the content router is used in direct mode, in which the user's local DNS server is configured to point directly to the content router.) If the content router cannot handle the user's request, it forwards the request on to the DNS server specified in the request. Otherwise, the content router handles the request in the same way as it does in direct mode, as described in the previous section.

The use of WCCP mode requires that WCCP be enabled both on the content router and on another router in the path between the user and the primary DNS server. This second router must be configured to send DNS address requests to the content router.

Figure 8-5 illustrates how a WCCP-mode content router interacts with other devices in the network.

Figure 8-5. A Content Router in WCCP Mode Receives Requests Intercepted by a WCCP-Enabled Router

The steps involved when a content router is operating in WCCP mode are as follows:

Step 1.	In Figure 8-5, the user at the workstation in Toronto requests a web page from a server. The user's workstation (the client) sends a DNS query for the IP address of the content that it is looking for.
Step 2.	This request is destined for a DNS server but is intercepted by the WCCP router.
Step 3.	The WCCP router forwards the request to the content router.
Step 4.	The content router forwards the request to a selection of the appropriate content engines (assuming that multiple content engines service the requested content). In this example, the request is forwarded to the content engines in San Jose and New York.
Step 5.	The content engines receive the request and then reply to the client. The first response is from the best content engine for this client.
Step 6.	The client communicates with the best content engine (which is in New York, in this example) and retrieves the requested web page for the user.