arrying e-mail from a certain Internet service provider (ISP) to all enterprise users. To make forwarding and routing decisions based on policy For example, an enterprise with multiple connections to the outside world may decide to route all voice over Internet Protocol (VoIP) and IP telephony traffic over an Asynchronous Transfer Mode (ATM) network link of carrier A instead of letting it go through the IP network of ISP B. To limit traffic based on rates A core network provider can stipulate to its equipment that no more than 75 Mbps traffic from a specific ISP can be accepted. They can also state that no more than 10 Mbps out of whatever comes in can be telnet sessions. To impose traffic shaping For example, an intercarrier network where specific agreements are signed with regional carriers might apply a rule that stipulates that no more traffic above 75 Mbps from carrier X will be allowed into the core pipeline. To provide accounting and billing per agreed levels of usage For example, some context could stipulate that all streaming video traffic destined to host X must be forwarded through the output interface Y, and accounting and billing must be performed on a per-packet basis according to some contract agreed to by the customer. Obviously, the possibilities are endless. Let us now go back and look at each one of these two major categories of classification in a little more detail. Destination Address Output Port Copies of the Lookup Lookup Table 123.14.21.xxx Table originally 98.13.xxx.xxx kept at each 98.13.20.xxx line card Switch/router 98.13.14.121 Card 1 Card 1 123.14.21.223 Card 2 Card 2 Destination addresses output from the headers of incoming packets backplane 98.13.20.77 Card N Card N FIGURE 13.1 Lookup and forwarding based on tables kept in local memory at each line card. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. CLASSIF