er generation networks, hardware-routers and switches-were key factors in operating local area and wide area networks. They were programmed to perform specific routing tasks such as routing packets to other networks. In software designed wide area networks (WA SDNs), software external to router and switching functions control the routing, switching, and other functions in the network. See Figure 5-11 for an overview of routing on WA SDNs. A controller is programmed with rules that are applied under specified conditions. For example, alternate routes are set up SO that if there is congestion or a malfunctioning section, traffic is automatically sent over an alternate route. WA SDN is used to manage traffic at branch offices, headquarters, and data centers as well as in carriers' core networks. www.TechnicalPDF.com Network Controller at telephone company with script for alternate routes if there's congestion on the main route Congested default route on data plane Control plane signals LLLLL Branch Router & Router Headquarters firewall firewall Alternate route Figure 5-11 An overview of routing on Wide Area Software Defined Networks. www.TechnicalPDF.com SDN software can be located in the cloud, or at the telephone company's data center-within their CORD. CORD is short for Central Office Re-architected as a Data Center. For information on CORD, see the section "Central Office Re-architected as a Data Center (CORD) -Streamlined Central Offices" in Chapter 4, "Managing Broadband Networks." Or, large enterprises may use WA SDN to manage traffic within their own broadband networks. WA SDNs are used to invoke commonly used operations under specified conditions. For example, configurations for branch offices can also be defined and tested centrally before being downloaded to each location. In large organizations with many branch or warehouse locations, network configurations can be downloaded sequentially to each location once there are circuits installed at the locations. An Application Program Interface (API) is used as