 to one switch is down, the Layer 3 switch can send traffic via another link. Layer 3 switches generally are located in wiring closets (connecting hundreds of users) or LAN data centers (connecting many wiring closets or buildings together). Most enterprises use Layer 3 switches to connect traffic on the LAN or campus backbone. 40/100Gbps Ethernet Switches The Institute of Electrical and Electronics Engineers (IEEE) has approved a 40/100Gbps standard for switches. Manufacturers have made these high-speed devices available. Older Layer 3 switches typically support 10Gbps speeds. However, for organizations that support the very largest web sites, this is actually too slow. To support that level of web site traffic, they network together 10Gbps Ethernet switches or purchase the higher-speed Ethernet switches. LAN Backbones in Heavy Traffic Areas LAN backbones are the portions of an organization's internal networks that carry the heaviest amount of traffic. This includes the copper or fiber-optic cabling run between buildings on a campus and the fiber-optic cabling that typically runs between floors within buildings. In multistory buildings, backbone cabling connects switches in wir- ing closets to Layer 3 backbone switches located in data centers. The devices that connect internal networks to a carrier's public networks are con- sidered edge devices. Routers are defined as edge devices because they connect sites to the Internet via dedicated links, such as T1, T3, and Carrier Gigabit Ethernet services. For more on T1, T3, and Carrier Gigabit Ethernet, see Chapter 5, "Broadband and Wide Area Network Services." In carrier networks, cell phones and smartphones are edge devices that connect users to either the Internet or other mobile devices. Switch Redundancy in the Backbone If a backbone switch crashes, it will cause many LAN segments to lose service, not just a single department. Organizations consider a variety of levels of redundancy for Layer 3 switches, including the following. Purchase of a separate, duplicate 