ter's memory. If a LAN- connected device such as a printer or PC is moved from one LAN to another, the router table must be updated or messages will not reach that device. The fact that routers transmit packets by IP address rather than individual device MAC addresses is one reason they are considered Layer 3 devices. Routers are critical. If they fail, all access to the Internet and other sites is lost. Because of this criticality, organizations typically purchase backup routers. They can balance the traffic between the two routers or keep a backup router on hand, which can be installed quickly in the event of a primary-device failure. The devices that connect internal networks to a carrier's public networks are considered edge devices. Routers are defined as edge devices because they connect sites to the Internet via dedicated links, such as Carrier Gigabit Ethernet service. For more on Carrier Gigabit Ethernet, see Chapter 5, "Broadband Network Services." In cellular networks, cell phones and smartphones are www.TechnicalPDF.com edge devices that connect users to either the Internet or other mobile devices. Network Functions Virtualization Enterprises are starting to implement previously hardware-based network devices as software on their network. For example, routing and switching functions may be represented as software in commodity, x86 servers. Additional Router Functionality Beyond simply acting as a connection point to the Internet, routers can provide other functionality, including the following: Firewall Routers can screen incoming and outgoing traffic. Deep Packet Inspection (DPI) DPI can manage internal traffic. See Chapter 1 for more information about DPI. Encryption protocols Routers employ IPsec and other protocols for sending packets over the Internet securely. Encryption is the use of complex algorithms to mathematically reorder bits in frames and packets. Carrier Gigabit Ethernet See Chapter 5 for more information about Carrier Gigabit Ethernet. Video Digital Signal Processor (DSP) This is used