routers communicate with each other by WAN connections. Routers are the backbone devices of large intranets and of the Internet. They operate at Layer 3 of the OSI model, making decisions based on network addresses. The two main functions of a router are the selection of best path for and the switching of frames to the proper interface. Routers accomplish this by building routing tables and exchanging network information with other routers. An administrator can maintain routing tables by configuring static routes, but generally routing tables are maintained dynamically through the use of a routing protocol that exchanges network topology (path) information with other routers. If, for example, computer (x) needs to communicate with computer (y) on one side of the world, and with computer (z) in another distant location, a routing feature for information flow is required as well as redundant paths for reliability. Many network design decisions and technologies can be traced to this desire for computers x, y, and z to be able to communicate. A correctly configured internetwork provides the following: Web Links How Routers Work http://www22.verizon.com/about/community/ learningcenter/articles/dslarticle1/ 0%2C16157%2C1080z1%2C00.html
Content 1.1 WANs 1.1.4 Router role in a WAN A WAN is said to operate at the physical layer and at the data link layer. This does not mean that the other five layers of the OSI model are not found in a WAN. It simply means that the characteristics that separate a WAN from a LAN are typically found at the physical layer and the data link layer. In other words, the standards and protocols used in WANs at Layer 1 and Layer 2 are different from those used in LANs at the same layers. The WAN physical layer describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Generally, the DCE is the service provider and the DTE is the attached device. In this model, the services offered to the DTE are made available through a modem or a CSU/DSU. The principal function of a router is routing. Routing occurs at the network layer, Layer 3, but if a WAN operates at Layers 1 and 2, is a router a LAN device or a WAN device? The answer is both, as is so often the case in the field of networking. A router may be exclusively a LAN device, it may be exclusively a WAN device, or it may sit at the boundary between a LAN and a WAN and be a LAN and WAN device at the same time. One of the roles of a router in a WAN is to route packets at Layer 3, but this is also a role of a router in a LAN. Therefore routing is not strictly a WAN role of a router. When a router uses the physical and data link layer standards and protocols that are associated with WANs, it is operating as a WAN device. The primary WAN roles of a router are therefore not routing, but providing connections to and between the various WAN physical and data-link standards. For example, a router may have an ISDN interface using PPP encapsulation and a serial interface terminating a T1 line using Frame Relay encapsulation. The router must be able to move a stream of bits from one type of service, such as ISDN, to another, such as a T1, and change the data link encapsulation from PPP to Frame Relay. Many of the details of WAN Layer 1 and Layer 2 protocols will be covered later in the course, but some of the key WAN protocols and standards are listed here for reference. WAN physical layer standards and protocols: WAN data link layer standards and protocols: Web Links Choosing a WAN Router http://www.naspa.com/PDF/97/ T9704009.pdf
Content 1.1 WANs 1.1.5 Academy approach to hands-on labs In the academy lab, all the networks will be connected with serial or Ethernet cables and the students can see and physically touch all the equipment. Unlike the academy lab setup, the serial cables in the real world are not connected back to back. In a real world situation, one router could be in New York, while another router could be in Sydney, Australia. An administrator located in Sydney would have to connect to the router in New York through the WAN cloud in order to troubleshoot the New York router. In the academy lab, devices that make up the WAN cloud are simulated by the connection between the back-to-back DTE-DCE cables. The connection from one router interface s0/0 to another router interface s0/1 simulates the whole circuit cloud. Interactive Media Activity Drag and Drop: Academy Lab Equipment Setup When the student completes this activity, the student should know the correct order in which to connect all devices and cables to create the CCNA lab equipment setup. Web Links Introduction to WAN Technologies http://www.cisco.com/univercd/cc/td/doc/ cisintwk/ito_doc/ introwan.htm
Content 1.2 Routers 1.2.1 Router internal components While the exact architecture of the router varies between router models, this section will introduce the major internal components. Figures and show the internal components of some of the Cisco router models. The common components are covered in the paragraphs below. CPU – The Central Processing Unit (CPU) executes instructions in the operating system. Among these functions are system initialization, routing functions, and network interface control. The CPU is a microprocessor. Large routers may have multiple CPUs. RAM – Random-access memory (RAM) is used for routing table information, fast switching cache, running configuration, and packet queues. In most routers the RAM provides run time space for executable Cisco IOS software and its subsystems. RAM is usually logically divided into main processor memory and shared input/output (I/O) memory. Shared I/O memory is shared among interfaces for temporary storage of packets. The contents of RAM are lost when power is removed. RAM is generally dynamic random-access memory (DRAM) and can be upgraded by adding additional Dual In-Line Memory Modules (DIMMs). Flash – Flash memory is used for storage of a full Cisco IOS software image. The router normally acquires the default IOS from flash. These images can be upgraded by loading a new image into flash. The IOS may be in uncompressed or compressed form. In most routers an executable copy of the IOS is transferred to RAM during the boot process. In other routers the IOS may be run directly from flash. Adding or replacing the flash Single In-Line Memory Modules (SIMMs) or PCMCIA cards can upgrade the amount of flash. NVRAM – Nonvolatile random-access memory (NVRAM) is used to store the startup configuration. In some devices, NVRAM is implemented using separate electronically erasable programmable read-only memory (EEPROMs) in some devices. In other devices it is implemented in the same flash device from which the boot code is loaded. In either case these devices retain contents when power is removed. Buses – Most routers contain a system bus and a CPU bus. The system bus is used for communication between the CPU and the interfaces and/or expansion slots. This bus transfers the packets to and from the interfaces. The CPU bus is used by the CPU for accessing components from router storage. This bus transfers instructions and data to or from specified memory addresses. ROM – Read-only memory (ROM) is used for permanently storing startup diagnostic code (ROM Monitor). The