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Overview

1.1

WANs

1.1.1

Introduction to WANs

A wide-area network (WAN) is a data communications network spanning a large geographic area such as a state, province, or country. WANs often use transmission facilities provided by common carriers, for example, telephone companies.

These are the major characteristics of WANs:

• They connect devices that are separated by wide geographical areas.
• They use the services of carriers such as the Regional Bell Operating Companies (RBOCs), Sprint, MCI, VPM Internet Services, Inc., and Altantes.net.
• They use serial connections of various types to access bandwidth over large geographic areas.

A WAN differs from a LAN in several ways. For example, unlike a LAN, which connects workstations, peripherals, terminals, and other devices in a single building or other small geographic area, a WAN makes data connections across a broad geographic area. Companies use a WAN to connect various company sites so that information can be exchanged between distant offices.

A WAN operates at the physical layer and the data link layer of the OSI reference model. It interconnects LANs that are usually separated by large geographic areas. WANs provide for the exchange of data packets and frames between routers and switches and the LANs they support.

The following devices are used in WANs:

• Routers offer many services, including internetworking and WAN interface ports.
• Switches in the WAN provide connectivity for voice, data, and video communication.
• Modems include interface voice-grade services, channel service units/digital service units (CSU/DSUs) that interface T1/E1 services, and Terminal Adapters/Network Termination 1 (TA/NT1s) that interface Integrated Services Digital Network (ISDN) services.
• Communication servers concentrate dial-in and dial-out user communication.

WAN data link protocols describe how frames are carried between systems on a single data link. They include protocols designed to operate over dedicated point-to-point, multipoint, and multi-access switched services such as Frame Relay. WAN standards are defined and managed by a number of recognized authorities, including the following agencies:

• International Telecommunication Union-Telecommunication Standardization Sector (ITU-T), formerly the Consultative Committee for International Telegraph and Telephone (CCITT).
• International Organization for Standardization (ISO).
• Internet Engineering Task Force (IETF).
• Electronic Industries Association (EIA).

1.1

WANs

1.1.2

Introduction to routers in a WAN

A router is a special type of computer. It has the same basic components as a standard desktop PC. It has a CPU, memory, a system bus, and various input/output interfaces. However, routers are designed to perform some very specific functions that are not typically performed by desktop computers. For example, routers connect and allow communication between two networks and determine the best path for data to travel through the connected networks.

Just as computers need operating systems to run software applications, routers need the Internetwork Operating System software (IOS) to run configuration files. These configuration files contain the instructions and parameters that control the flow of traffic in and out of the routers. Specifically, by using routing protocols, routers make decisions regarding the best path for packets. The configuration file specifies all the information for the correct set up and use of the selected, or enabled, routing and routed protocols on the router. 

This course will demonstrate how to build configuration files from the IOS commands in order to get the router to perform many essential network functions. The router configuration file may at first glance appear complex, but it will seem much less so by the end of the course.

The main internal components of the router are random access memory (RAM), nonvolatile random-access memory (NVRAM), flash memory, read-only memory (ROM), and interfaces.

RAM, also called dynamic RAM (DRAM), has the following characteristics and functions:

• Stores routing tables
• Holds ARP cache
• Holds fast-switching cache
• Performs packet buffering (shared RAM)
• Maintains packet-hold queues
• Provides temporary memory for the configuration file of the router while the router is powered on
• Loses content when router is powered down or restarted

NVRAM has the following characteristics and functions:

• Provides storage for the startup configuration file
• Retains content when router is powered down or restarted

Flash memory has the following characteristics and functions:

• Holds the operating system image (IOS)
• Allows software to be updated without removing and replacing chips on the processor
• Retains content when router is powered down or restarted
• Can store multiple versions of IOS software
• Is a type of electronically erasable, programmable ROM (EEPROM)

Read-only memory (ROM) has the following characteristics and functions:

• Maintains instructions for power-on self test (POST) diagnostics
• Stores bootstrap program and basic operating system software
• Requires replacing pluggable chips on the motherboard for software upgrades

Interfaces have the following characteristics and functions:

• Connect router to network for frame entry and exit
• Can be on the motherboard or on a separate module

1.1

WANs

1.1.3

Router LANs and WANs

While a router can be used to segment LANs, its major use is as a WAN device. Routers have both LAN and WAN interfaces. In fact, WAN technologies are frequently used to connect routers and these 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:

• Consistent end-to-end addressing
• Addresses that represent network topologies
• Best path selection
• Dynamic or static routing
• Switching

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:

• EIA/TIA-232
• EIA/TIA-449
• V.24
• V.35
• X.21
• G.703
• EIA-530
• ISDN
• T1, T3, E1, and E3
• xDSL
• SONET (OC-3, OC-12, OC-48, OC-192)

WAN data link layer standards and protocols:

• High-level data link control (HDLC)
• Frame Relay
• Point-to-Point Protocol (PPP)
• Synchronous Data Link Control (SDLC)
• Serial Line Internet Protocol (SLIP)
• X.25
• ATM
• LAPB
• LAPD
• LAPF

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.

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 main tasks for ROM are hardware diagnostics during router bootup and loading the Cisco IOS software from flash to RAM. Some routers also have a scaled down version of the IOS that can be used as an alternative boot source. ROMs are not erasable. They can only be upgraded by replacing the ROM chips in the sockets.

Interfaces – The interfaces are the router connections to the outside. The three types of interfaces are local-area network (LANs), wide-area network (WANs), and Console/AUX. The LAN interfaces are usually one of several different varieties of Ethernet or Token Ring. These interfaces have controller chips that provide the logic for connecting the system to the media. The LAN interfaces may be a fixed configuration or modular.

The WAN interfaces include serial, ISDN, and integrated Channel Service Unit (CSUs). As with LAN interfaces, WAN interfaces also have special controller chips for the interfaces. The WAN interfaces may be a fixed configuration or modular.

The Console/AUX ports are serial ports used primarily for the initial configuration of the router. These ports are not networking ports. They are used for terminal sessions from the communication ports on the computer or through a modem.

Power Supply – The power supply provides the necessary power to operate the internal components. Larger routers may use multiple or modular power supplies. In some of the smaller routers the power supply may be external to the router.

1.2

Routers

1.2.2

Router physical characteristics

It is not critical to know the location of the physical components inside the router to understand how to use the router. However in some situations, such as adding memory, it can be very helpful.

The exact components used and their location varies between router models. Figure identifies the internal components of a 2600 router.

Figure shows some of the external connectors on a 2600 router.

1.2

Routers

1.2.3

Router external connections

The three basic types of connections on a router are LAN interfaces, WAN interfaces, and management ports. LAN interfaces allow the router to connect to the Local Area Network media. This is usually some form of Ethernet. However, it could be some other LAN technology such as Token Ring or Asynchronous Transfer Mode (ATM).

Wide Area Network connections provide connections through a service provider to a distant site or to the Internet. These may be serial connections or any number of other WAN interfaces. With some types of WAN interfaces, an external device such as a CSU is required to connect the router to the local connection of the service provider. With other types of WAN connections, the router may be directly connected to the service provider.

The function of management ports is different from the other connections. The LAN and WAN connections provide network connections through which frame packets are passed. The management port provides a text-based connection for the configuration and troubleshooting of the router. The common management interfaces are the console and auxilliary ports. These are EIA-232 asynchronous serial ports. They are connected to a communications port on a computer. The computer must run a terminal emulation program to provide a text-based session with the router. Through this session the network administrator can manage the device.

1.2

Routers

1.2.4

Management port connections

The console port and the auxiliary (AUX) port are management ports. These asynchronous serial ports are not designed as networking ports. One of these two ports is required for the initial configuration of the router. The console port is recommended for this initial configuration. Not all routers have an auxiliary port.

When the router is first put into service, there are no networking parameters configured. Therefore the router cannot communicate with any network. To prepare for initial startup and configuration, attach an RS-232 ASCII terminal, or a computer emulating an ASCII terminal, to the system console port. Then configuration commands can be entered to set up the router.

Once this initial configuration is entered into the router through the console or auxiliary port, the router can then be connected to the network for troubleshooting or monitoring.

The router can also be configured from a remote location by dialing to a modem connected to the console or auxiliary port on the router.

The console port is also preferred over the auxiliary port for troubleshooting. This is because it displays router startup, debugging, and error messages by default. The console port can also be used when the networking services have not been started or have failed. Therefore, the console port can be used for disaster and password recovery procedures.

1.2

Routers

1.2.5

Connecting console interfaces

The console port is a management port used to provide out-of-band access to the router. It is used for the initial configuration of the router, monitoring, and disaster recovery procedures.

To connect to the console port, a rollover cable and a RJ-45 to DB-9 adapter are used to connect a PC. Cisco supplies the necessary adapter to connect to the console port.

The PC or terminal must support VT100 terminal emulation. Terminal emulation software such as HyperTerminal is usually used.

To connect the PC to a router:

1. Configure terminal emulation software on the PC for:
• The appropriate com port
• 9600 baud
• 8 data bits
• No parity
• 1 stop bit
• No flow control
2. Connect the RJ-45 connector of the rollover cable to the router console port.
3. Connect the other end of the rollover cable to the RJ-45 to DB-9 adapter.
4. Attach the female DB-9 adapter to a PC.

1.2

Routers

1.2.6

Connecting LAN interfaces

In most LAN environments, the router is connected to the LAN using an Ethernet or Fast Ethernet interface. The router is a host that communicates with the LAN via a hub or a switch. A straight-through cable is used to make this connection. A 10/100BaseTX router interface requires Category 5 or better, unshielded twisted-pair (UTP) regardless of the router type.

In some cases the Ethernet connection of the router is connected directly to the computer or to another router. For this type of connection, a crossover cable is required.

The correct interface must be used. If the wrong interface is connected, damage can result to the router or other networking devices. Many different types of connections use the same style of connector. For example Ethernet, ISDN BRI, Console, AUX, integrated CSU/DSU, and Token Ring interfaces use the same eight-pin connector, RJ-45, RJ-48, or RJ-49.

To help distinguish the connections on the router, Cisco uses a color code scheme to identify connector use. Figure shows some of these for a 2600 router.

1.2

Routers

1.2.7

Connecting WAN interfaces

WAN connections may take any number of forms. A WAN makes data connections across a broad geographic area using many different types of technology. These WAN services are usually leased from service providers. Among these WAN connection types are leased line, circuit-switched, and packet-switched.

For each type of WAN service, the customer premises equipment (CPE), often a router, is the data terminal equipment (DTE). This is connected to the service provider using a data circuit-terminating equipment (DCE) device, commonly a modem or channel service unit/data service unit (CSU/DSU). This device is used to convert the data from the DTE into a form acceptable to the WAN service provider.

Perhaps the most commonly used router interfaces for WAN services are serial interfaces. Selecting the proper serial cable is as easy as knowing the answers to four questions:

• What is the type of connection to the Cisco device? Cisco routers may use different connectors for the serial interfaces. The interface on the left is a Smart Serial interface. The interface on the right is a DB-60 connection. This makes the selection of the serial cable connecting the network system to the serial devices a critical part in setting up a WAN.
• Is the network system being connected to a DTE or DCE device? DTE and DCE are the two types of serial interfaces that devices use to communicate. The key difference between these two is that the DCE device provides the clock signal for the communications on the bus. The device documentation should specify whether it is DTE or DCE.
• What signaling standard does the device require? For each different device, a different serial standard could be used. Each standard defines the signals on the cable and specifies the connector at the end of the cable. Device documentation should always be consulted for the signaling standard.
• Is a male or female connector required on the cable? If the connector has visible projecting pins, it is male. If the connector has sockets for projecting pins, it is female.

Summary

An understanding of the following key points should have been achieved:

• WAN and LAN concepts
• Role of a router in WANs and LANs
• WAN protocols
• Configuring encapsulation
• The identification and description of the internal components of a router
• The physical characteristics of a router
• The common ports on a router
• How to connect router console, LAN, and WAN ports