Module Overview
12.1 WANs
12.1.1 Introduction to WANs
12.1.2 Introduction to routers in a WAN
12.1.3 Router LANs and WANs
12.1.4 Role of routers in a WAN
12.1.5 Academy approach to hands-on labs
12.2 Routers
12.2.1 Introduction to WANs
12.2.2 Router physical characteristics
12.2.3 Router external connections
12.2.4 Management port connections
12.2.5 Console port connections
12.2.6 Connecting router LAN interfaces
12.2.7 Connecting WAN interfaces
Module: Summary
overview
A wide-area network (WAN) is a data
communications network that connects user networks over a large geographical
area. WANs have several important characteristics that distinguish them from
LANs. The first lesson in this module will provide an overview of WAN
technologies and protocols. It will also explain how WANs and LANs are
different, and ways in which they are similar.
It is important to understand the physical
layer components of a router. This knowledge builds a foundation for other
information and skills that are needed to configure routers and manage routed
networks. This module provides a close examination of the internal and external
physical components of the router. The module also describes techniques for physically
connecting the various router interfaces.
This module covers some of the objectives
for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams. -
Students who complete this module should be
able to perform the following tasks:
- Identify
organizations responsible for WAN standards
- Explain
the difference between a WAN and LAN and the type of standards and
protocols each uses
- Describe
the role of a router in a WAN
- Identify
internal components of the router and describe their functions
- Describe
the physical characteristics of the router
- Identify
LAN and management ports on a router
- Properly
connect Ethernet, serial WAN, and console ports
12.1 WANs
12.1.1 Introduction to WANs
A WAN is a data communications network that
spans a large geographic area such as a state, province, or country. WANs often
use transmission facilities provided by common carriers such as 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, and VPM Internet Services, Inc. to establish the
link or connection between sites.
- 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, 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.
- 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.
The Interactive Media Activity will help
students become more familiar with WAN devices.
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)
The next page will describe routers. This
information is important to further understand WANs.
12.1
WANs
12.1.2
Introduction to routers in a WAN
This page will provide a brief review of
routers.
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 (IOS)
software to run configuration files. These configuration files contain the
instructions and parameters that control the flow of traffic in and out of the
routers. Routers use routing protocols to determine the best path for packets.
The configuration file specifies all the information for the correct setup and
use of the selected, or enabled, routing and routed protocols on a 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 seem
complex at first, but it will be easier to understand 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 has the following characteristics and
functions:
- Stores
routing tables
- Holds
ARP cache
- Holds
fast-switching cache
- Performs
packet buffering as shared RAM
- Maintains
packet-hold queues
- Provides
temporary memory for the configuration file of a router while the router
is powered on
- Loses
content when a router is powered down or restarted
NVRAM has the following characteristics and
functions:
- Provides
storage for the startup configuration file
- Retains
content when a router is powered down or restarted
Flash memory has the following
characteristics and functions:
- Holds
the IOS image
- Allows
software to be updated without removing and replacing chips on the
processor
- Retains
content when a router is powered down or restarted
- Can
store multiple versions of IOS software
- Is
a type of electrically erasable programmable read-only memory (EEPROM)
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
routers to a network for packet entry and exit
- Can
be on the motherboard or on a separate module
The next page will describe the role of
routers in WANs and LANs.
12.1
WANs
12.1.3
Router LANs and WANs
Routers can be used to segment LANs, but
they are mainly used as WAN devices. This page will explain how routers are
used in a network.
Routers have both LAN and WAN interfaces.
WAN technologies are frequently used to connect routers. Routers use WAN
connections to communicate with each other.
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
and the switching of packets to the proper interface. To accomplish this,
routers build routing tables and exchange network information with other
routers.
An administrator can configure static
routes to maintain routing tables. However, most routing tables are maintained
dynamically through the use of a routing protocol that exchanges network
topology information with other routers.
For example, if Computer X needs to
communicate with Computer Y and Computer Z in Figure , this requires a routing
feature for information flow and redundant paths for reliability. Many network
design decisions and technologies can be traced to this desire for Computers X,
Y, and Z 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
The next page will explain the function of
routers in a WAN.
12.1
WANs
12.1.4
Role of routers in a WAN
This page will review WANs in relation to
the OSI model and explain the functions of a router.
The standards and protocols or primary
functions of a WAN 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 standards and protocols that define a WAN
connection are typically found at the physical and data link layers. In other
words, the Layer 1 and Layer 2 WAN standards and protocols are different than
the Layer 1 and Layer 2 LAN standards and protocols.
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 main function of a router is to
transmit data using Layer 3 addresses. This process is also called routing.
Routing occurs at the network layer, which is Layer 3. If a WAN operates at
Layers 1, 2, and 3, 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. Therefore, the main role of a
router in a WAN is not to route. It is to provide connections between the
various WAN physical and data-link standards. These standards and protocols
that define and structure a WAN connection operate at Layers 1 and 2. For
example, a router may have an ISDN interface that uses PPP encapsulation and a
serial interface at the end of a T1 line that uses 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.
Here is a list of 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)
Here is a list of 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
The next page will describe how a WAN is
simulated in a lab environment.
12.1
WANs
12.1.5
Academy approach to hands-on labs
This page will help students understand how
a lab is configured to simulate a WAN.
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.
Students can use the Interactive Media
Activity to practice the connection of lab devices.
This page concludes the discussion about
WANs. The next lesson will describe routers in greater detail.
12.2
Routers
12.2.1
Introduction to WANs
While the exact architecture of the router varies between
router models, this page 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 – RAM is used for routing table
information, fast switching caches, running configurations, 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 with the addition of 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. The flash single in-line memory modules
(SIMMs) or PCMCIA cards can be added or replaced to upgrade the amount of
flash.
NVRAM – NVRAM is used to store the startup
configuration. In some devices, EEPROMs can be used to implement NVRAM. 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 to communicate between the CPU and the
interfaces 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 – ROM is used to permanently store the
startup diagnostic code, which is called the 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 LANs, WANs, and
console or auxiliary (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 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 and AUX ports are serial ports
that are used primarily for the initial configuration of a router. 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.
Students can use the Interactive Media
Activity to test their knowledge of router components.
The next page will describe the components
of a Cisco 2600 router.
12.2
Routers
12.2.2
Router physical characteristics
This page will help students identify the
location of different components on a router.
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.
Students can use the Interactive Media
Activities to learn more about the Cisco 1721 and 2621 routers.
The next page will describe the external
connections on a router.
12.2
Routers
12.2.3
Router external connections
This page will describe the three basic types of
connections on a router, which are LAN interfaces, WAN interfaces, and
management ports.
LAN interfaces allow routers to connect to
the LAN media. This is usually some form of Ethernet. However, it could be some
other LAN technology such as Token Ring or FDDI.
WANs 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 packets are forwarded. The management port
provides a text-based connection for the configuration and troubleshooting of
the router. The common management interfaces are the console and auxiliary
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.
The next page will provide a detailed
explanation of management ports.
12.2
Routers
12.2.4
Management port connections
This page will introduce the console and
auxiliary (AUX) ports, which are also known as the management ports. These
asynchronous serial ports are not designed as networking ports. The console
port is required for the configuration of the router. 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 attach
the rollover cable to a personal computer running terminal emulating software
such as HyperTerminal, to the system console port. Then configuration commands
can be entered to set up the router.
After the initial configuration is entered
into the router through the console or auxiliary port, the router can be
connected to the network to troubleshoot or monitor it.
The router can also be remotely configured
through the configuration port across an IP network using Telnet or 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.
The next page contains more information
about console ports.
12.2
Routers
12.2.5
Console port connections
This page will provide more information about the console
port.
The console port is a management port that
is used to provide out-of-band access to a router. It is used to set up the
initial configuration of a router and to monitor it. The console port is also
used for disaster recovery procedures.
A rollover cable and an RJ-45 to DB-9 adapter
are used to connect a PC to the console port.
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.
The following are steps to connect a PC to
a router:
- Configure
terminal emulation software on the PC for the following:
The appropriate
COM port
·
9600 baud
·
8 data bits
·
No parity
·
1 stop bit
·
No flow control
- Connect
the RJ-45 connector of the rollover cable to the router console port.
- Connect
the other end of the rollover cable to the RJ-45 to DB-9 adapter.
- Attach
the female DB-9 adapter to a PC.
Students can use the Lab Activity to further practice the
steps listed above.
The next page will explain how LAN
interfaces are connected.
12.2
Routers
12.2.6
Connecting router LAN interfaces
This page will teach students how to
connect LAN interfaces.
A router is usually connected to a LAN
through an Ethernet or Fast Ethernet interface. The router is a host that
communicates with the LAN through a hub or a switch. A straight-through cable
is used to make this connection. A 10BASE-TX or 100BASE-TX router interface
requires Category 5, or better, unshielded twisted-pair (UTP) cable, 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, it can damage 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, which is RJ-45, RJ-48, or
RJ-49. Students can use the Lab Activity and the Interactive Media Activity to
practice LAN interface connections.
Cisco uses a color code scheme to help
distinguish the connections that are used on a router. Figure shows some of these for a 2600 router.
The next page will discuss WAN interface
connections.
12.2 Routers
12.2.7
Connecting WAN interfaces
This page discusses the different forms of
WAN connections.
A WAN uses many different technologies to
make data connections across a broad geographic area. WAN communication
services are usually leased from service providers. WAN connection types
include leased line, circuit-switched, and packet-switched.
For each type of WAN service, the customer
premises equipment (CPE), which is often a router, is the DTE. This is
connected to the service provider through a DCE device, which is commonly a
modem or 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. Answer the following
questions to select the proper serial cable:
- 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. It is
important to select the correct serial cable to connect the network system
to the serial devices. This is a critical part in setting up a WAN.
- Is
the network system 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.
- Which
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.
Students can use the Lab Activity and the
Interactive Media Activity to practice WAN connections.
This page concludes Module 1. The next page
will provide a summary of the main points from this module.
Summary
This page summarizes the topics discussed
in this module.
The major difference between a WAN and a
LAN is the geographical area that is covered. A LAN connects workstations,
printers, servers, and other devices within a building or other small area. A
WAN is used to connect multiple LANs, typically over a large geographical area.
The primary characteristics of a WAN include the ability to connect devices
separated by wide geographical areas, the use of service companies to make these
connections, and the serial connections used to access bandwidth.
There are several organizations that define
and manage the standards used for WAN design such as ITU-T, ISO, IETF, and EIA.
WANs operate at the physical layer and the
data link layer, which are Layers 1 and 2 of the OSI reference model. The
devices used in a WAN, such as routers, CSU/DSUs, modems, and communication
servers, operate at the physical layer. At the data link layer, the protocols
determine how frames are carried between systems. A router can act as a LAN or
a WAN device because it operates at the network layer, which is Layer 3.
Routers are specialized computers that use
the Cisco IOS software to run configuration files. The main internal components
of a router are as follows:
- The
CPU, which executes instructions in the operating system
- RAM
or DRAM to store the routing tables
- NVRAM
to provide storage for the startup configuration file
- Flash
memory to hold the IOS
- ROM
for the POST
- Interfaces
to connect to a PC or modem
There are three basic external connections
on a router:
- LAN
interface
- WAN
interface
- Management
interface
Management is used for the initial setup of
the router and for troubleshooting. Most routers provide a console port, which
is an EIA-232 asynchronous serial port. Some routers include an auxiliary port.
A rollover cable and an RJ-45 to DB-9 adapter are used to connect the router
console port to a PC.
In a LAN environment, the router is a host
that communicates with the LAN through a hub or a switch. It is connected using
a straight-through cable. A WAN is a little more complicated. The DTE is
connected from the CPE to the service provider through a DCE device, which is
typically a modem or CSU/DSU. This device converts the data from the DTE to a form
recognized by the service provider. WAN services include leased line,
circuit-switched, or packet-switched. Four considerations are used to select
the proper cable:
- The
type of connection to the Cisco device
- The
type of network system that will be connected, which is DTE or DCE
- The
signaling standard
- The
type of connector on the cable
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