WAN Routers 539
More Information: Roles of the Router in WANs (Continued)
■ Autonomous system—An autonomous system (AS) is a network or internetwork (either
LAN or WAN) that is relatively self-contained. It is administered by a single person (or
organization), features a single routed protocol, has an address architecture, and usually
involves just one routing protocol. An autonomous system can support connections to
other autonomous systems owned and operated by the same organization. Alternatively,
an AS might have connections to other networks, such as the Internet, yet it retains auton-
omy of operation. This term is usually used in conjunction with specific routing protocols,
such as Open Shortest Path First (OSPF) or Border Gateway Protocol (BGP), that enable a
network to be carved into numbered subsections.
Given these definitions, it is possible to better define the functional classes of routers. An inte-
rior router is one that can be used by end systems in a network to access other end systems
within the same network. The interior router supports no connections to any other network.
Figure 10-11 illustrates a small network and identifies those devices that function as interior
routers.
Figure 10-11 Interior Routers in a Network
An exterior router is one that lies beyond the boundaries of any given network. Figure 10-12,
although not pretending to depict the Internet’s actual topology, presents a highly simplified
Internet topology that is solely intended to demonstrate what an exterior router is.
continues
Router
Router
Router
Router
Router
Router
Router
Router
Router
Router

Interior Routers
Internetwork
Private Network A
Private Network B
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540 Chapter 10: WANs and Routers
More Information: Roles of the Router in WANs (Continued)
Figure 10-12 Exterior Routers
The last functional class of router is the border router. As the name implies, border routers
interconnect a network with other networks. It is important to note that a single entity might
own and operate multiple autonomous systems. Therefore, a border router may denote the
boundary between two autonomous systems rather than the border between a private network
and some other network. Figure 10-13 identifies the border routers in the sample network that
was used in Figure 10-11 and Figure 10-12.
Figure 10-13 Border Routers
Router
Router
Router
Router
Router
Router
Router
Router
Router
Router
Exterior
Routers
Internetwork
Private Network A Private Network B
Router

Router
Router
Router
Router
Router
Router
Router
Router
Router
Internetwork
Border Routers
Private Network A Private Network B
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WAN Routers 541
Lab WAN Simulation
A viable WAN connection can connect routers from around the world. In an academy
lab, all the networks are connected with a serial or Ethernet cable, and the students
can see and physically touch the equipment. 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 to troubleshoot the New York router.
In the academy lab, the whole dedicated circuit cloud has been made extremely small
and put in the “crack” between the back-to-back DTE-DCE cables. The connection
from one router’s interface s0/0 to another's router interface s0/1 simulates the entire
circuit cloud.
Note that clocking must be configured, or this will not work properly for the use of
CSU/DSU.
More Information: Internetworking Scenarios
Having examined the concepts underlying routing and internetworking, as well as some of the
terminology inherent with these topics, you can see how they are used by examining three

internetworking scenarios. Each scenario demonstrates some of the issues that need to be
addressed in any network or internetwork:
■ Routing within a network
■ Routing between adjacent networks
■ Routing between nonadjacent networks
These three generic aspects encompass virtually every form of internetworking that you are
likely to encounter. Each one holds different implications for the network administrator, includ-
ing such routing aspects as route calculation and distribution, convergence, and security. The
following sections provide an overview of each internetworking scenario and highlight the
areas of concern for a network administrator. The various potential resolutions to these routing
concerns are presented throughout this book.
Routing Within a Network
The simplest form of routing is routing within the confines of a single network. Such a network
contains just interior routers. In theory, this form of network uses just one routed protocol, one
address architecture, and a minimum number of destinations. This structure greatly reduces
the workload of each router and maximizes potential network performance. Therefore, the
routing issues in intranetwork routing are more closely related to the network’s size and topol-
ogy than to its address architectures and routing protocols. The topology must be carefully
selected to match the requirements of the user community.
continues
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542 Chapter 10: WANs and Routers
More Information: Internetworking Scenarios (Continued)
If the network is small enough, it might be feasible for the administrator to preprogram all the
possible routes statically rather than introduce the complexity of a dynamic routing protocol.
Statically programmed routes, however, can become an onerous burden in a growing or con-
stantly changing network. You have to go to each router and manually update each time a
change occurs in the network rather than having routers automatically update.
Routing Between Adjacent Networks
A small step up in complexity from intranetwork routing is internetwork routing between adja-

cent networks. Physical adjacency means that the two networks are directly connected to each
other. Such an adjacency might have been designed to promote rapid convergence, improve
security, or satisfy any number of other performance criteria.
The logical separation of the multiple networks implies that the border routers must summa-
rize and redistribute routing information between them. In this fashion, the end systems of one
network can directly address the end systems in another network. Figure 10-14 illustrates this
type of routing.
Figure 10-14 Routing Between Adjacent Networks
The routing issues you need to address in this type of internetwork arise from the differences
between the two networks. Some of the potential differences that must be identified include
the following:
■ Do the networks belong to the same organization? If not, the border routers need to
secure the network’s perimeter.
■ Do the networks use the same routing protocol? If not, you need to find some mutually
acceptable metric.
■ Do the networks use the same routed protocol? If not, you might have to support a second
routed protocol in your network to facilitate internetworking.
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WAN Routers 543
More Information: Internetworking Scenarios (Continued)
Additionally, topology can affect routing between adjacent networks. Using a single point of
interconnection between the two networks, for example, makes it easy to control the calculation
and redistribution of routing information between the networks. This convenience introduces
a single point of failure, however, and it might not be acceptable to your users. Introducing a
second (or more) interconnection point solves the single point of failure problem but can
create the opportunity for infinite routing loops to occur. Resolving such a dilemma requires an
understanding of your users’ tolerance for downtime and risk. Armed with this understanding,
you can evaluate specific routing protocols for their capabilities to converge quickly and com-
pensate for potential routing problems.
Routing Between Nonadjacent Networks

Routing between nonadjacent networks is simultaneously the most complicated and useful
type of routing. Two networks can use a third network as an intermediary. It is highly likely that
the three different networks will use different routing protocols, routed protocols, and address
architectures. Therefore, the boundary router’s job is to overcome these obstacles to commu-
nication while also guarding the border of its network. Figure 10-15 illustrates routing between
nonadjacent small networks.
Figure 10-15 Routing Between Nonadjacent Networks
The border router of each private network in this illustration needs to protect the border of its
network from unwanted intrusion. Because the two networks that need to communicate aren’t
adjacent and the intermediary network is beyond their control, the risks of unwanted intrusion
are much higher than if the networks were directly internetworked. Therefore, the network
administrators must develop a set of criteria for allowing specific external users into their net-
work, while disallowing access to everyone else. The border router implements these criteria
in an access control list.
continues
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544 Chapter 10: WANs and Routers
Summary
In this chapter, you learned the following the key points:
■ The Internet is the interconnection of thousands of large and small networks all
over the world.
■ A WAN is used to interconnect LANs that are separated by a large geographic
distance.
■ Dedicated line, circuit-switching, packet-switching, and cell-switching are some
of the common WAN connection types.
■ Many types of WAN services are available to the WAN subscriber, who must
know how to interface to the WAN provider’s service.
■ WAN devices include routers, WAN switches, modems, CSUs/DSUs, and access
servers.
■ WAN physical layer protocols describe how to provide electrical, mechanical,

operational, and functional connection for WAN services.
■ WAN data link layer protocols describe how frames are carried between systems
on a single data link.
To supplement all that you’ve learned in this chapter, refer to the chapter-specific Videos,
PhotoZooms, and e-Lab activities on the CD-ROM accompanying this book.
More Information: Internetworking Scenarios (Continued)
Another responsibility of the border router is to summarize the internal routes and redistribute
this information to the networks beyond. This process enables users outside the bounds of the
private network to access its end systems. If this routing information isn’t distributed, no one
outside that private network is able to access its end systems.
Finally, it is highly likely that the border routers will have to be configured to use multiple routing
protocols. An interior gateway protocol will likely be selected for intranetwork routing
purposes. Calculating routes across the internetwork, however, might require a different proto-
col—one that features stronger support for route summarization. Route summarization can
reduce routing table size by aggregating routes to multiple networks into one or two routing
entries.
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Key Terms 545
Key Terms
cell-switched services Provide a dedicated-connection switching technology that
organizes digital data into cell units and transmits them over a physical medium using
digital signal technology.
circuit switching A WAN switching method in which a dedicated physical circuit
through a carrier network is established, maintained, and terminated for each commu-
nication session. ISDN is an example of a circuit-switched WAN technology.
DCE (data circuit-terminating equipment) (ITU-T expansion) Devices and connections
of a communications network that comprise the network end of the user-to-network
interface. The DCE provides a physical connection to the network, forwards traffic,
and provides a clocking signal used to synchronize data transmission between DCE
and DTE devices. Modems and interface cards are examples of DCE.

DTE (data terminal equipment) Device at the user end of a user-network interface
that serves as a data source, destination, or both. DTE connects to a data network
through a DCE device (for example, a modem) and typically uses clocking signals
generated by the DCE. DTE includes such devices as computers, protocol translators,
and multiplexers.
modem Device that converts digital and analog signals. At the source, a modem
converts digital signals to a form suitable for transmission over analog communication
facilities. At the destination, the analog signals are returned to their digital form.
Modems allow data to be transmitted over voice-grade telephone lines. The term
modem is also used to describe various digital devices such as CSU/DSUs and ISDN
terminal adapters.
packet-switched services Routes small units of data called packets through a
network based on the destination address contained within each packet.
propagation delay Time required for data to travel over a network from its source
to its ultimate destination.
router Network layer device that uses one or more metrics to determine the optimal
path along which network traffic should be forwarded. Routers forward packets from
one network to another based on network layer information.
switch Network device that filters, forwards, and floods frames based on the destina-
tion address of each frame. The switch operates at the data link layer of the OSI
model.
uptime The amount of time that the device is functional and in service relative to the
users’ requirements for its availability.
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546 Chapter 10: WANs and Routers
WAN (wide-area network) A data communications network spanning a large geo-
graphic area, such as a state, province or country. WANs often use transmission facili-
ties provided by common carriers, for example, telephone companies.
Check Your Understanding
Complete all the review questions to test your understanding of the topics and con-

cepts in this chapter. Answers are listed in Appendix C, “Check Your Understanding
Answer Key.”
1. Which of the following best describes a WAN?
A. Connects LANs that are separated by a large geographic area
B. Connects workstations, terminals, and other devices in a metropolitan area
C. Connects LANs within a large building
D. Connects workstations, terminals, and other devices within a building
2. How do WANs differ from LANs?
A. WANs emphasize access over serial interfaces operating at lower speeds.
B. WANs provide high-speed multiple access services.
C. WANs typically exist in small geographic areas.
D. WANs use tokens to regulate network traffic.
3. Which of the following are examples of WAN technologies?
A. Token Ring, ARCNet
B. Frame Relay, ISDN
C. Star, Banyan VINES
D. CSU/DSU, ARCView
4. Which layers of the OSI model do WAN standards describe?
A. Data link and network
B. Data link and presentation
C. Physical and application
D. Physical and data link
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Check Your Understanding 547
5. Which best describes data circuit-terminating equipment (DCE)?
A. Consists of the user device at the end of a network
B. Serves as the data source or destination
C. Consists of physical devices such as protocol translators and multiplexers
D. Consists of physical devices at the end of a WAN connection
6. Which of the following components provides interface voice-grade services,

channel service units/digital service units (CSUs/DSUs) that interface T1/E1
services, and terminal adapters/Network Termination 1 (TAs/NT1s) that
interface Integrated Services Digital Network (ISDN) services?
A. Switches
B. Routers
C. Modems
D. Communication servers
7. Which of the following concentrates the dial-in and dial-out user connections?
A. Switches
B. Routers
C. Modems
D. Communication servers
8. Some WAN physical and data link layer standards are:
A. EIA/TIA-232
B. PPP
C. Frame Relay
D. All of the above
9. Match the functions with the components.
1) RAM/DRAM
2) NVRAM
3) ROM
4) Flash memory
5) Interface
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548 Chapter 10: WANs and Routers
Options:
a. RAM that retains its contents when a unit is powered off
b. Volatile memory that can be read and written by the microprocessor
c. Volatile memory that can be read but not written by a microprocessor
d. Nonvolatile storage that can be electrically erased and reprogrammed so

that software images can be stored, booted, and rewritten as necessary
e. Connection between two systems or devices
Select your answer:
A. 1-b, 2-a, 3-c, 4-e, 5-d
B. 1-a, 2-b, 3-c, 4-e, 5-d
C. 1-b, 2-a, 3-e, 4-c, 5-d
D. 1-b, 2-a, 3-c, 4-d, 5-e
10. Any internetwork will probably include the following:
A. Consistent end-to-end addressing and priority-level bandwidth allocation
capabilities
B. Addresses that represent network topologies and assurance of quality of
service
C. Best path selection and dynamic routing
D. All of the above
11. Which of the following are data link encapsulations for WAN?
A. High-Level Data Link Control (HDLC)
B. Frame Relay
C. Point-to-Point Protocol (PPP)
D. All of the above
12. What are the main functions of routers?
A. The determination of best paths for incoming data packets and the switching
of packets to the proper outgoing interface
B. Replying to ARP requests when two nodes are on different LANs
C. Building routing tables and exchanging the network information contained
within them with other routers
D. Both A and B
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