WAN Routers 529
WAN Routers
Just as computers need operating systems to run software applications, routers need
Cisco 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.
Specifically, by using routing protocols to direct routed protocols and routing tables,
routers make decisions regarding the best path for packets. The configuration file spec-
ifies all the information the router needs to make these decisions.
Router Internal Components
Table 10-9 outlines the main internal configuration components of the router.
More Information: Resource Utilization Rates (Continued)
These rates can be tricky to analyze and might even be misleading. It is not uncommon, for
example, for network-management software packages to capture utilization data in time inter-
vals. These intervals can be 1 hour, 5 minutes, or just about any other interval. If set too
coarsely, the sampling frequency can miss short-duration fluctuations in bandwidth consump-
tion. If the sampling is too frequent, you can find yourself mired in a meaningless morass of
data points. The trick is finding the right frequency that provides meaningful data about how
the network is performing relative to the users’ expectations.
Beyond merely selecting the sampling rate is the issue of sampling window. The sampling
window should be determined by the users’ requirements for WAN availability. If the utilization
samples are spread over a 24-hour day and a 7-day week, whereas the users work only
10 hours per day, 5 days per week, the statistical data is not indicative of how well the users’
requirements are being met.
Utilization rates are a wonderful statistical tool for monitoring and measuring the status of
transmission facilities. However, they are not the only metric for assessing a network’s perfor-
mance. The network is successful only if it satisfies the users’ requirements. Therefore, a com-
bination of performance metrics that provides a multifaceted, composite perspective is likely to
provide a better assessment of the network’s success than any single metric can offer.
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Table 10-9 Main Internal Configuration Components of a Router

Internal Component Characteristics
Random-access
memory (RAM/
DRAM)
Stores routing tables.
Address Resolution Protocol (ARP) cache.
Fast-switching cache.
Packet buffering (shared RAM).
Packet-hold queues.
Provides temporary and running memory for the configu-
ration file of the router while the router is powered on.
RAM content is lost when powered down or restarted.
Nonvolatile random-
access memory
(NVRAM)
Stores the backup/startup configuration file for the router.
Retains content when the router is powered down or
restarted.
Flash memory Erasable, programmable read-only memory (EPROM).
Holds the operating system image and microcode.
Allows software to be updated without removing and
replacing chips on the processor.
Content remains when powered down or restarted.
Multiple versions of Cisco IOS Software can be stored in
Flash memory.
Read-only memory
(ROM)
Maintains instructions for power-on self test (POST)
diagnostics.
Stores a bootstrap program, and basic operating system

software.
Software upgrades in ROM require replacing pluggable
chips on the CPU.
Interface Network connection through which packets enter and exit
a router.
Located on the motherboard or on a separate interface
module.
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WAN Routers 531
More Information: A Closer Look at Routers
Routers are designed to interconnect multiple networks. This interconnection enables machines
on different networks to communicate with each other. Interconnected networks can be
co-located or geographically dispersed. Networks that are geographically dispersed are usually
interconnected via a WAN. WANs are constructed of numerous different technologies, includ-
ing routers, transmission facilities, and line drivers. It is the router’s capability to interconnect
networks in a WAN that has made it indispensable.
A router is an intelligent network device that operates predominantly at the first three layers of
the OSI reference model. Routers, like any host, are actually capable of operating at all seven
layers of the OSI reference model. Depending on your particular configuration, you might or
might not use all seven layers of functionality. However, the need for the first three layers is
virtually universal. Communication across the first two layers allows routers to communicate
directly with LANs (data link layer constructs). More importantly, routers can identify routes
through networks based on Layer 3 addresses. This enables routers to internetwork multiple
networks by using network layer addressing, regardless of how near or far they might be rela-
tive to each other.
Understanding routers and routing requires examining a router from two different perspec-
tives: physical and logical. From a physical perspective, routers contain myriad parts, each of
which has a specific function. From a logical perspective, routers perform many functions,
including finding other routers in the network, learning about potential destination networks
and hosts, discovering and tracking potential routes, and forwarding datagrams toward their

specified destination. Together, these physical components and logical functions enable you to
build and use internetworks, including WANs.
Physical Components of a Router
A router is a remarkably complex device. Its complexity lies in its routing engine logic that
enables the physical device to perform the various routing functions. The complexity of routing
logic is hidden by the relative simplicity of the router’s physical form. The most common type
of router is actually a highly specialized type of computer; it contains the same basic compo-
nents as any other computer. These components include the following:
■ A central processing unit (CPU)
■ Random-access memory (RAM)
■ A basic input/output system (BIOS)
■ An operating system (OS)
■ A motherboard
■ Physical input/output (I/O) ports
■ A power supply, chassis, and sheet-metal skin
continues
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532 Chapter 10: WANs and Routers
More Information: A Closer Look at Routers (Continued)
The vast majority of a router’s components will remain forever shielded from the eyes of net-
work administrators by the chassis’ sheet-metal skin. These components are extremely reliable
and, under normal operating conditions, shouldn’t see the light of day. The obvious exceptions
to this general statement are born of expansion. Any time you need to add more resources to
the router, you might have to take off its cover. Such resources usually include either memory
or I/O ports.
The components that a network administrator will encounter most often are the OS and the I/O
ports. A router’s OS (in Cisco Systems’ case, the Internetwork Operating System or IOS) is the
software that controls the various hardware components and makes them usable. Network
administrators mostly use a command-line interface to develop a logical configuration. The
configuration is a profile of the system: the numbers, the locations, the types of each I/O port,

and details such as addressing and bandwidth information. A router’s configuration can also
include security information such as which users are permitted access to specific I/O ports and
configuration modes.
The I/O ports are the one physical router component that network administrators see on a rou-
tine basis. These ports bear out the router’s unique capability to interconnect seemingly end-
less combinations of LAN and WAN transmission technologies. Each one of these ports,
whether LAN or WAN, must have its own I/O port on the router. These ports function like a net-
work interface card (NIC) in a LAN-attached computer; they are related to the medium and
framing mechanisms expected and provide the appropriate physical interfaces. Many of these
physical interfaces appear quite similar to each other. This physical similarity belies the differ-
ences between the higher-layer functions of those technologies. Therefore, you will find it more
useful to examine transmission technologies than to examine specific physical interfaces.
Functions of a Router
The logical functions that a router performs are just as important as providing physical inter-
connectivity for multiple networks. These functions make the physical interconnections usable.
For example, internetworked communications require that at least one physical path intercon-
nect the source and destination machines. However, having and using a physical path are two
very different things. Specifically, the source and destination machines must speak a common
language (a routed protocol). It also helps if the routers that lie between them also speak a
common language (a routing protocol) and agree on which specific physical path is the best
one to use.
Therefore, some of the more salient functions that a router provides are
■ Physical interconnectivity
■ Logical interconnectivity
■ Route calculation and maintenance
■ Security
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WAN Routers 533
More Information: A Closer Look at Routers (Continued)
Physical Interconnectivity

A router has a minimum of two (and frequently many more) physical I/O ports. I/O ports, or
interfaces as they are better known, are used to physically connect network transmission facili-
ties to a router. Each port is connected to a circuit board that is attached to the router’s mother-
board. Thus, the motherboard actually provides the interconnectivity among multiple networks.
The network administrator must configure each interface via the router’s console. Configura-
tion includes defining the interface’s port number in the router, the specific transmission tech-
nology and bandwidth available on the network connected to that interface, and the types of
protocols that will be used through that interface. The actual parameters that must be defined
vary based on the type of network interface.
Note that on the higher-end platforms (7500 and 12000) the interfaces (VIP2 or line card) can
forward packets without interrupting the main CPU.
Logical Interconnectivity
As soon as a router interface is configured, it can be activated. The interface’s configuration
identifies the type of transmission facility that it connects to, the interface’s IP address, and the
address of the network that it connects to. Upon activation of a port, the router immediately
begins monitoring all the packets that are being transmitted on the network attached to the
newly activated port. This monitoring allows it to “learn” about network and host IP addresses
that reside on the networks that can be reached via that port. These addresses are stored in
tables called routing tables. Routing tables correlate the port number of each interface in the
router with the network layer addresses that can be reached (either directly or indirectly) via
that port.
A router can also be configured with a default route. A default route associates a specific router
interface with all unknown destination addresses. This association allows a router to forward a
datagram to destinations that it has not yet learned of. Default routes can be useful in other
ways, too. Default routes can be used to minimize the growth of routing tables, for example, or
can be used to reduce the amount of traffic generated between routers as they exchange rout-
ing information.
However, consider your default route carefully because it could potentially cause problems if
not thoroughly thought out as for traffic flow.
Route Calculation and Maintenance

Routers communicate with each other using a predetermined protocol, a routing protocol.
Routing protocols enable routers to do the following:
■ Identify potential routes to specific destination networks
■ Perform a mathematical calculation, based on the routing protocol’s algorithm, to deter-
mine the best path to each destination
■ Continuously monitor the network to detect any topology changes that might render
known routes invalid
continues
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More Information: A Closer Look at Routers (Continued)
Many different types of routing protocols exist. Some, such as the Routing Information Proto-
col (RIP), are quite simple. Others, such as Open Shortest Path First (OSPF), are remarkably
powerful and feature-rich but complicated. In general, routing protocols can use two approaches
to make routing decisions: distance vectors and link states. A distance-vector routing protocol
makes its decisions based on some measurement of the distance between source and destination
machines. A link-state protocol bases its decisions on various states of the links, or transmis-
sion facilities, that interconnect the source and destination machines. Neither one is right or
wrong; they are just different ways of making the same decisions. However, they result in
different levels of performance, including convergence times.
You can evaluate routing protocols using numerous, more-specific criteria than just which
approaches they use. Some of the more meaningful criteria include the following:
■ Optimality—Optimality describes a routing protocol’s capability to select the best avail-
able route. Unfortunately, the word best is ambiguous. Many different ways exist to evalu-
ate different routes to any given destination. Each way could result in the selection of a
different “best” route depending on the criteria used. The criteria used by routing proto-
cols to calculate and evaluate routes are known as routing metrics. A wide variety of met-
rics are used, and they vary widely by routing protocol. One simple metric is hop count,
the number of hops, or routers, that lie between the source and destination machines.
■ Efficiency—Another criterion to consider when evaluating routing protocols is their opera-

tional efficiency. Operational efficiency can be measured by examining the physical
resources, including router RAM and CPU time, and network bandwidth required by a
given routing protocol. You might need to consult your router manufacturer or vendor to
determine the relative efficiencies of any protocols you are considering.
■ Robustness—A routing protocol should perform reliably at all times, not just when the
network is stable. Error conditions, including hardware or transmission-facility failures,
router configuration errors, and even heavy traffic loads, adversely affect a network.
Therefore, that a routing protocol functions properly during periods of network failure or
instability is critical.
■ Convergence—Because they are intelligent devices, routers can automatically detect
changes in the internetwork. When a change is detected, all the routers involved must con-
verge on a new agreement of the network’s topology and recalculate the routes to known
destinations accordingly. This process of reaching mutual agreement is called conver-
gence. Each routing protocol uses different mechanisms for detecting and communicating
network changes. Therefore, each one converges at a different rate. In general, the slower
a routing protocol converges, the greater the potential for disrupting service across the
internetwork.
■ Scalability—A network’s scalability is its capability to grow. Although growth isn’t a
requirement in every organization, the routing protocol that you select should be capable
of scaling upward to meet your network’s projected growth.
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WAN Routers 535
Router External Connections
The three basic types of connections on a router are LAN interfaces, WAN interfaces,
and management ports, as illustrated by Figure 10-10. LAN interfaces allow the router
to connect to the LAN media, which is usually some form of Ethernet. However, it
could be some other LAN technology such as Token Ring or ATM.
Figure 10-10 Router External Connections
WAN connections provide connections through a service provider to a distant site or
to the Internet. These connections can 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 service provider’s local connection. With
other types of WAN connections, the router might be directly connected to the service
provider.
The function of management ports is different from that of 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 configura-
tion 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 will run a terminal
emulation program to provide a text-based session with the router. Through this ses-
sion the network administrator can manage the device.
Management Port Connections
The console port and the auxiliary (AUX) port are management ports. These asynchro-
nous 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.
Management Port ConnectionsLAN Connections
WAN Connections
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When the router is first put into service, no networking parameters are configured.
Therefore, the router cannot communicate with any network. To prepare for initial star-
tup and configuration, attach an RS-232 ASCII terminal, or a computer emulating an
ASCII terminal, to the system console port. Then you can enter configuration com-
mands to set up the router.
Once you enter this initial configuration into the router through the console or auxil-
iary port, you can then connect the router to the network for troubleshooting or
monitoring.
You can also configure the router from a remote location by dialing to a modem con-

nected to the console or auxiliary port on a router.
The console port is also preferred over the auxiliary port for troubleshooting. This
preference for the console port is because it displays router startup, debugging, and
error messages by default. You can use the console port when the networking services
have not been started or have failed. Therefore, you can use the console port for disas-
ter and password recovery procedures.
The Function of a Router in a WAN
While you can use routers to segment LANs, the major use for a router is as a WAN
device. Routers have both LAN and WAN interfaces. In fact, WAN technologies are
frequently used to connect routers. These routers communicate with each other through
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 net-
work addresses (on the Internet, by using IP).
The two main functions of a router are as follows:
■ The selection of best paths for incoming data packets
■ The switching of packets to the proper outgoing interface
Routers accomplish these functions by building routing tables and exchanging the net-
work information contained within them with other routers.
An administrator can maintain routing tables by configuring static routes. However,
routing tables are generally maintained dynamically through the use of a routing pro-
tocol that exchanges network topology, or path, information with other routers.
An example is when computer x needs to communicate with computer y and computer z.
Both computer y and computer z are located in distant parts of the world. For com-
puter x to communicate with the other computers, a routing feature for information
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WAN Routers 537
flow is required, as well as redundant paths for reliability. Many network design deci-
sions and technologies can be traced back to the need for computers x, y, and z to
communicate, or internetwork.
Note, however, that an internetwork must also include the following:

■ Consistent end-to-end addressing
■ Addresses that represent network topologies
■ Best path selection
■ Dynamic routing and/or static routing
■ Switching
Lab Activity Connecting Console Interface
In this activity, you identify the console interfaces on the router. You then iden-
tify and locate the proper cable to connect a PC to the router to serve as its
management console.
Lab Activity Connecting LAN Interfaces
In this activity, you identify the Fast Ethernet interfaces on the router. You then
identify and locate the proper cables to connect the routers. Finally, you use
the cables to connect the routers and computer to the hubs.
Connecting WAN Interfaces
In this activity, you identify the serial interfaces on the router. You then identify
and locate the proper cables to interconnect the routers. Finally, you use the
cables to connect the routers.
More Information: Roles of the Router in WANs
More often than not, internetworks are extensive in terms of the number of routers, transmis-
sion facilities, and attached end systems. In an extensive internetwork, such as the Internet or
even large private networks, it is virtually impossible for any given machine to know about
every other machine. Therefore, some semblance of hierarchy is needed. Hierarchical organiza-
tion of internetworked machines creates the need for specialized routing functions.
continues
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More Information: Roles of the Router in WANs (Continued)
Routers can specialize in learning about and distributing routing information within their
domain. These routers are called interior gateways. Alternatively, routers can specialize in col-
lecting routing information about networks that lie beyond their domain. These routers are

known as exterior gateways.
Networking is often used as a generic or universal term. However, networked machines com-
municate in tremendously different ways. Routers can function in different capacities in an
internetwork, for example, as interior, exterior, or border routers.
As you might have noticed in the previous paragraphs, finding interior routers, exterior routers,
and border routers described as interior gateways, exterior gateways, and border gateways,
respectively, is not uncommon. The term gateway is as old as routing itself. Over time, this
term has lost some of its descriptive value. Consequently, both sets of terms are technically
correct, except in the presence of technological purists. Then you have to determine which ter-
minology they consider correct!
These functional specializations are more than merely academic. Understanding the differ-
ences among them requires examining them in the context of a WAN. Therefore, a logical start-
ing point is an examination of the context. The terms WAN, network, internetwork, and
autonomous system are all used interchangeably, yet each has a slightly different meaning:
■ WAN—A WAN is a collection of related LANs linked via routers and serial transmission
facilities, such as leased lines or Frame Relay circuits. Implicit in this definition is that the
LANs in the WAN might be geographically dispersed, but they still fall under the auspices
of a single organization such as a company or school.
■ Network—Network is a more nebulous term that defies specificity. Everything from LANs
to WANs can be classified as a network. Consequently, for the purposes of this book, a
network identifies a generic collection of related networking mechanisms. Therefore,
a network can be a LAN or a WAN, but it must belong to a single organization and feature a
consistent addressing architecture. This term is sometimes used to indicate an internet-
work or even the Internet.
■ Internetwork—Internetwork is only slightly more concrete than network. An internetwork
is a collection of loosely related networks that are interconnected. The interconnected net-
works can belong to different organizations. For example, two companies can use the
Internet to interconnect their private WANs. The resulting internetwork consists of one
public network and two private networks linked together. The most common definition of
internetwork is a set of networks linked by routers. This internetwork is not necessarily a

loosely related set of networks, although the term is applied to both a single domain or
autonomous system (AS) internetwork or an internetwork comprised of separate autono-
mous systems.
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