Routed, Routable, and Routing Protocols 439
Logical addressing occurs at the network layer. Recall the analogy that compares net-
work addresses to telephone numbers. The first portions of a phone number are the
area code and the first three digits. The last four digits of a phone number tell the phone
company equipment which specific phone to ring. This is similar to the function of
the host portion of an address. The host portion tells the router the specific device to
which it should deliver a packet.
Without network layer addressing, routing cannot take place. Routers require network
addresses to ensure proper delivery of packets. Without some hierarchical addressing
structure, packets could not travel across an internetwork. Similarly, without some
hierarchical structure to telephone numbers, postal addresses, or transportation systems,
there would be no smooth delivery of goods and services.
A MAC address can be compared to your name, and a network layer address can be
compared to your mailing address (network and host address). For example, if you
were to move to another town, your name would remain unchanged, but your mailing
address would indicate your new location. Network devices (routers as well as individual
computers) have both a MAC address and a protocol (network layer) address. When
you move a computer to a different network, the computer maintains the same MAC
address, but you must assign it a new network layer address.
The Communication Path
The function of the network layer is to find the best path through the network. To be
truly practical, a network must consistently represent the paths available between routers.
As Figure 8-13 shows, each line between the routers has a number that the routers use
as a network address. These addresses must convey information that can be used by a
routing process. This means that an address must have information about the path of
media connections that the routing process uses to pass packets from a source toward
a destination.
Figure 8-13 Network Media Connections
1
3
4

5
6
7
8
9
10
11
2
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440 Chapter 8: Routing Fundamentals and Subnets
Using these addresses, the network layer can provide a relay connection that intercon-
nects independent networks. The consistency of Layer 3 addresses across the entire
internetwork also improves the use of bandwidth by preventing unnecessary broadcasts.
Broadcasts invoke unnecessary process overhead and waste capacity on any devices or
links that do not need to receive the broadcast. By using consistent end-to-end address-
ing to represent the path of media connections, the network layer can find a path to the
destination without unnecessarily burdening the devices or links on the internetwork
with broadcasts.
Routing Tables
To aid in the process of path determination, routing protocols build and maintain
routing tables, which contain route information, as shown in Figure 8-14. Route infor-
mation varies, depending on the routing protocol used. Routing protocols fill routing
tables with a variety of information.
Figure 8-14 Routing Tables
Routers keep track of important information in their routing tables:
■ Protocol type—The type of routing protocol that created the routing table entry.
■ Destination/next-hop associations—Tell a router that a particular destination is
either directly connected to the router or that it can be reached via another router
called the next hop on the way to the final destination. When a router receives an
incoming packet, it checks the destination address and attempts to match it with

a routing table entry.
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Routed, Routable, and Routing Protocols 441
■ Routing metrics—Different routing protocols use different routing metrics. Routing
metrics are used to determine a route’s desirability. For example, RIP uses hop
count as its routing metric. IGRP uses bandwidth, load, delay, and reliability to
create a composite metric value. This is covered in more depth in CCNA2.
■ Outbound interface—The interface that the data must be sent out to reach the
final destination.
Routers communicate with one another to maintain their routing tables through the
transmission of routing update messages. Depending on the particular routing protocol,
routing update messages can be sent periodically or only when there is a change in the
network topology. The routing protocol also determines whether just the changed
routes or the entire routing table is sent in the routing update. By analyzing the routing
updates from the neighboring routers, a router can build and maintain its routing table.
Routing Algorithms and Metrics
Routing protocols often have one or more of the following design goals:
■ Optimization—Optimization describes the capability of the routing protocol/
algorithm to select the best route, depending on metrics and metric weightings
used in the calculation. For example, one algorithm might use hop count and
delay for its metric but might weigh delay more heavily in the calculation.
■ Simplicity and low overhead—Ideally efficient routing algorithm functionality is
achieved if the routers have minimum CPU and memory overhead. This is impor-
tant so that the network can scale to large proportions, such as the Internet.
■ Robustness and stability—A routing algorithm should perform correctly in the
face of unusual or unforeseen circumstances, such as hardware failures, high
load conditions, and implementation errors.
■ Rapid convergence—Convergence is the process of all routers agreeing on routes.
When a network event causes changes in router availability, recalculations are
needed to reestablish network connectivity. Routing algorithms that converge

slowly can cause data to not be delivered.
■ Flexibility—A routing algorithm should quickly adapt to a variety of network
changes. These changes include router availability, changes in bandwidth, queue
size, and network delay.
■ Scalability—Some routing protocols are better designed for scalability than others.
It is important to keep in mind that if the network is intended to grow (or even if
this option is to be left open), a routing protocol such as EIGRP rather than RIP
should be used.
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442 Chapter 8: Routing Fundamentals and Subnets
When a routing algorithm updates a routing table, its primary objective is to determine
the best information to include in the table. Routing algorithms use different metrics
to determine the best route. Each routing algorithm interprets what is best in its own
way. The routing algorithm generates a number, called the metric value, for each path
through the network. Sophisticated routing algorithms can base route selection on
multiple metrics, combining them in a single composite metric, as shown in Figure 8-15.
Typically, the smaller the metric, the better the path.
Figure 8-15 Routing Metrics
Metrics can be based on a single characteristic of a path or can be calculated based on
several characteristics. The metrics that are most commonly used by routing protocols
are as follows:
■ Bandwidth—A link’s data capacity. (Normally, a 10-Mbps Ethernet link is pref-
erable to a 64-kbps leased line.)
■ Delay—The length of time required to move a packet along each link from
source to destination. Delay depends on the bandwidth of intermediate links,
port queues at each router, network congestion, and physical distance.
■ Load—The amount of activity on a network resource such as a router or link.
■ Reliability—Usually refers to the error rate of each network link.
■ Hop count—The number of routers that a packet must travel through before
reaching its destination. Whenever data goes through a router, this is one hop.

A path that has a hop count of 4 indicates that data traveling along that path
passes through four routers before reaching its final destination. If there are mul-
tiple paths to a destination, the router chooses the path with the fewest hops.
B
A
56 Kbps
56 Kbps
T1
T1
Hop count
Ticks
Cost
Bandwidth
Delay
Load
Reliability
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Routed, Routable, and Routing Protocols 443
■ Cost—An arbitrary value, usually based on bandwidth, monetary expense, or
another measurement, that is assigned by a network administrator.
Interior and Exterior Routing Protocols
Routers use routing protocols to exchange routing information. In other words, routing
protocols determine how routed protocols are routed. Two families of routing protocols
are the Interior Gateway Protocols (IGPs) and the Exterior Gateway Protocols (EGPs),
as shown in Figure 8-16. These families are classified based on how they operate with
regard to autonomous systems.
Figure 8-16 IGPs and EGPs
An autonomous system is a network or set of networks that are under the administra-
tive control of a single entity, such as the cisco.com domain. An autonomous system
consists of routers that present a consistent view of routing to the external world. The

Internet Assigned Numbers Authority (IANA) allocates autonomous system numbers
to the regional registries. These registries are ARIN () for the
Americas, the Caribbean, and Africa; RIPE-NCC () for Europe; and
AP-NIC () for the Asia Pacific region. This autonomous system is a
16-bit number. A routing protocol such as BGP requires that you specify this unique,
assigned autonomous system number in your configuration.
IGPs route data within an autonomous system. Here are some examples of IGPs:
■ RIP and RIP V2
■ IGRP
■ EIGRP
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444 Chapter 8: Routing Fundamentals and Subnets
■ OSPF
■ Intermediate System-to-Intermediate System (IS-IS) protocol
EGPs route data between autonomous systems. BGP is the most pervasive example of
an EGP.
Routing Protocols
Routing protocols can be classified in many different ways, such as IGPs or EGPs. Another
classification that describes routing protocols is distance-vector or link-state. Whereas IGP
and EGP describe the physical relationships of routers, the distance-vector and link-state
categories describe how routers interact with each other in terms of routing updates.
Distance-Vector Protocols
The distance-vector routing approach determines the direction (vector) and distance
(hop count) to any link in the internetwork. Distance-vector algorithms periodically
(such as every 30 seconds) send all or some portion of their routing table to their adjacent
neighbors. Routers running a distance-vector routing protocol send periodic updates
even if there are no changes in the network. By receiving a neighbor’s routing table, a
router can verify all the known routes and make changes to the local routing table based
on updated information received from the neighboring router. This process is called
“routing by rumor” because the understanding that a router has of the network is

based on the neighbor’s perspective of the network topology. Distance-vector protocols
use the Bellman-Ford Algorithm to calculate the best paths.
Examples of distance-vector protocols include the following:
■ Routing Information Protocol (RIP)—The most common IGP in the Internet,
RIP uses hop count as its routing metric.
■ Interior Gateway Routing Protocol (IGRP)—Cisco developed this IGP to
address the issues associated with routing in large, heterogeneous networks.
Link-State Protocols
Link-state routing protocols were designed to overcome the limitations of distance-
vector routing protocols. Link-state routing protocols respond quickly to network changes,
send trigger updates only when a network change has occurred, and send periodic
updates (called link-state refreshes) at long time intervals, such as every 30 minutes.
When a link changes state, the device that detected the change creates a link-state
advertisement (LSA) concerning that link (route), and that LSA is propagated to all
neighboring devices. Each routing device takes a copy of the LSA, updates its link-
state (topological) database, and forwards the LSA to all neighboring devices. This
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Routed, Routable, and Routing Protocols 445
flooding of the LSA is required to ensure that all routing devices update their data-
bases before creating an updated routing table that reflects the new topology, as
shown in Figure 8-17.
Figure 8-17 Link-State Routing Protocols
The link-state database is used to calculate the best paths through the network. Link-
state routers find the best paths to destinations by applying the Dijkstra Shortest Path
First (SPF) algorithm against the link-state database to build the SPF tree. The best
(shortest) paths are then selected from the shortest-path-first tree and are placed in the
routing table.
Examples of link-state protocols are OSPF and IS-IS, as shown in Figure 8-18.
Routing Protocol Characteristics
The following sections describe the metrics, network usability, and other significant

characteristics of the most commonly used routing protocols.
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446 Chapter 8: Routing Fundamentals and Subnets
Figure 8-18 Link-State Routing Protocols: OSPF and IS-IS
RIP
RIP uses hop count to determine the direction and distance to any link in the internet-
work, as shown in Figure 8-19. If there are multiple paths to a destination, RIP selects
the path with the fewest hops. However, because hop count is the only routing metric
RIP uses, it does not necessarily select the fastest path to a destination. RIP-1 uses only
classful routing. This means that all devices in the network must use the same subnet
mask, because RIP-1 does not include the subnet information with the routing update.
RIP-2 provides what is called prefix routing and sends subnet mask information with
the route updates. This supports the use of classless routing. With classless routing
protocols, different subnets within the same network can have different subnet masks.
The use of different subnet masks within the same network is called variable-length
subnet masking (VLSM).
IGRP
IGRP is a distance-vector routing protocol developed by Cisco specifically to address
problems associated with routing in large networks that are beyond the scope of pro-
tocols such as RIP. IGRP can select the fastest path based on the delay, bandwidth, load,
and reliability. By default, IGRP uses bandwidth and delay metrics only and uses a
24-bit metric. IGRP also has a much higher maximum hop-count limit than RIP to
allow the network to scale. IGRP uses only classful routing.
Distance Vector
Link State
RIP
Distance Vector
Using Hop Count
IGRP
Distance Vector

Developed by Cisco
Addressing Problems in Large,
Heterogeneous Network
OSPF
Link-State, Hierarchical
Successor to RIP
Using Least-Cost Routing,
Multipath Routing, and Load
Balancing
Derived from IS-IS
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Routed, Routable, and Routing Protocols 447
Figure 8-19 RIP Uses Hop Count as Its Metric
EIGRP
Like IGRP, EIGRP is a proprietary Cisco protocol. EIGRP is an advanced version of
IGRP and uses a 32-bit metric. Specifically, EIGRP provides superior operating effi-
ciency such as faster convergence and lower overhead bandwidth. It is an advanced
distance-vector protocol. EIGRP also uses some of the link-state protocol functions.
Hence, the term hybrid is also used to describe EIGRP.
OSPF
OSPF is a link-state routing protocol. The Internet Engineering Task Force (IETF)
developed OSPF in 1988. The most recent version, OSPF Version 2, is described in
RFC 2328. OSPF is an IGP, which means that it distributes routing information
between routers belonging to the same autonomous system. OSPF was written to
address the needs of large, scalable internetworks that RIP could not.
IS-IS
Intermediate System-to-Intermediate System (IS-IS) is the dynamic link-state routing
protocol for the OSI protocol stack. As such, it distributes routing information for
routing Connectionless Network Protocol (CLNP) data for the ISO Connectionless
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448 Chapter 8: Routing Fundamentals and Subnets
Network Service (CLNS) environment. Integrated IS-IS is an implementation of the
IS-IS protocol for routing multiple network protocols. Integrated IS-IS tags CLNP
routes with information about IP networks and subnets. It provides an alternative to
OSPF in the IP world, mixing ISO CLNS and IP routing in one protocol. It can be used
purely for IP routing, purely for ISO routing, or for a combination of the two.
BGP
Border Gateway Protocol (BGP) is an example of an EGP. BGP exchanges routing
information between autonomous systems while guaranteeing loop-free path selection.
It is the principal route advertising protocol used by major companies and ISPs on the
Internet. BGP-4 is the first version of BGP that supports classless interdomain routing
(CIDR) and route aggregation. Unlike common IGPs such as RIP, OSPF, and EIGRP,
BGP does not use metrics such as hop count or bandwidth or delay. Instead, BGP
makes routing decisions based on network policies or rules using various BGP path
attributes.
IP as a Routed Protocol
IP is the most widely used implementation of a hierarchical network addressing scheme.
IP is a connectionless, unreliable, best-effort delivery system protocol used on the Inter-
net. The term connectionless means that no dedicated circuit connection is required, as
there would be for a telephone call. There is no call setup before data is transferred
between hosts. The IP protocol takes whichever route is the most efficient based on the
routing protocol decision. Unreliable and best-effort do not mean that the system is
unreliable and doesn’t work well, but that the IP protocol does not make any effort to
see if the packet was delivered. This function is handled by the upper-layer protocols.
As information flows down the layers of the OSI model, the data is processed at each
layer. At the network layer, the data is encapsulated within packets called datagrams,
as shown in Figure 8-20.
Lab Activity Small Router Purchase (Cable/DSL Router)
The purpose of this lab is to introduce the variety and prices of network
components in the market. This lab looks specifically at small routers used

by telecommuters when working from home.
NOTE
CLNP refers to the
OSI network layer
protocol that does not
require a circuit to be
established before
data is transmitted.
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