Routed, Routable, and Routing Protocols 429
■ Routing protocol—Supports a routed protocol by providing mechanisms for shar-
ing routing information. Routing protocol messages move between the routers. A
routing protocol allows the routers to communicate with other routers to update
and maintain tables. Here are some TCP/IP examples of routing protocols:
— Routing Information Protocol (RIP)
— Interior Gateway Routing Protocol (IGRP)
— Enhanced Interior Gateway Routing Protocol (EIGRP)
— Open Shortest Path First (OSPF)
For a protocol to be routable, it must provide the capability to assign a network number,
as well as a host number, to each individual device. Some protocols, such as Internet-
work Packet Exchange (IPX), only require that an administrator assign a network
number, because they use a host’s Media Access Control (MAC) address for the physical
number. Other protocols, such as IP, require that a complete address be provided, as
well as a network mask.
Both the
IP address and network mask are required to have a routed network. A net-
work mask separates the network and host portions of a 32-bit IP address. IPX uses
the MAC address concatenated with an administrator-assigned network address to
create the complete address and does not use a network mask. With IP addresses, the
network address is obtained by comparing the address with the network mask.
A network mask allows groups of sequential IP addresses to be treated as a single unit.
If this grouping were not allowed, each host would have to be mapped individually
for routing. That would not be possible with the millions of hosts on the Internet. As
shown in Figure 8-2, all 254 addresses in the sequence of 192.168.10.1 to 192.168.10.254
can be represented by the network address 192.168.10.0. This allows data to be sent
to any one of these hosts just by locating the network address. This means that routing
tables need to contain only one entry of 192.168.10.0 instead of all 254 individual
entries. This is according to the Internet Software Consortium (www.isc.org). For
routing to function, this process of grouping must be used.
The following sections describe the router’s use and operations in performing the key

internetworking function of the Open System Interconnection (OSI) reference model’s
network layer, Layer 3. In addition, you’ll learn about the difference between routing
and routed protocols and how routers track distance between locations. Finally, you’ll
learn more about distance-vector, link-state, and hybrid routing approaches and how
each resolves common routing problems.
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430 Chapter 8: Routing Fundamentals and Subnets
Figure 8-2 Network and Host Addresses
Routing Overview
Routing is an OSI Layer 3 function. It functions as a hierarchical organizational scheme
that allows individual addresses to be grouped and treated as a single unit until the
individual address is needed for final delivery of the data. Routing is the process of
finding the most efficient path from one device to another, as shown in Figure 8-3. The
main device that performs this process is the
router.
Figure 8-3 Network Layer Protocol Operation
X
Y
A
B
C
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Routed, Routable, and Routing Protocols 431
A router has two key functions:
■ To maintain routing tables and make sure other routers know of changes in
the network topology. This function is performed using a routing protocol to
communicate network information to other routers.
■ When packets arrive at an interface, the router must use the routing table to
determine where to send the packets. It switches them to the appropriate inter-
face, adds the necessary framing for the interface, and then transmits the frame.

A router is a network layer device that uses one or more routing metrics to determine
the optimal path along which network traffic should be forwarded. The routing metric
is a value used to determine the route’s desirability. Routing protocols use various
combinations of criteria for determining the routing metric, as shown in Figure 8-4.
Figure 8-4 Routing Protocol Metrics
The metrics of hop count, bandwidth, delay, reliability, load, and cost are calculated
in various combinations to determine the best path through an internetwork. Routers
interconnect network segments or entire networks. They pass data frames between
networks based on Layer 3 information. Routers make logical decisions regarding
the best path for the delivery of data on an internetwork and then direct packets to
the appropriate output port to be encapsulated for transmission. The encapsulation/
de-encapsulation process occurs each time a packet passes through a router and data is
sent from one device to another, as shown in Figure 8-5. Encapsulation breaks the data
stream into segments, adds the appropriate headers and trailers, and transmits the data. The
de-encapsulation process is the opposite, removing the headers and trailers and then
recombining the data into a seamless stream. Routers take frames from LAN devices
(for example, workstations) and, based on Layer 3 information, forward them through
the network.
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432 Chapter 8: Routing Fundamentals and Subnets
Figure 8-5 Data Encapsulation
This chapter and the rest of the book focus on the most commonly used routable (or
routed) protocol—IP. Even though the focus is on IP, it is important to know that there
are other routable protocols, such as IPX/SPX and AppleTalk.
Protocols such as IP, IPX/SPX, and AppleTalk provide Layer 3 support and therefore
are routable. Protocols that do not support Layer 3 are called nonroutable protocols.
The most common of these is NetBIOS Extended User Interface (NetBEUI)—a small,
fast, efficient protocol that is limited to running on one segment.
Routing Versus Switching
Routing is often contrasted with Layer 2 switching, which might seem to perform the

same function to the casual observer. The primary difference between the two is that
switching occurs at Layer 2 (the data link layer) of the OSI model, and routing occurs
at Layer 3. This distinction means that routing and switching use different information
in the process of moving data from source to destination.
The relationship between switching and routing parallels that of local and long distance
telephone calls. When a telephone call is made to another local number (a number
with the same area code), a local switch handles the call. However, the local switch
cannot keep track of all the telephone numbers in the world—only its own local num-
bers. When the switch receives a request for a call outside its area, it switches the call
to a higher-level switch that recognizes area codes. The higher-level switch switches the
call so that it eventually gets to the local switch for the area code dialed.
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Routed, Routable, and Routing Protocols 433
The router performs a function similar to that of the higher-level switch in the telephone
network, as shown in Figure 8-6. Layer 2 switching takes place within the LAN, also
called a broadcast domain. Layer 3 routing moves traffic between broadcast domains.
This requires the hierarchical addressing scheme that a Layer 3 addressing scheme like
IP provides. The Layer 2 switch cannot recognize the Layer 3 IP address—only local
MAC addresses. When a host has data for a nonlocal IP address, it sends the frame to
its default gateway, the router, by using the router’s MAC address.
Figure 8-6 Layer 2 Switching and Layer 3 Routing
A Layer 2 switch interconnects segments belonging to the same logical network or
subnetwork. If Host X needs to send a frame to another host on a different network
or subnetwork, Host X sends the frame to the router that is also connected to the
switch. Host X knows the router IP address because the Host IP configuration includes
the IP address of the default gateway, but it does not know its MAC address. Host X
learns the router’s MAC address by using an Address Resolution Protocol (ARP) request,
which translates IP addresses to MAC addresses. The switch forwards the frame to the
router based on the router’s destination MAC address. The router examines the packet’s
Layer 3 destination address to make the forwarding decision. The default gateway is

the router that is on the same network or subnetwork as Host X.
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434 Chapter 8: Routing Fundamentals and Subnets
Much like the way a Layer 2 switch keeps a table of known MAC addresses, the router
keeps a table of IP network addresses called a routing table, as shown in Figure 8-7.
Each computer and router Ethernet interface maintains an ARP table for Layer 2 com-
munication. The ARP table is effective only for the broadcast domain that it is connected
to. The router also maintains a routing table that allows it to route data outside the
broadcast domains. Each ARP table contains the IP-MAC address pair. (The MAC
address in Figure 8-7 is represented by the acronym MAC because the actual addresses
are too long to fit in the figure.) The routing tables show how the route was learned—
in this case, directly connected (C) and RIP (R)—the network IP address for reachable
networks, the hop count to get those networks, and the interface the data must be sent
out to get to the destination network. The difference between these two types of addresses
is that the MAC address is not organized in any particular way. This is OK, though,
because any individual network segment does not have a large number of hosts, so it is
manageable. If the IP network addresses were treated the same way, the Internet simply
would not work. There would be no way to organize all the addresses and the directions
on how to get to them (hierarchically or otherwise). Organizing IP addresses hierarchi-
cally lets you group addresses to be treated as a single unit until you need to locate
an individual host. A way to understand this is to think of a library that contains only
millions of individual pages in a large pile. This material is useless, because it is impossible
to locate an individual document. If the pages were organized into books, and each
page were individually identified, and if the books were listed in an index, it would be
much easier to find and use the data.
Figure 8-7 Router ARP Tables
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Routed, Routable, and Routing Protocols 435
Another difference between switched and routed networks is that Layer 2 switched
networks do not block Layer 3 broadcasts. As a result, they can be swamped by

broadcast storms. Routers normally block broadcasts so that a broadcast storm affects
only the broadcast domain it originated on. Routers also provide higher security and
bandwidth control than Layer 2 switches because they block broadcasts.
Table 8-1 compares routers and switches.
Routed Versus Routing
There are two categories of protocols at the network layer—routed and routing (see
Figure 8-8). Routed protocols transport data across a network, and routing protocols
allow routers to properly direct the data from one location to another.
Protocols that transfer data from one host to another across a router are routed or
routable protocols.
A routed protocol operates as shown in Figure 8-9:
■ It includes any network protocol suite that provides enough information in its
network layer address to allow a router to forward it to the next device and
ultimately to its destination.
■ It defines the format and use of the fields within a packet. Packets generally are
conveyed from end system to end system.
IP and IPX are examples of routed protocols. Other examples include DECnet, Apple-
Talk, Banyan VINES, and Xerox Network Systems (XNS).
Routers use routing protocols to exchange routing tables and share routing information.
In other words, routing protocols let routers route routed protocols after a path has
been determined.
Table 8-1 Router and Switch Feature Comparison
Feature Router Switch
Speed Slower Faster
OSI layer Layer 3 Layer 2
Addressing used IP MAC
Broadcasts Blocks Forwards
Security Higher Lower
Segment networks Segment broadcast domains Segment collision domains
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436 Chapter 8: Routing Fundamentals and Subnets
Figure 8-8 Routed and Routing Protocols
Figure 8-9 Routed Protocol
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Routed, Routable, and Routing Protocols 437
A routing protocol operates as shown in Figure 8-10:
■ It provides processes for sharing routing information.
■ It allows routers to communicate with other routers to update and maintain
the routing tables.
Examples of routing protocols that support IP routed protocols include RIP, IGRP,
OSPF, Border Gateway Protocol (BGP), and EIGRP.
Figure 8-10 Routing Protocol
Path Determination
Path determination occurs at Layer 3 (the network layer). It lets a router evaluate the
available paths to a destination and establish the preferred handling of a packet. Routing
services use network topology information when evaluating network paths, as shown
in Figure 8-11. Path determination is the process that a router uses to choose the next
hop in a path toward a packet’s ultimate destination. This process is also called routing
the packet.
Figure 8-11 Choosing a Path
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438 Chapter 8: Routing Fundamentals and Subnets
Path determination for a packet can be compared to a person driving a car from one
side of a city to another. The driver has a map that shows the streets to take to get to
the destination. The drive from one intersection to another is a hop. Similarly, a router
uses a map that shows the available paths to a destination. Routers can also make their
decisions based on the traffic density and the link’s speed (bandwidth), just as a driver
might choose a faster path (a highway) or use less-crowded back streets. In this section,
you’ll see how a router determines the best path for packets traveling from one net-
work to another.

Network Layer Addressing
A network address helps the router identify a path within the network cloud and also
provides hierarchical or subnet information. The router uses the network address to
identify the destination network of a packet within an internetwork. In addition to
the network address, network protocols use some form of host, or node, address. For
some network layer protocols, a network administrator assigns host addresses accord-
ing to some predetermined network addressing plan. For other network layer protocols,
assigning host addresses is partially or completely dynamic or automatic. Figure 8-12
shows three devices in Network 1 (two workstations and a router), each with its own
unique host address. (The figure also shows that the router is connected to two other
networks—networks 2 and 3.)
Figure 8-12 Network Addresses
1.1
3.1
2.1
Network
1
2
3
1
2
3
1
1
Host
1.2
1.3
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