Cisco Networking Academy Program: Engineering Journal

102 Pages·2001·0.38 MB·English

by  Cisco User

Cisco Networking Academy Program: Engineering Journal
and Workbook, Volume II, Second Edition
Engineering Journal and Workbook Questions and Answers
Chapter 1
Review: The OSI Reference Model and Routing
Introduction
Networks are complex environments that involve multiple media, multiple protocols, and
interconnections to networks outside an organization’s central office. Well-designed and
carefully installed networks can reduce the problems associated with growth as a
networking environment evolves.
Designing, building, and maintaining a network can be a challenging task. Even a small
network that consists of only 50 nodes can pose complex problems that lead to
unpredictable results. Large networks that feature thousands of nodes can pose even
more complex problems. Despite improvements in equipment performance and media
capabilities, designing and building a network is difficult.
This chapter reviews the Open System Interconnection (OSI) reference model and
overviews network planning and design considerations related to routing. Much of this
information should be familiar because you were introduced to these concepts in the
first year of the Cisco Networking Academy Program. Using the OSI reference model as
a reference for network design can facilitate changes. Using the OSI reference model as
a hierarchical structure for network design enables you to design networks in layers.
The OSI reference model is at the heart of building and designing networks, with every
layer performing a specific task in order to promote data communications. In the world
of networking, Layers 1 through 4 are the focus. These four layers define the following:
• The type and speed of LAN and WAN media to be implemented
• How data is sent across the media
• The type of addressing schemes used
• How data is reliably sent across the network and how flow control is
accomplished
• The type of routing protocol implemented
1 - 101 Engineering Journal and Workbook, Vol. II, 2nd Ed. – Chapter 1 Copyright © 2002 Cisco Systems, Inc.
Concept Questions
Demonstrate your knowledge of these concepts by answering the following questions in
the space provided.
• By using layers, the OSI model simplifies the task required for two computers to
communicate. Can you explain why?
Each layer focuses on specific functions, thereby allowing the networking
designer to choose the right networking devices and functions for the layer.
• Each layer’s protocol exchanges information, called protocol data units (PDUs),
between peer layers. Can you explain how this is done?
Host A has information to send to host B. The application program in host A
communicates with host A’s application layer, which communicates with host A’s
presentation layer, which communicates with host A’s session layer, and so on,
until host A’s physical layer is reached. The physical layer puts information on
(and takes information off) the physical network medium. After the information
traverses the physical network medium and is picked up by host B, it ascends
through host B’s layers in reverse order (first the physical layer, then the data
link layer, and so on) until it finally reaches host B’s application layer.
• Can you explain the concept of encapsulation?
Specific requests are stored as control information, which is passed between
peer layers in a header block that is attached to the actual application
information. Each layer depends on the service function of the OSI reference
model layer below it. To provide this service, the lower layer uses encapsulation
to put the PDU from the upper layer into its data field; then, it can add whatever
headers and trailers the layer will use to perform its function.
• Can you explain what the term Ethernet means?
The term Ethernet refers to the family of LAN implementations that includes
three principal categories:
⇒ Ethernet and IEEE 802.3—LAN specifications that operate at 10 Mbps
over coaxial and twisted-pair cable.
⇒ 100-Mbps Ethernet—A single LAN specification, also known as Fast
Ethernet, that operates at 100 Mbps over twisted-pair cable.
⇒ 1000-Mbps Ethernet—A single LAN specification, also known as Gigabit
Ethernet, that operates at 1000 Mbps (1 Gbps) over fiber and twisted-pair
cables.
• What is a datagram?
Logical grouping of information sent as a network layer unit over a transmission
medium without prior establishment of a virtual circuit. IP datagrams are the
primary information units in the Internet.
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• What is ARP and how does it work?
To communicate on an Ethernet network, the source station must know the
destination station’s IP and MAC addresses. When the source has determined
the IP address for the destination, the source’s Internet protocol looks into its
ARP table to locate the MAC address for the destination. If the Internet protocol
locates a mapping of destination IP address to destination MAC address in its
table, it binds the IP address with the MAC address and uses them to
encapsulate the data. The data packet is then sent out over the networking
media to be picked up by the destination. If the MAC address is not known, the
source must send out an ARP request. To determine a destination address for a
datagram, the ARP table on the router is checked. If the address is not in the
table, ARP sends a broadcast looking for the destination station. Every station
on the network receives the broadcast.
• Most protocols can be classified into one of two basic protocols: routed or
routing. What are the differences between the two types of protocols?
⇒ Routed protocol—Any network protocol that provides enough
information in its network layer address to allow a packet to be forwarded
from host to host based on the addressing scheme. Routed protocols
define the format and use of the fields within a packet. Packets generally
are conveyed from end system to end system. IP is an example of a
routed protocol.
⇒ Routing protocol—A protocol that supports a routed protocol by
providing mechanisms for sharing 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.
• Examples of IP routing protocols include RIP, IGRP, OSPF, and EIGRP. Explain
the differences between these different types of protocols.
IP Routing Protocols:
At the network layer (Layer 3) of the OSI reference model, a router can use IP
routing protocols to accomplish routing through the implementation of a specific
routing protocol. Examples of IP routing protocols include:
⇒ RIP—A distance-vector routing protocol
⇒ IGRP—Cisco’s distance-vector routing protocol
⇒ OSPF—A link-state routing protocol
⇒ EIGRP—A balanced-hybrid routing protocol
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• Classes of Routing Protocols:
Most interior routing protocols can be classified as one of three basic types:
distance vector, link state, or balanced-hybrid routing. The distance-vector
routing protocol determines the direction (vector) and distance to any link in the
network. The link-state routing protocol (also called the shortest path first
[SPF] protocol) approach re-creates the exact topology of the entire network (or
at least the partition in which the router is situated). The balanced-hybrid
protocol combines aspects of the link-state and distance-vector protocols.

Vocabulary Exercise Chapter 1
Define the following terms as completely as you can. Use the online Chapter 1 or the
Cisco Networking Academy Program: Second-Year Companion Guide, Second Edition
material for help.
Application layer Layer 7 of the OSI reference model. This layer provides network
services to user applications. For example, a word processing application is serviced by
file transfer services at this layer.
ARP (Address Resolution Protocol) An Internet protocol used to map an IP
address to a MAC address. Defined in RFC 826. Compare with RARP.
Cisco IOS (Internetwork Operating System) software Cisco system software that
provides common functionality, scalability, and security for all products under the
CiscoFusion architecture. The Cisco IOS software allows centralized, integrated, and
automated installation and management of internetworks, while ensuring support for a
wide variety of protocols, media, services and platforms.
Data link layer Layer 2 of the OSI reference model. This layer provides reliable
transit of data across a physical link. The data link layer is concerned with physical
addressing, network topology, line discipline, error notification, ordered delivery of
frames, and flow control. The IEEE has divided this layer into two sublayers: the MAC
sublayer and the LLC sublayer. Sometimes simply called link layer.
Datagram A logical grouping of information sent as a network layer unit over a
transmission medium without prior establishment of a virtual circuit. IP datagrams are
the primary information units in the Internet. The terms cell, frame, message, packet,
and segment are also used to describe logical information groupings at various layers of
the OSI reference model and in various technology circles.
Default route A routing table entry that is used to direct frames for which a next hop
is not explicitly listed in the routing table.
Distance-vector routing protocol A routing protocol that iterates on the number of
hops in a route to find a shortest-path spanning tree. Distance-vector routing protocols
call for each router to send its entire routing table in each update, but only to its
neighbors. Distance-vector routing protocols can be prone to routing loops, but are
computationally simpler than link-state routing protocols.
Dynamic routing Routing that adjusts automatically to network topology or traffic
changes.
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EIGRP (Enhanced Interior Gateway Routing Protocol) An advanced version of
IGRP developed by Cisco. Provides superior convergence properties and operating
efficiency, and combines the advantages of link-state protocols with those of distance-
vector protocols.
Flow control A technique for ensuring that a transmitting entity does not overwhelm
a receiving entity with data. When the buffers on the receiving device are full, a
message is sent to the sending device to suspend the transmission until the data in the
buffers has been processed.
ICMP (Internet Control Message Protocol) A network layer Internet protocol that
reports errors and provides other information relevant to IP packet processing.
IGRP (Interior Gateway Routing Protocol) A protocol developed by Cisco to
address the problems associated with routing in large, heterogeneous networks.
IP address A 32-bit address assigned to hosts by using TCP/IP. An IP address
belongs to one of five classes (A, B, C, D, or E) and is written as 4 octets separated by
periods (that is, dotted-decimal format). Each address consists of a network number, an
optional subnetwork number, and a host number. The network and subnetwork numbers
together are used for routing, and the host number is used to address an individual host
within the network or subnetwork. A subnet mask is used to extract network and
subnetwork information from the IP address.
MAC (Media Access Control) The part of the data link layer that includes the 6-byte
(48-bit) address of the source and destination, and the method of getting permission to
transmit.
Network A collection of computers, printers, routers, switches, and other devices that
can communicate with each other over some transmission medium.
Network layer Layer 3 of the OSI reference model. This layer provides connectivity
and path selection between two end systems. The network layer is the layer at which
routing occurs.
NIC (network interface card) A board that provides network communication
capabilities to and from a computer system.
Packet A logical grouping of information that includes a header containing control
information and (usually) user data. Packets are most often used to refer to network
layer units of data. The terms datagram, frame, message, and segment are also used to
describe logical information groupings at various layers of the OSI reference model and
in various technology circles.
RARP (Reverse Address Resolution Protocol) A protocol in the TCP/IP stack that
provides a method for finding IP addresses based on MAC addresses. Compare with
ARP.
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Focus Questions
1. List each of the layers of the OSI model and identify their function. Indicate
what networking and internetworking devices operate at each of the layers.
Be specific.
Layer 7: Application. This layer provides services to application processes
(such as electronic mail, file transfer, and terminal emulation) that are outside of
the OSI model. The application layer identifies and establishes the availability of
intended communication partners (and the resources required to connect with
them), synchronizes cooperating applications, and establishes agreement on
procedures for error recovery and control of data integrity.
Layer 6: Presentation. This layer ensures that information sent by the
application layer of one system will be readable by the application layer of
another. The presentation layer is also concerned with the data structures used
by programs and therefore negotiates data transfer syntax for the application
layer.
Layer 5: Session. This layer establishes, manages, and terminates sessions
between applications and manages data exchange between presentation layer
entities.
Layer 4: Transport. This layer is responsible for reliable network communication
between end nodes. The transport layer provides mechanisms for the
establishment, maintenance, and termination of virtual circuits, transport fault
detection and recovery, and information flow control.
Layer 3: Network. This layer provides connectivity and path selection between
two end systems. The network layer is the layer at which routing occurs. Routers
are Layer 3 devices.
Layer 2: Data link. This layer provides reliable transit of data across a physical
link. The data link layer is concerned with physical addressing, network topology,
line discipline, error notification, ordered delivery of frames, and flow control. The
IEEE has divided this layer into two sublayers: the MAC sublayer and the LLC
sublayer. Bridges and switches are Layer 2 devices.
Layer 1: Physical. The physical layer defines the electrical, mechanical,
procedural and functional specifications for activating, maintaining, and
deactivating the physical link between end systems. Hubs and repeaters are
Layer 1 devices.
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2. Define the following terms:
SPF (shortest path first) protocol. Routing algorithm that iterates on length of
path to determine a shortest-path spanning tree. Commonly used in link-state
routing algorithms.
Static routing. Routing that is explicitly configured and entered into the routing
table. Static routes take precedence over routes chosen by dynamic routing
protocols.
Stub network. A network that has only a single connection to a router.
Presentation layer. Layer 6 of the OSI reference model. This layer provides
data representation and code formatting, along with the negotiation of data
transfer syntax. It ensures that the data that arrives from the network can be
used by the application, and it ensures that information sent by the application
can be transmitted on the network.
RARP (Reverse Address Resolution Protocol). A protocol in the TCP/IP stack
that provides a method for finding IP addresses based on MAC addresses.
3. Outline a presentation that you might give to your parents that explains the
OSI model. What examples might you use to do this?
Answers will vary
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CCNA Exam Review Questions
The following questions help you review for the CCNA exam. Answers to these
questions also appear in Appendix C, “Answers to the CCNA Exam Review Questions,”
from the Cisco Networking Academy Program: Engineering Journal and Workbook,
Volume II, Second Edition.
1. Which OSI layer supports file transfer capability?
a. Application layer
b. Network layer
c. Presentation layer
d. Session layer
e. Physical layer
2. What OSI layer negotiates data transfer syntax such as ASCII?
a. Network layer
b. Transport layer
c. Application layer
d. Physical layer
e. Presentation layer
3. Which OSI layer deals with session and connection coordination?
a. Physical layer
b. Data link layer
c. Transport layer
d. Session layer
e. Presentation layer
4. What OSI layer supports reliable connections for data transport services?
a. Application layer
b. Session layer
c. Presentation layer
d. Physical layer
e. Transport layer
5. At what layer does routing occur?
a. Session layer
b. Application layer
c. Network layer
d. Transport layer
e. Data link layer
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Chapter 2
LAN Switching

Introduction
Today, network designers are moving away from using bridges and hubs to primarily
using switches and routers to build networks. Chapter 1, “Review: The OSI Reference
Model and Routing,” provided a review of the OSI reference model and an overview of
network planning and design considerations related to routing.
This chapter discusses problems in a local-area network (LAN) and possible solutions
that can improve LAN performance. You learn about LAN congestion, its effect on
network performance, and the advantages of LAN segmentation in a network. In
addition, you learn about the advantages and disadvantages of using bridges, switches,
and routers for LAN segmentation and the effects of switching, bridging, and routing on
network throughput. Finally, you learn about Ethernet, Fast Ethernet, and VLANs and
the benefits of these technologies.

Concept Questions
Demonstrate your knowledge of these concepts by answering the following questions in
the space provided.
• The combination of more powerful computers/workstations and network-
intensive applications has created a need for bandwidth that is much greater
than the 10 Mbps available on shared Ethernet/802.3 LANs. What technology
offers a solution to this bandwidth problem?
The performance of a shared-medium LAN can be improved by using one or
more of the following solutions:
⇒ Full-duplex Ethernet
⇒ LAN segmentation
Full-Duplex Ethernet:
Full-duplex Ethernet allows the transmission of a packet and the reception of a
different packet at the same time. This simultaneous transmission and reception
requires the use of two pairs of wires in the cable and a switched connection
between each node. This connection is considered point-to-point and is collision
free. Because both nodes can transmit and receive at the same time, there are
no negotiations for bandwidth. Full-duplex Ethernet can use an existing shared
medium as long as the medium meets minimum Ethernet standards.
Ethernet usually can only use 50 percent to 60 percent of the 10 Mbps available
bandwidth because of collisions and latency. Full-duplex Ethernet offers 100
percent of the bandwidth in both directions. This produces a potential 20-Mbps
throughput (10-Mbps TX and 10-Mbps RX).

9 - 101 Engineering Journal and Workbook, Vol. II, 2nd Ed. – Chapter 2 Copyright © 2002 Cisco Systems, Inc.