networking

General Networking Topics

This chapter covers general networking concepts listed in the CCIE Security blueprint for

the written exam. The CCIE blueprint lists some example topics that define general

networking, including switching, TCP/IP, routed and routing protocols, PPP, ISDN, and

asynchronous communications.

The CCIE Security written exam contains approximately 50 percent security questions and

approximately 50 percent general networking questions. This chapter prepares you for the

general networking questions. Although the CCIE Security written exam blueprint lists

some specific networking topics, it does not, for example, mention Frame Relay, which

might appear on the exam. This chapter covers many of the listed and a few of the unlisted

general networking topics.

Although these topics are not extensively defined in the blueprint, the CCIE Security

written exam might include topics taken from the CCIE Routing and Switching written

exam blueprint. This chapter endeavors to cover all bases and provide quality test examples

to ensure that you are well prepared to tackle the general networking questions you

encounter in the examination.

This chapter covers the following topics:

•Networking basics—The OSI model, concepts, and functions. Topics include the

seven layers of the OSI model and common examples (TCP/IP).

•Switching and bridging—The process today’s networks use to switch packets and

traditional bridging methods. Virtual LANs, spanning tree, and Ethernet Channel are

discussed.

•Routing IP—The most widely used routed protocol in today’s Internet, IP, and the

routing protocols available on Cisco routers, such as RIP, EIGRP, OSPF, and BGP.

IOS commands and configuration examples demonstrate the power of routing IP on

Cisco routers.

•PPP, ISDN, Frame Relay, IP Multicast, and Async—Two of the most widely used dialup protocols are PPP and ISDN. Frame Relay is covered brie y to ensure that you

have a good understanding of the common terminology used in today’s networks. IP

multicast and async protocols are also covered.

Foundation Topics

Networking Basics—The OSI Reference Model

This section covers the Open Systems Interconnection (OSI) seven layer model theory and

common examples. CCIE candidates must fully understand and appreciate the model because

almost every routed protocol in use today is based on the architecture of the seven layer model.

The OSI model was developed by a standards body called the International Organization for

Standardization (ISO) to provide software developers a standard architecture to develop proto-

cols (such as IP). For example, the OSI model allows a PC to communicate with a UNIX device.

ISO developed the OSI model in 1984. Layers 1 and 2 are implemented in hardware and Layers 3

The OSI Seven Layer Model

Layer Name Layer Number

Application Layer 7

Presentation Layer 6

Session Layer 5

Transport Layer 4

Network Layer 3

Data Link Layer 2

Physical Layer 1

The following sections cover each layer and provide protocol examples for each.

Layer 1: The Physical Layer

The physical layer consists of standards that describe bit ordering, bit transmission rates,

connector types, and electrical and other specifications. Information at Layer 1 is transmitted in

binary (1s and 0s). For example, the letter A is transmitted as 00001010. Examples of physical

layer standards include the following:

• RS-232

• V.24

• V.35

• RJ-45

• RJ-12

Layer 2: The Data Link Layer

The data link layer focuses on getting data reliably across any particular kind of link. Flow

control and error notifications are also functions of the data link layer. The data link layer

applies to all access methods, whether they are LAN or WAN methods. Information being

processed at this layer is commonly known as frames.

The IEEE further complicated matters by subdividing the data link layer into to sublayers: the

Logical Link Control (LLC) sublayer and the MAC sublayer.

IEEE Sublayers Versus ISO Definitions

IEEE 802 Definition

ISO Standard

Logical Link Control, LLC

Data Link Layer

MAC Sublayer

Physical Medium (Layer 1)

The LLC sublayer manages and ensures communication between end devices, and the Mac

sublayer manages protocol access to the physical layer.

Examples of data link frame types include the following:

•ISDN

•SDLC

•HDLC

•PPP

•Frame Relay

•Ethernet Version II

•Spanning tree protocol

•NetBEUI

Layer 3: The Network Layer

The network layer determines the best path to a destination. Device addressing, packet

fragmentation, and routing all occur at the network layer. Information being processed at

this layer is commonly known as packets. Examples of network layer protocols include the

following:

• Internet Protocol (IP)

• Open Shortest Path First (OSPF)

• Cisco’s EIGRP routing protocol

Routing protocols (OSPF, EIGRP, and BGP, for example) provide the information required to

determine the topology of the internetwork and the best path to a remote destination. A routed

protocol is one that is transported by a routing protocol (such as RIP). For example, IP is a

routed protocol that can be advertised by a number of routing algorithms, such as RIP, OSPF,

and BGP.

Connection-oriented and connectionless protocols are commonly used terms to describe Layer 3

and 4 (lower layers of the OSI model) protocols, such as IP or TCP.

A connection-oriented protocol, such as TCP, ensures delivery of all information, whereas a

connectionless protocol, such as IP, only packages the data and sends it without guaranteeing

delivery. Connection-oriented protocols exchange control information (also called Handshake)

before transmitting data. A telephone call can be considered a connection-oriented service

because the call is established before conversation can take place, much the same way that TCP

sets up a data connection before data is sent. FTP is another example of a connection-oriented

protocol. IP is an example of connectionless service.

Layer4:TheTransport Layer

The transport layer is responsible for segmenting upper-layer applications and establishing end-

to-end connections between devices. Other transport layer functions include providing data

reliability and error-free delivery mechanisms. Information being processed at this layer is

commonly known as segments. Examples of transport layer protocols include the following:

• Transmission Control Protocol (TCP)

• Real-time transport protocol (RTP)

• User Datagram Protocol (UDP)

Layer 5: The Session Layer

The session layer performs several major functions, including managing sessions between

devices and establishing and maintaining sessions. Examples of session layer protocols include

the following:

•Database SQL

•NetBIOS Name Queries

• H.323 (Supports video as well; it is the packet switch voice standard)

• Real Time Control Protocol

Layer 6: The Presentation Layer

The presentation layer handles data formats and code formatting. The layer’s functions are

normally transparent to the end user because this layer takes care of code formats and presents

them to the application layer (Layer 7), where the end user can examine the data. Examples of

presentation layer protocols include the following:

•GIF

•JPEG

•ASCII

•MPEG

•TIFF

•MIDI

•HTML

Layer 7: The Application Layer

The application layer is closest to the end user, which means that the application will be

accessed by the end user. This layer’s major function is to provide services to end users.

Examples of application layer services include the following:

• File Transfer Protocol (FTP)

• Telnet

• Ping

• Trace route

• SMTP

• Mail clients

TCP/IP and OSI Model Comparison

TCP/IP is the most widely used networking protocol and is often compared to the industry-

defined OSI model.

Figure 2-2 displays the TCP/IP model in relation to the OSI model and where the protocol suite

of TCP/IP lines up with the ISO standard. This comparison is provided to demonstrate that

TCP/IP does not exactly conform to the OSI model. For example, the TCP/IP model has no

Layer 5 or 6.

Example of Peer-to-Peer Communication

Each layer of the OSI or TCP model has its own functions and interacts with the layer above and

below it. Furthermore, the communication between each layer’s end devices also establishes

peer-to-peer communication; this means that each layer of the OSI model communicates with

the corresponding peer.

Consider the normal communication that occurs between two IP hosts over a wide-area network

(WAN) running Frame Relay,.

The data from one (Host A) is encapsulated inside a TCP header and passed down to Layer 3

(the IP layer) for address configuration, where an IP header is also added. Information included

here is the source IP address and destination address. Layer 3 (the network layer) passes the

data to the local router acting as the gateway via the Ethernet connection in raw binary.

Router A strips the 802.3 header and encapsulates the IP, TCP, and data in a Frame Relay packet

for delivery over the WAN. A CRC is added here to ensure the packet is not corrupted over

the WAN. Frame Relay is connectionless so, if an error occurs, it’s up the to upper layers to

retransmit; Frame Relay will not retransmit the packet. Similarly, HDLC (Layer 2 protocol)

is connectionless and depends on upper layers to resubmit damaged data packets. PPP

(connection-oriented), on the other hand, resubmits packets damaged in transmission over

the WAN.

Router B receives the Layer 2 frames and strips the Frame Relay header/CRC and encapsulates

the IP, TCP, and data frame back into an 802.2 header (with its own CRC; Ethernet checks only

for errors and cannot repair them; once more, upper layers, such as TCP, ensure data delivery)

for binary transmission across the Ethernet to Host B. The data is passed up the layers through

IP, TCP, and finally to the application, where the application layer reads and acts upon the data.

The good news for security candidates is that Token Ring and legacy technologies are not

covered in the written exam, so this chapter concentrates only on Ethernet switching. Before

covering switching, the next section summarizes the evolution of Ethernet so that you are aware

of the standards that have developed since Xerox first introduced Ethernet.

Ethernet Overview

Ethernet networks are based on a development made by Xerox, Digital, and Intel. The two

versions of Ethernet are commonly referred to as Ethernet I and Ethernet II (or version 2).

Ethernet uses Carrier Sense Multiple Access Collision Detection (CSMA/CD) to transmit

frames on the wire. In an Ethernet environment, all hosts can transmit as long as no other

devices are transmitting. CSMA/CD is used to detect and warn other devices of any collisions,

and colliding stations will use a back off algorithm and wait a random amount of time before

trying again. Colliding devices send a jam signal to advise all stations that a collision has

occurred. When a jam signal is sent (a jam signal is detected by all devices because the voltage

is that of the combined colliding devices), all stations also stop transmitting. A device will

attempt to transmit up to 16 times before a user is notified of the collisions; typically, an

application error will inform the user that data could not be delivered. Microsoft’s famous

words are “Network is busy.”

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