Hierarchical Network Model

Cisco has developed a three-layer hierarchical model to help you design campus networks. Cisco uses this model to simplify designing, implementing, and managing large-scale networks. With traditional network designs, it was common practice to place the networking services at the center of the network and the users at the periphery.

However, many things in networking have changed over the past decade, including advancements in applications, developments in graphical user interfaces (GUIs), the proliferation of multimedia applications, the explosion of the Internet, and fast-paced changes in

Going Down and Up the Protocol Stack

Going Down the Protocol Stack
This section covers the basic mechanics as to how information is processed as it is sent down the protocol stack on a computer. I’ll use the diagram shown in Figure 2-10 to illustrate this process as PC-A sends information to PC-B. In this example, assume that the data link layer is Ethernet and the physical layer is copper.

The first thing that occurs on PC-A is that the user, sitting in front of the computer, creates some type of information, called data, and then

Transferring Information Between Computers

Before delving into the mechanics of how information is transferred between computers, you must grow familiar with the terminology used to describe the transmitted data. Many of the layers of the OSI Reference Model use their own specific terms to describe data transferred back and forth. As this information is passed from higher to lower layers, each layer adds information to the original data—typically a header and possibly a trailer. This process is called encapsulation. Generically speaking, the term protocol data unit (PDU) is

Transport Layer, Unreliable Connections

One of the issues of connection-oriented services is that they must always go through a three-way handshake before you can transfer data. In some instances, like file transfers, this makes sense, because you want to make sure that all data for the file is transferred successfully. However, in other cases, when you want to send only one piece of information and get a reply back, going through the three-way handshake process adds additional
overhead that isn’t necessary.

A DNS query is a good example where using a connection-oriented service doesn’t make sense. With a DNS query, a device is trying to resolve a fully qualified domain name to an IP address. The device sends the single query to a DNS server and waits for the server’s response.

In this process, only two messages are generated: the client’s query and the server’s response. Because of the minimal amount of information shared between these two devices, it makes no sense to establish a reliable connection first before sending the query. Instead, the device should just send its information and wait for a response.

If a response doesn’t come back, the application can send the information again or the user can get involved. Again, with DNS, you can configure two DNS servers in the Microsoft Windows operating system. If you don’t get a reply from the first server, the application can use the second configured server.

Because no “connection” is built up front, this type of connection is referred to as a connectionless service. The TCP/IP protocol stack uses the User Datagram Protocol (UDP) to provide unreliable connections.

Transport Layer, Reliable Connections

The fourth layer of the OSI Reference Model is the transport layer. The transport layer has four main functions:
■ It sets up and maintains a session connection between two devices.
■ It can provide for the reliable or unreliable delivery of data across this connection.
■ It can implement flow control through ready/not ready signals or windowing to ensure one device doesn’t overflow another device with too much data on a connection.
■ It multiplexes connections, allowing multiple applications to simultaneously send and receive data.

The following sections cover these processes.


Reliable Connections
The transport layer can provide reliable and unreliable transfer of data between networking devices. TCP/IP’s Transmission Control Protocol (TCP) is an example of a transport layer protocol that

Advantages of Routers

Because routers operate at a higher layer than the network layer and use logical addressing, they provide many advantages over bridges and switches, including:

■ Logical addressing at layer-3 allows you to build hierarchical networks that scale to very large sizes. This is discussed in Chapter 12.
■ They contain broadcasts and multicasts. When a broadcast or multicast is received on an interface, it is not forwarded to

Routing Tables

Routers are devices that function at the network layer; they use network numbers to make routing decisions: how to get a packet to its destination. Routers build a routing table, which contains path information. This information includes the network number, which interface the router should use to reach the network number, the metric of the path (what it costs to reach the destination), and how the router learned about this network number. Metrics are used to weight the different paths to a destination. If there is more than one way to

Network Layer, Layer-3 Addressing

Network Layer
Layer 3 of the OSI Reference Model is the network layer. This layer is responsible for three main functions:
■ Defines logical addresses used at layer-3
■ Finds paths, based on the network numbers of logical addresses, to reach destination devices
■ Connects different data link types together, such as Ethernet, FDDI, Serial, and Token Ring

The following sections cover the network layer in more depth.

Layer-3 Addressing
Many protocols function at the network layer: AppleTalk, DECnet, IP, IPX, Vines, XNS, and others. Each of these protocols has its own method of defining logical addressing. Correct assignment of these addresses on devices across your network allows you to build a hierarchical design that can scale to very large sizes. This provides an advantage over layer-2 addresses, which use a flat design and are not scalable.

All layer-3 addressing schemes have two components: network and host (or node). Each segment (physical or logical) in your network needs a unique network number. Each host on these segments needs a unique host number from within the assigned network number. The combination of the network and host number assigned
to a device provides a unique layer-3 address throughout the entire network. For example, if you had 500 devices in your network that were running IP, each of these devices would need a unique IP layer-3 address.

This process is different with MAC addresses, which are used at layer-2. MAC addresses need to be unique only on a physical (or logical) segment. In other words, within the same broadcast domain, all of the MAC addresses must be unique. However, MAC addresses do not need to be unique between two different broadcast domains.

An example of this appears later in this chapter.
To understand the components of layer-3 addresses, let’s look at a few examples. TCP/IP addresses are 32 bits in length. To make these addresses more readable, they are broken up into four bytes, or octets, where any two bytes are separated by a period. This is commonly referred to as dotted decimal notation. Here’s a simple example of an IP address: 10.1.1.1. An additional value, called a subnet mask, determines the boundary between the network and host components of an address. When comparing IP addresses to other protocols’ addressing schemes, IP is the most complicated. IP addressing is thoroughly covered in Chapter 3.

Most other protocols have a much simpler format. For example, IPX addresses are 80 bits in length. The first 32 bits are always the network number, and the last 48 bits are always the host address. IPX addresses are represented in hexadecimal. Here’s an example: ABBA.0000.0000.0001. In this example, ABBA is the network number and 0000.0000.0001 is the host number. Every protocol has its own addressing scheme. However, each scheme always begins with a network component followed by a host component.

Bridge, Data Link Devices

Bridges are data link layer devices that switch frames between different layer-2 segments. They perform their switching in software, and their switching decisions are based on the destination MAC address in the header of the data link layer frame.

Bridges perform three main functions:

■ They learn where devices are located by placing the MAC address of a device and the identifier of the port it is connected to in a port address table.
■ They forward traffic intelligently, drawing on information they have in their port address table.
■ They remove layer-2 loops by running the Spanning Tree Protocol (STP).

Actually, these three functions are implemented in bridges that perform transparent bridging. There are other types of

Ethernet II’s Version of Ethernet

Ethernet II is the original Ethernet frame type. Ethernet II and 802.3 are very similar: they both use CSMA/CD to determine their operations. Their main difference is the frames used to transmit information between NICs. The bottom part of earlier Figure 2-3 shows the fields in an Ethernet II frame. Here are the two main differences between an Ethernet II and IEEE:

■ Ethernet II does not have any sublayers, while IEEE 802.2/3 have two: LLC and MAC.

■ Ethernet II has a type field instead of a length field (used in 802.3). IEEE 802.2 defines the type for IEEE Ethernet.

If you examine the IEEE 802.3 frame and the Ethernet II frame, you can see that they are very similar. NICs differentiate them by examining the value in the type field for an Ethernet II frame and the value in the

IEEE’s Version of Ethernet

There are actually two variants of Ethernet: IEEE’s implementation and the DIX implementation. Ethernet was developed by three different companies in the early 1980s: Digital, Intel, and Xerox, or DIX for short. This implementation of Ethernet has evolved over time; its current version is called Ethernet II. Devices running TCP/IP typically use the Ethernet II implementation.

The second version of Ethernet was developed by IEEE and is standardized in the IEEE 802.2 and 802.3 standards. IEEE has split the data link layer into two components: MAC and LLC. These components are

Data Link Layer

Layer 2 of the OSI Reference Model is the data link layer. This layer is responsible for defining the format of layer-2 frames as well as the mechanics of how devices communicate with each other over the physical layer. Here are the components the data link layer is responsible for:

■ Defining the Media Access Control (MAC) or hardware addresses
■ Defining the physical or hardware topology for connections
■ Defining how the network layer protocol is encapsulated in the

Wireless Concept Basic

Wireless transmission has been used for a very long time to transmit data by using infrared radiation, microwaves, or radio waves through a medium like air. With this type of connection, no wires are used. Typically, three terms are used to group different wireless technologies: narrowband, broadband, and circuit/packet data. Whenever you are choosing a wireless solution for your WAN or LAN, you should always consider the following criteria: speed, distance, and number of devices to connect.

Narrowband solutions typically require a license and operate at a low data rate. Only one frequency is used for transmission: 900 MHz, 2.4 GHz, or 5 GHz. Other technologies—household wireless phones, for instance—also use these technologies. Through the use of spread spectrum, higher data rates can be achieved by spreading the signal across multiple frequencies. However, transmission of these signals is

Fiber Cabling

LANs typically use either copper or fiber-optic cabling. Copper cabling is discussed in more depth in the section “Ethernet” later in this chapter.

Fiber-optic cabling uses light-emitting diodes (LEDs) and lasers to transmit data. With this transmission, light is used to represent binary 1’s and 0’s: if there is light on the wire, this represents a 1; if there is no light, this represents a 0. Fiber-optic cabling is typically used to provide very high speeds and to span connections across very large distances. For example,

OSI Reference Model 3

Network Layer

The third layer of the OSI Reference Model is the network layer. The network layer provides quite a few functions. First, it provides for a logical topology of your network using logical, or layer-3, addresses. These addresses are used to group machines together. The network component is used to group devices together. Layer-3 addresses allow devices that are on the same or different media types to communicate with each other. Media types define types of connections, such as Ethernet, Token Ring, or serial.

To move information between devices that have different network numbers, a router is used. Routers use information in the logical address to make intelligent decisions about how to

OSI Reference Model 2

Layer Definitions

There are seven layers in the OSI Reference Model, shown in Figure 2-1: application, presentation, session, transport, network, data link, and physical. The functions of the application, presentation, and session layers are typically part of the user’s application. The transport, network, data link, and physical layers are

Unicast, Multicast, Broadcast

Unicast
A frame with a destination unicast MAC address is intended for just one device on a segment. The top part of Figure 2-2 shows an example of a unicast. In this example, PC-A creates an Ethernet frame with a destination MAC address that contains PC-C’s address. When PC-A places this data link layer frame on the wire, all the devices on the segment receive. Each of the NICs of PC-B, PC-C, and PC-D examine the destination MAC address in the frame. In this instance, only PC-C’s NIC will process the frame, since the destination MAC address in the frame matches the MAC address of its NIC. PC-B and PC-D will ignore the frame.

Multicast

Unlike a unicast address, a multicast address represents a group of devices on a segment. The multicast group can contain anywhere from no devices to every device on a segment. One of the interesting things about multicasting is that the membership of a group is dynamic—devices can join and leave as they please. The detailed process of multicasting is beyond the scope of this book, however.

The middle portion of Figure 2-2 shows an example of a multicast. In this example, PC-A sends a data link layer frame to a multicast group on its segment. Currently, only PC-A, PC-C, and PC-D are members of this group. When each of the PCs receives the frame, its NIC examines the destination MAC address in the data link layer frame. In this example, PC-B ignores the frame, since it is not a member of the group. However, PC-C and PC-D will process the frame.

Broadcast

A broadcast is a data link layer frame that is intended for every networking device on the same segment. The bottom portion of Figure 2-2 shows an example of a broadcast. In this example, PC-A puts a broadcast address in the destination field of the data link layer frame. For MAC broadcasts, all of the bit positions in the address are enabled, making the address FFFF.FFFF.FFFF in hexadecimal. This frame is then placed on the wire. Notice that in this example, when PC-B, PC-C, and PC-D receive the frame, they all process it.

Broadcasts are mainly used in two situations. First, broadcasts are more effective than unicasts if you need to send the same information to every machine. With a unicast, you would have to create a separate frame for each machine on the segment; with a broadcast, you could accomplish the same thing with one frame. Second, broadcasts are used to discover the unicast address of a device. For instance, when you turn on your PC, initially, it doesn’t know about any MAC addresses of any other machines on the network. A broadcast can be used to discover the MAC addresses of these machines, since they will all process the broadcast frame. In IP, the Address Resolution Protocol (ARP) uses this process to discover another device’s MAC address.

OSI Reference Model 1

The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) Reference Model to describe how information is transferred from one machine to another, from the point when a user enters information using a keyboard and mouse to when that information is converted to electrical or light signals transferred along a piece of wire or radio waves transferred through the air. It is important to understand that the OSI Reference Model describes concepts and terms in a general manner, and that many network protocols, such as IP and IPX, fail to fit nicely into the scheme explained in ISO’s model. Therefore, the OSI Reference Model is most often used as a teaching and troubleshooting tool. By understanding the basics of the OSI Reference Model, you can apply these to real protocols to gain a better understanding of them as well as