Data Link
The Tcpip Network Interface Layer
The network interface layer defines the protocols and hardware required to deliver data across some physical network. The term network interface refers to the fact that this layer defines how to connect the host computer, which is not part of the network, to the network it is the interface between the computer and the network. For instance, Ethernet is one example protocol at the TCP IP network interface layer. Ethernet defines the required cabling, addressing, and protocols used to create an...
Ethernet Addressing
Ethernet LAN addressing identifies either individual devices or groups of devices on a LAN. Unicast Ethernet addresses identify a single LAN card. Each address is 6 bytes long, is usually written in hexadecimal, and, in Cisco devices, typically is written with periods separating each set of four hex digits. For example, 0000.0C12.3456 is a valid Ethernet address. The term unicast addresses, or individual addresses, is used because it identifies an individual LAN interface card. (The term...
What Is the Subnet Number and What Are the IP Addresses in the Subnet
One of the most common things you need to figure out is that after you know an IP address and subnet mask, you must answer questions about them. The question might be straightforward, such as What is the subnet number , or it might be more subtle, such as Which of the following IP addresses are in the same subnet as the stated address In either case, if you can dissect an IP address as described in this chapter, you can answer any variation of this type of question. In the next several...
Do I Know This Already Quiz
Which of the following describes the functions of OSI Layer 3 protocols Answer A and C 2. Imagine that PC1 needs to send some data to PC2, and PC1 and PC2 are separated by several routers. What are the largest entities that make it from PC1 to PC2 3. Which of the following does a router normally use when making a decision about routing TCP IP 4. Imagine a network with two routers that are connected with a point-to-point HDLC serial link. Each router has an Ethernet, with PC1 sharing the...
DSL Standards
DSL comes in many flavors to meet different needs. For instance, DSL has limits on how long the local loop can be (the length of the local loop is simply the length of the combined cables that stretch from a house to the CO). Some DSL variants allow the local loop to be much longer, while others allow for only a shorter local loop. For the standards with a shorter local loop, the transmission rates tend to be much higher a simple design trade-off. ADSL and SDSL tend to be the most popular...
Unshielded Twisted Pair and Shielded Twisted Pair Cabling and Connectors
The Telecommunications Information Associatation (TIA) defines standards for LAN cabling. For copper-wire LAN cabling, two main branches have been defined Unshielded twisted pair (UTP) Shielded twisted pair (STP) Figure 11-5 shows a conceptual diagram of each type of cable. The figure shows a side view of each cable and a straight-on view of a UTP cable. All the parts of the figure show the cable cut open so that you can see the internal components of the cables. Figure 11-5 UTP and STP Cable...
Multiplexing Using TCP Port Numbers
TCP provides a lot of features to applications, at the expense of requiring slightly more processing and overhead, as compared to UDP. However, TCP and UDP both use a concept called multiplexing. So, this section begins with an explanation of multiplexing with TCP and UDP. Afterward, the unique features of TCP and UDP are explored. Multiplexing by TCP and UDP involves the process of how a computer thinks when receiving data. The computer might be running many applications, such as a web...
OSI Terminology
First, remembering the names of the OSI layers is just an exercise in memorization. You might benefit from the following list of mnemonic phrases, with the first letters in each word being the same as the first letters of the OSI layer names, in order All People Seem To Need Data Processing (Layers 7 to 1) Please Do Not Take Sausage Pizzas Away (Layers 1 to 7) Pew Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7) You also should know how to use the names of the layers when discussing...
Typical Features of OSI Layer
The transport layer (Layer 4) defines several functions, the most important of which are error recovery and flow control. Routers discard packets for many reasons, including bit errors, congestion and instances in which no correct routes are known. As you have read already, most data-link protocols notice errors but then discard frames that have errors. The OSI transport layer might provide for retransmission (error recovery) and help to avoid congestion (flow control) or it might not. It...
Math Operations Used to Answer Subnetting Questions
Computers, especially routers, do not think about IP addresses in terms of the conventions shown in Table 12-2. They think in terms of 32-bit binary numbers, which is fine because, technically, that's what IP addresses really are. Also, computers use a mask to define the structure of these binary IP addresses. A full understanding of what that means is not too difficult. However, getting accustomed to doing the binary math in your head is challenging for most of us, particularly if you don't do...
How Many Hosts and How Many Subnets
You also should know how to figure out how many network, subnet, and host bits are used with that subnetting scheme. From those facts, you easily can figure out how many hosts exist in the subnet and how many subnets you can create in that network using that subnet mask. You already have learned that Class A, B, and C networks have either 8, 16, or 24 bits in their network fields, respectively. Those rules do not change. You also already have read that, without subnetting, Class A, B, and C...
CCNA Exam Topics
Interestingly, the CCNA (640-801) exam topics posted by Cisco are not simply the combination of the INTRO exam topics and the ICND exam topics. If you look closely, the CCNA exam topics match more closely to the ICND exam topics than they do to the INTRO exam topics. So, for those of you planning to take the single CCNA exam, what does that mean Well, for practical purposes, the CCNA exam covers all the topics covered on both the INTRO and ICND exams. However, the length of the CCNA exam does...
Integrated Services Digital Network
Integrated Services Digital Network (ISDN) provides switched (dialed) digital WAN services in increments of 64 kbps. Before ISDN, the only widely-available method to dial a circuit for data communication between two computers was to use analog modems. When ISDN was created, analog modem speeds typically did not even exceed 9600 bps. The phone companies of the world wanted to have a dialed service that not only allowed faster transmission rates, but also was pervasive as a simple analog line...
ATM and SONET
Asynchronous Transfer Mode (ATM) and Synchronous Optical Network (SONET) together provide the capability for a telco to provide high-speed services for both voice and data over the same network. SONET defines a method for transmitting digital data at high speeds over optical cabling, and ATM defines how to frame the traffic, how to address the traffic so that DTE devices can communicate, and how to provide error detection. In short, SONET provides Layer 1 features, and ATM provides Layer 2...
The User Datagram Protocol
UDP provides a service for applications to exchange messages. Unlike TCP, UDP is connectionless and provides no reliability, no windowing, and no reordering of the received data. However, UDP provides some functions of TCP, such as data transfer, segmentation, and multiplexing using port numbers, and it does so with fewer bytes of overhead and with less processing required. UDP multiplexes using port numbers in an identical fashion to TCP. The only difference in UDP (compared to TCP) sockets is...
Basic Administrative Configuration
Chapter 7 focused on the configuration process more than the actual configuration commands that happened to be in the chapter. Before you configure IP, this short section reviews some of the basic commands you typically will configure on any router. On most routers, you would configure at least the following Reference to a DNS so that commands typed on the router can refer to host names instead of IP addresses Set a password for those Telnetting to the router Set the enable secret password to...
Data Encapsulation
The term encapsulation describes the process of putting headers and trailers around some data. A computer that needs to send data encapsulates the data in headers of the correct format so that the receiving computer will know how to interpret the received data. You have seen several examples of encapsulation in this chapter already. The web server encapsulated the home page inside an HTTP header in Figure 2-2. The TCP layer encapsulated the HTTP headers and data inside a TCP header in Figure...
Trademark Acknowledgments
All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capitalized. Cisco Press or Cisco Systems, Inc., cannot attest to the accuracy of this information. Use of a term in this book should not be regarded as affecting the validity of any trademark or service mark. Publisher John Wait Editor-In-Chief John Kane Executive Editor Brett Bartow Managing Editor Patrick Kanouse Development Editor Christopher Cleveland Project Editor Marc Fowler Copy...
Internet Control Message Protocol
Earlier in this chapter, you read about how to configure IP addresses and how to perform some basic troubleshooting. For troubleshooting, you have seen how to look at the routing table with the show ip route command, how to look at interface status with several options on the show interfaces command, and how to use standard and extended ping commands for basic troubleshooting. TCP IP includes a protocol specifically to help manage and control the operation of a TCP IP network, called the...
Comparing and Contrasting IP Routing Protocols
Routing protocols can be categorized in several ways. One distinction is whether the protocol is more useful between two companies or inside a single company. Only one IP routing protocol that is popular today, the Border Gateway Protocol (BGP), is designed specifically for use between two different organizations. In fact, BGP distributes routing information between ISPs worldwide today and between ISPs and their customers as need be. Routing protocols that are best used to distribute routes...
The CCNA Certification Exams What Theres More than One Exam
For the first time since Cisco announced CCNA in 1998, the CCNA certification has an option for multiple exams. Before Cisco announced these latest changes around summer of 2003, to get your CCNA certification, you just passed a single CCNA exam. With this latest generation of the CCNA, you can either take a single exam to get your CCNA, or you can take two exams, with each of these exams covering a subset of the CCNA exam topics. Table I-1 lists the exams. Table I-1 CCNA Exam Names and Numbers...
About the Ccna Intro Exam Certification Guide and Ccna Icnd Exam Certification Guide
As mentioned earlier in this introduction, you can take both the INTRO and ICND exams to acquire CCNA certification, or you can take a single CCNA exam. Because of the significantly expanded topics as compared with the previous CCNA exam, there was simply too much material for a single book. So, we simply created two books one for the INTRO exam and one for the ICND exam. The contents of the two books, however, were actually designed for both the single-exam and dual-exam audience. For those of...
How to Use These Books to Prepare for the CCNA Exam
If you are using this book to study for the INTRO exam, just follow the plan outlined in the last few pages. However, to use this book to study for the CCNA exam, you really should use both this book and the CCNA ICND Exam Certification Guide. (By the way, if you've not bought this book yet and you want both, you can generally get the pair cheaper by buying a set, called the CCNA Certification Library.) These two books were designed to be used together to help those who want to get their CCNA...
Classes of Networks
In Figure 5-3 and the surrounding text, I claimed that the IP addresses of devices attached to the Token Ring all started with 8 and that the IP addresses of devices attached to the Ethernet all started with 130.4. Why only one number for the prefix on the Token Ring and two numbers on the Ethernet Well, it all has to do with IP address classes. RFC 790 defines the IP protocol, including multiple different classes of networks. IP defines three different network classes, called A, B, and C, from...
Boot Protocol BOOTP
Dynamic Host Configuration Protocol (DHCP) RARP and BOOTP work using the same basic process. To use either protocol, a PC needs a LAN interface card. The computer sends a LAN broadcast frame announcing its own MAC address and requests that someone assign it an IP address. Figure 5-11 outlines the process for both RARP and BOOTP. Hey Everybody My MAC Address Is 0200.1111.1111. If You Are a RARP Server, Please Tell Me My IP Address 0200.1111.1111 10.1.1.1 0200.1234.567810.1.1.2 0200.5432.1111...
The Need for Spanning Tree
Without the Spanning Tree Protocol (STP), frames would loop for an indefinite period of time in networks with physically redundant links. To prevent looping frames, STP blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segments (collision domains). The result of STP is good Frames do not loop infinitely, which makes the LAN usable. However, the network uses some redundant links in case of a failure, but not for balancing traffic. To avoid...
Perspectives on Networking
You might have seen or heard about different topics relating to networking, but you are only just now getting serious about learning the details. Like many people, your perspective about networks might be that of a user of the network, as opposed to the network engineer who builds networks. For some, that perspective is as a dialup user of the Internet. Others might use a computer at a job or at school that computer is typically connected to a network via some...
What Subnet Masks Meet the Stated Design Requirements
So far in this chapter, the text has explained how to answer questions that provide the subnet number. However, some questions do not supply the subnet number, but instead ask you to choose the correct subnet mask, given a set of requirements. The most common of these questions reads something like this You are using Class B network X, and you need to have 200 subnets, with at most 200 hosts per subnet. Which of the following subnet masks can be used (followed by some subnet masks that you can...
Format of the CCNA Exams
The INTRO, ICND, and CCNA exams all follow the same general format. When you get to the testing center and check in, the proctor will give you some general instructions and then take you into a quiet room with a PC. When you're at the PC, you have a few things to do before the timer starts on your exam for instance, you can take a sample quiz, just to get accustomed to the PC and to the testing engine. Anyone who has user-level skills in getting around a PC will have no problems with the...
The Tcpip Internetwork Layer
Imagine that you just wrote a letter to your favorite person on the other side of the country and that you also wrote a letter to someone on the other side of town. It's time to send the letters. Is there much difference in how you treat each letter Not really. You put different addresses on the envelope for each letter because the letters need to go to two different places. You put stamps on both letters and put them in the same mailbox. The postal service takes care of all the details of...
LAN Segmentation
LAN segmentation simply means breaking one LAN into parts, with each part called a segment. The term LAN segment comes from the original use of a physical bus with 10BASE2 and 10BASE5. A single Ethernet segment consisted of the devices connected serially with coaxial cable. When 10BASE-T came along, the term segment still was used, this time to identify a hub with multiple devices connected to it. Figure 9-7 repeats an earlier figure, but with the term segment noted on the figure. Figure 9-7...
Forwarding Unknown Unicasts and Broadcasts
What do you suppose the bridge did with Fred's first frame in Figure 9-4, the one that occurred when there were no entries in the bridging table As it turns out, when there is no matching entry in the table, bridges forward the frame out all interfaces. Bridges were designed to forward what are called unknown unicast frames (frames whose destination MAC addresses are not yet in the bridging table), with the hope that the unknown device will be on some other Ethernet segment and will reply, and...
Navigating Through a Cisco 2950 Switch
You can buy a Cisco 2950 switch or any other model of Cisco switch, turn on the power, and plug in the Ethernet cables to your computers, and the LAN you just created typically works just fine. There is no need to ever perform any specific configuration on the switch. However, you will probably want to configure the switch and possibly investigate and troubleshoot problems when they occur. So, knowing how to configure and troubleshoot a switch can help you in almost any networking job. The nice...
Network Topologies
You already have been introduced to several different network topologies as you have read through this book. For instance, 10BASE2 networks use a physical bus topology, whereas 10BASE-T networks use a physical star topology. This section introduces you to several other types of network topologies. Figure 11-1 shows the different types of Ethernet topologies covered earlier in the book, with some specific terms used to describe the topology for each design. Figure 11-1 Different Types of Network...
Layer 4 Switching
The term Layer 4 switches (L4 switches) refers to a type of switching in which the switch considers the information in the Layer 4 headers when forwarding the packet. In some cases, the forwarding decision is based upon information inside the Layer 4 headers. In other cases, L3 forwarding is used, but the switch does accounting based on the Layer 4 headers. Both are considered to be Layer 4 switching. The key to understanding L4 switching is to remember the function of TCP and UDP port numbers,...
The Tcpip Application Layer
Arguably, the most popular TCP IP application today is the web browser. Many major software vendors either have already changed or are changing their software to support access from a web browser. And thankfully, using a web browser is easy you start a web browser on your computer and select a web site by typing in the name of the web site, and the web page appears. What really happens to allow that web page to appear on your web browser These next few sections take a high-level look at what...
The Tcpip Transport Layer
The TCP IP application layer includes a relatively large number of protocols, with HTTP being only one of those. The TCP IP transport layer consists of two main protocol options the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). To get a true appreciation for what TCP IP transport layer protocols do, read Chapter 6, Fundamentals of TCP and UDP. However, in this section, you will learn about one of the key features of TCP, which enables us to cover some more general...
OSI Layer 1 for Pointto Point WANs
The OSI physical layer, or Layer 1, defines the details of how to move data from one device to another. In fact, many people think of OSI Layer 1 as sending bits. Higher layers encapsulate the data, as described in Chapter 2, The TCP IP and OSI Networking Models. No matter what the other OSI layers do, eventually the sender of the data needs to actually transmit the bits to another device. The OSI physical layer defines the standards and protocols used to create the physical network and to send...
Link State Protocols OSPF and Integrated ISIS
Link-state and distance vectors share a common goal to fill the routing tables with the current best routes. They differ significantly in how they each accomplish the task. The largest difference between the two is that distance vector protocols advertise sparse information in fact, distance vector protocols know only that other routers exist if the other router broadcasts a routing update to them. When a distance vector protocol in a router hears a routing update, the update says nothing about...
Frame Relay
Point-to-point WANs can be used to connect a pair of routers at multiple remote sites. However, an alternative WAN service, Frame Relay, has many advantages over point-to-point links, particularly when you connect many sites via a WAN. To introduce you to Frame Relay, I focus on a few of the key benefits compared to leased lines. One of the benefits is seen easily by considering Figures 4-7. Figure 4-7 Two Leased Lines to Two Branch Offices In Figure 4-7, a main site is connected to two branch...
Router Memory Processors and Interfaces
The configuration file contains the configuration commands that you have typed, as well as some configuration commands entered by default by the router. The configuration file can be stored in a variety of places, including two inside a router. The router has a couple of other types of memory as well RAM Sometimes called DRAM for dynamic random-access memory, RAM is used by the router just as it is used by any other computer for working storage. The running or active configuration file is...
Scenario Part A Planning
This scenario has three parts, listed as Parts A, B, and C. Part A begins with some planning guidelines that mainly consist of planning an IP addressing scheme for a network. After you complete Part A, Part B of the scenario asks you to configure the three routers and one switch to implement the planned design. Finally, Part C asks you to examine router command output and answer questions about the details of current operation of the network. Part C also lists some questions related to the user...
Solutions to Part A Planning
It's a good idea to keep the design as simple as possible, without making it so simple that it will not be useful as the network evolves. In this case, any subnet mask with at least 7 host bits would work, including the easy mask of 255.255.255.0. Any choice of mask between 255.255.224.0 and 255.255.255.128 would have allowed for 6 subnets and 100 hosts per subnet. Table 16-3 shows one solution for the subnet numbers chosen, using mask 255.255.255.128, with Table 16-4 showing some sample IP...
Routing Protocol Overview
IP routing protocols have one primary goal to fill the IP routing table with the current best routes it can find. The goal is simple, but the process and options can be complicated. Terminology can get in the way when you're learning about routing protocols. This book's terminology relating to routing and routing protocols is consistent with the authorized Cisco courses, as well as with most Cisco documentation. So, just to make sure you have the terminology straight before diving into the...
Configuring IP Addresses
Before you configure IP address, you first must decide what IP addresses to configure. Figure 13-1 not only outlined the network diagram, but it also listed the IP addresses chosen for the network. In this case, network 10.0.0.0 hs been subnetted with six subnets and a mask of 255.255.255.0. After you have chosen the IP addresses and masks, configuration is simple. Examples 13-2, 13-3, and 13-4 show the IP configuration details for the three routers in Figure 13-1. The full configuration for...
Routing Path Selection
Routing focuses on the end-to-end logic of forwarding data. Figure 5-1 shows a simple example of how routing works. The logic seen in the figure is relatively simple. For PC1 to send data to PC2, it must send something to R1, when sends it to R2, then on to R3, and finally to PC2. However, the logic used by each device along the path varies slightly. PC1's Logic Sending Data to a Nearby Router In this example, PC1 has some data to send data to PC2. Because PC2 is not on the same Ethernet as...
Multilayer Switching
A switch does not have to just perform switching at one layer or another. For instance, for traffic between ports in the same VLAN, L2 switching is needed. For traffic between two different VLANs, L3 switching is needed. So, often a switch performs switching at multiple layers. These switches are called multilayer switches. Most of the time today, when you hear of a Layer 3 switch, it is really a multilayer switch because it almost always is also a Layer 2 switch. Table 10-2 summarizes the key...
Accessing the Cisco 2950 Switch CLI
The 2950 CLI works just like the router IOS CLI. Some of the commands you use are different because switches perform different tasks than routers, but the process and the look and feel are the same. For instance It uses user exec and priviledged (enable) exec modes. It uses the enable and disable commands to move between the two. It uses a console password and telnet (vty) password, configured just like a router. It uses an enable secret or just plain enable password, with the enable secret...
Switch LEDs During POST
Power-On Self Test (POST) defines the series of steps that a device goes through to test the hardware and find out what is working before moving on to loading the operating system. POST processing is performed by boot code that is loaded into ROM. Because a full operating system has not yet been loaded when the switch performs POST, it needs a way to tell the human user if POST worked well, if it failed partly, or if the switch is totally unusable. To communicate the status, the switch uses the...
Initial Configuration Mode
The 2950 switch OS uses the same concepts of an initial configuration dialogue as does a router. When the switch initializes and finds no configuration file in NVRAM, it presents the console user with a question, asking whether to enter the initial configuration dialogue. The only real difference between the router initial configuration dialogue and the 2950 switch initial configuration dialogue is in the things the switch lets you configure. Otherwise, the process is identical. Example 8-1...
Cable Modems
Of all the access technologies covered in this chapter, cable modems are the only one not using a phone line from the local telco for physical connectivity. Many homes also have a cable TV service supplied by an analog electrical signal entering the home over a coaxial cable in other words, over the cable TV cabling. Cable modems provide an always-on Internet access service, while allowing you to surf the Internet over the cable and make all the phone calls you want over your telephone line and...
Example Configuration Process
Example 7-1 illustrates how the console password is defined gives host name, prompt, and interface descriptions and shows the finished configuration. The lines beginning with are comment lines that highlight significant processes or command lines within the example. The show running-config command output also includes comment lines with just a to make the output more readable many comment lines in the examples in this book were added to explain the meaning of the configuration. You should...
Typical LAN Features for OSI Layer
The OSI physical layer, or Layer 1, defines the details of how to move data from one device to another. In fact, many people think of OSI Layer 1 as sending bits. Higher layers encapsulate the data and decide when and what to send. But eventually, the sender of the data needs to actually transmit the bits to another device. The OSI physical layer defines the standards used to send and receive bits across a physical network. To keep some perspective on the end goal, consider the example of the...
The Tcpip Protocol Architecture
TCP IP defines a large collection of protocols that allow computers to communicate. TCP IP defines the details of each of these protocols inside document called Requests For Comments (RFCs). By implementing the required protocols defined in TCP IP RFCs, a computer can be relatively confident that it can communicate with other computers that also implement TCP IP. An easy comparison can be made between telephones and computers that use TCP IP. I can go to the store and buy a phone from one of a...
BASET Ethernet
10BASE-T solved several problems with the early Ethernet specifications. 10BASE-T allowed the use of telephone cabling that was already installed, or simply allowed the use of cheaper, easier-to-install cabling when new cabling was required. 10BASE-T networks make use of devices called hubs, as shown in Figure 3-5. 10BASE-T, Using Shared Hub - Acts Like Single Bus 10BASE-T, Using Shared Hub - Acts Like Single Bus Solid Lines Represent Twisted Pair Cabling Solid Lines Represent Twisted Pair...
Network Layer Interaction with the Data Link Layer
In Figure 5-1, four different types of data links were used to deliver the data. When the network layer protocol is processing the packet, it decides to send the packet out the appropriate network interface. Before the actual bits can be placed onto that physical interface, the network layer must hand off the packet to the data link layer protocols, which, in turn, ask the physical layer to actually send the data. And as was described in Chapter 3, Fundamentals of Ethernet LANs, the data link...
IP Routing Protocols
IP routing protocols fill the IP routing table with valid, (hopefully) loop-free routes. Each route includes a subnet number, the interface out which to forward packets so that they are delivered to that subnet, and the IP address of the next router that should receive packets destined for that subnet (if needed). Before examining the underlying logic, you need to consider the goals of a routing protocol. The goals described in the following list are common for any IP routing protocol,...
Distance Vector Protocols RIP and IGRP
Distance vector protocols advertise routing information by sending messages, called routing updates, out the interfaces on a router. These updates contain a series of entries, with each entry representing a subnet and a metric. The metric represents how good the route is from that router's perspective, with a smaller number being a better route. Any routers that receive a copy of a distance vector routing update receive that information and possibly add some routes to their routing table. The...
ISDN Channels
ISDN includes two types of lines Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Both BRI and PRI provide multiple digital bearer channels (B channels) over which data can be sent and received. Because both BRI and PRI have multiple B channels, a single BRI or PRI line can have concurrent digital dial circuits to multiple sites. Alternately, you can create multiple circuits to the same remote site to increase available bandwidth to that site. B channels transport data. They operate...
The Flintstones Network The First Computer Network
The Flintstones are a cartoon family that, according to the cartoon, lived in prehistoric times. Because I want to discuss the thought process behind some imaginary initial networking standards, the Flintstones seem to be the right group of people to put in the example. Fred is the president of FredCo, where his wife (Wilma), buddy (Barney), and buddy's wife (Betty) all work. They all have phones and computers, but they have no network because no one has ever made up the idea of a network...
IP Addressing Review
Chapter 5 explained the concepts behind IP addressing Class A, B, and C networks and subnetting. Before looking at the math behind IP addressing, a quick review will be helpful. Many different Class A, B, and C networks exist. Table 12-2 summarizes the possible network numbers, the total number of each type, and the number of hosts in each Class A, B, and C network. Table 12-2 List of All Possible Valid Network Numbers* Table 12-2 List of All Possible Valid Network Numbers* Size of Network Part...
Modem Installation and Cabling
PC modems can be located internally or externally. Internal modems are placed inside the PC itself, whereas external modems are external to the PC. Laptops might come with a modem built in or simply might use a convenient type of internal modem called a PCMCIA card, or simply PC card. PC cards are roughly the size of a credit card and easily can be inserted and removed from a PC. Most PC hardware comes with either a serial communications port, called a COM port, or a Universal Serial Bus (USB)...
Comparison of Remote Access Technologies
This chapter scratches the surface of how modems, ISDN, cable, and DSL work. Consumers choose between these options for Internet access all the time, and network engineers choose between these options for supporting their work-at-home users as well. So, it seems appropriate to close the chapter by listing some of the key comparison points for these options. Comparison of Remote Access Technologies 465 The remote access technologies in this chapter provide services at Layer 1, and possibly Layer...
Identifying the Data Inside an Ethernet Frame
Each data-link header has a field in its header with a code that defines the type of protocol header that follows. For example, in the first frame in Figure 3-13, the Destination Service Access Point (DSAP) field has a value of E0, which means that the next header is a Novell IPX header. Why is that Well, when the IEEE created 802.2, it saw the need for a protocol type field that identified what was inside the field called data in an IEEE Ethernet frame. Figure 3-13 802.2 SAP and SNAP Type...
VLAN Basics
A virtual LAN (VLAN) is a broadcast domain created by one or more switches. The switch creates a VLAN simply by putting some interfaces in one VLAN and some in another. So, instead of all ports on a switch forming a single broadcast domain, the switch separates them into many, based on configuration. It's really that simple. The first two figures in this chapter compare two networks. First, before VLANs existed, if a design specified two separate broadcast domains, two switches would be used...
IP Naming Commands
When using the IOS CLI, you will want to refer to names instead of IP addresses. Particularly for the trace, ping, and telnet commands, the IP address or host name must be supplied. This section describes the use of host names on an IOS-based device. Along the way, some nuances of the use of Telnet are covered. IOS can use statically configured names as well as refer to one or more DNSs. Example 13-9 shows some names statically configured, with configuration pointing to two different DNSs....
The Transmission Control Protocol
Each TCP IP application typically chooses to use either TCP or UDP based on the application's requirements. For instance, TCP provides error recovery, but to do so, it consumes more bandwidth and uses more processing cycles. UDP does not do error recovery, but it takes less bandwidth and uses fewer processing cycles. Regardless of which of the two TCP IP transport layer protocols the application chooses to use, you should understand the basics of how each of the protocols works. TCP provides a...
Summary of Interior Routing Protocols
Before finishing your study for the ICND or CCNA exam, you will learn a lot more about RIP-1, IGRP, EIGRP, and OSPF. This chapter has introduced you to some of the key terms and points of comparison for these routing protocols, as well as a few others. Table 14-6 summarizes the most important points of comparison between the interior routing protocols, and Table 14-7 lists some of the key terminology. Table 14-6 Interior IP Routing Protocols Compared Summary Table 14-6 Interior IP Routing...
Link Speeds Offered by Telcos
No mater what you call them telcos, PTTs, service providers these companies do not simply let you pick the exact speed of a WAN link. Instead, standards define how fast a point-to-point link can run. For a long time, the telcos of the world made more money selling voice services. That is no longer the case for any of these companies in the United States, except for the companies that provide local residential telephone service. So, years ago, the telcos of the world developed a standard for...
Layer 3 Forwarding Using a Router
Switches do not forward frames between different VLANs. So, when you have multiple VLANs, what do you do when the hosts in each VLAN want to communicate with each other Well, you use a router. Figure 10-7 outlines the general idea in a network with one switch and three VLANs. Although the switch cannot forward frames between two VLANs, a router can. First, notice that three VLANs are shown, and each VLAN corresponds to a different subnet. The router needs an interface in each subnet to forward...
ISDN Call Setup and Data Link Protocols
Call setup differs between ISDN and modems. With a telephone call and with analog modems, DTMF tones are sent across the analog local loop to the telco. The telco switch at the local CO interprets the dialed digits and sets up the call. However, with ISDN, there is no analog local loop over which the analog DTMF tones can be sent. ISDN devices send and receive signaling messages to and from the local ISDN switch to which it is connected. In telco terminology, signaling refers to any type of...
Layer 2 Ethernet Summary
As mentioned earlier in this chapter, physical layer protocols define how to deliver data across a physical medium. Data-link protocols make that physical network useful by defining how and when the physical network is used. Ethernet defines the OSI Layer 1 functions for Ethernet, including cabling, connectors, voltage levels, and cabling distance limitations, as well as many important OSI Layer 2 functions. In this section, four of these data link features were emphasized, as shown in Table...
The Cisco IOS Software Boot Sequence
Cisco routers perform the same types of tasks that a typical computer performs when you power it on or reboot (reload) it. Of course, most of us do not think about these details very often. The router performs some somewhat obvious steps, with one of those being tricky namely, the process of choosing the location of the software to load and use in the router. And that software might not be IOS. The boot process follows this basic litany 1. The router performs a power-on self-test (POST) to...
Analog Modems
Analog modems allow two computers to send and receive a serial stream of bits, with no physical changes required on the typical analog local loop between a residence and the telco CO. Because the switch in the CO expects to send and receive analog voice signals over the local loop, modems simply send an analog signal to the PSTN and expect to receive an analog signal from the PSTN. However, that analog signal represents some bits that the computer needs to send to another computer, instead of...
Modulation and Demodulation
Most people can fully appreciate the concept of the speed of a dialed circuit in terms of bits per second. However, another term, baud, often is used to describe the speed of a modem. In fact, some people say things like That modem runs at 33 kilo baud per second, really meaning 33 kilobits per second (kbps), thinking that bits per second and baud per second are the same thing. But the two terms are not synonymous, as you will read shortly. Modems create an analog signal when sending data. As...
Modem Standards
Modems have been around for more than 30 years, so as you might imagine, a lot of standards have evolved. Table 15-3 summarizes some of the modem standards. Mainly used outside the United States First widely deployed worldwide standard Backward compatible with V.32bis and V.32 Same speed as V.34, but with error-correction features 56 kbps (downstream), 33 kbps (upstream) Created from two earlier competing standards, X2 and K56Flex 56 kbps 33 kbps (downstream upstream) or 48 kbps (each...
Connection Establishment and Termination
TCP connection establishment occurs before any of the other TCP features can begin their work. Connection establishment refers to the process of initializing sequence and acknowledgment fields and agreeing to the port numbers used. Figure 6-8 shows an example of connection establishment flow. Figure 6-8 TCP Connection Establishment SYN, ACK, DPORT 1027, SPORT 80 < - SEQ 201, ACK 1451 ACK, DPORT 80, SPORT 1027 This three-way connection-establishment flow must complete before data transfer can...
Fiber Optic Cabling and Connectors for Ethernet
Fiber cabling, also called optical cabling or fiber optics, provides another option for cabling Ethernet. The main differences, in terms of function, between optical cabling for Ethernet and electrical cabling are as follows Longer distances supported by optical cabling Less magnetic interference, making it slightly more secure Only type of cabling supported by 10 Gig Ethernet For instance, network engineers might choose to use optical interfaces and cabling for Ethernet when building a campus...
Layer 3 Forwarding Using a Layer 3 Switch
The term Layer 3 switch (L3 switch) refers to a switch that also has routing features. So, instead of requiring a router external to the switch, as in Figure 10-8, the router internal to the switch performs the same routing function. The only difference between routing using a router, as in Figure 10-8, and using a Layer 3 switch lies in the internal processing. Outwardly, nothing is different. For instance, Figure 10-9 shows routing and L3 switching between two interfaces in two different...
Clock Rates DCE and DTE
When a network engineer needs to add a point-to-point leased line between two routers, he contacts a service provider and orders the circuit. As part of that process, the customer specifies how fast the circuit should run, in kilobits per second (kbps). While the circuit is being set up by the telco, the engineer purchases two CSU DSUs, installs one at each site, and configures each CSU DSU. He also cables each router to the respective CSU DSU using the cables shown in the previous section....
WAN Cabling Standards
Cisco offers a large variety of different WAN interface cards for its routers, including synchronous and asynchronous serial interfaces. For any of the point-to-point serial links or Frame Relay links in this chapter, the router uses an interface that supports synchronous communication. Synchronous serial interfaces in Cisco routers use a variety of proprietary physcial connector types, such as the 60-pin D-shell connector shown in Figure 4-4. The cable connecting the router to the CSU uses a...
Converting Analog Voice to Digital Voice
The last step in understanding how the PSTN supports voice across a digital PSTN relates to how the PSTN converts the analog electrical signals to digital signals, and vice versa. To see the need for the conversion, examine Figure 15-4. Figure 15-4 Analog Voice Calls Through a Digital PSTN Figure 15-4 Analog Voice Calls Through a Digital PSTN When Andy calls Barney in Raleigh, the circuit is set up by the telco. (Yes, Barney moved to Raleigh since the last example.) And it works It works...
Data Segmentation and Ordered Data Transfer
Sometimes the data is small in some cases, a single byte. In other cases, for instance, with a file transfer, the data might be millions of bytes. Each different type of data link protocol typically has a limit on the maximum transmission unit (MTU) that can be sent. MTU refers to the size of the data, according to the data link layer in other words, the size of the Layer 3 packet that sits inside the data field of a frame. For many data link protocols, Ethernet...
Internal Processing on Cisco Switches
Switches use a couple of different types of internal processing variations. Almost of the more recently released switches use store-and-forward processing, but all three types of switching are supported in at least one type of currently available Cisco switch. Some switches, and transparent bridges in general, use store-and-forward processing. With store-and-forward, the entire frame is received by the switch before the first bit of the frame is forwarded. However, Cisco also offers two other...
Cisco Discovery Protocol
The Cisco Discovery Protocol (CDP) discovers basic information about neighboring routers and switches, without needing to know the passwords for the neighboring devices. CDP supports any LAN, HDLC, Frame Relay, and ATM interface in fact, it supports any interface that supports the use of SNAP headers. The router or switch can discover Layer 2 and Layer 3 addressing details of neighboring routers without even configuring that Layer 3 protocol this is because CDP is not dependent on any...
Network Layer Layer 3 Addressing
One key feature of network layer addresses is that they were designed to allow logical grouping of addresses. In other words, something about the numeric value of an address implies a group or set of addresses, all of which are considered to be in the same grouping. In TCP IP, this group is called a network or a subnet. In IPX, it is called a network. In AppleTalk, the grouping is called a cable range. These groupings work just like U.S.P.S. ZIP codes, allowing the routers (mail sorters) to...
Flow Control Using Windowing
TCP implements flow control by taking advantage of the Sequence and Acknowledgment fields in the TCP header, along with another field called the Window field. This Window field implies the maximum number of unacknowledged bytes allowed outstanding at any instant in time. The window starts small and then grows until errors occur. The window then slides up and down based on network performance, so it is sometimes called a sliding window. When the window is full, the sender will not send, which...
Managing Configuration Files
IOS on a router uses a configuration file for the initial configuration at router startup and the active, running configuration file. The startup configuration file is in NVRAM the other file, which is in RAM, is the one that the router uses during operation. When the router first comes up, the router copies the stored configuration file from NVRAM into RAM, so the running and startup configuration files are identical at that point. Also, exterior to the router, configuration files can be...
What Are the Other Subnet Numbers
The final general type of IP addresing and subnetting question covered in this chapter asks you to list all the subnets of a particular network. You could use a long process, which requires you to count in binary and convert many numbers from binary to decimal. However, because most people would either learn the shortcut or use a subnet calculator in their normal jobs, I decided to just show you the shortcut method for this particular type of question. First, the question needs a better...
Balanced Hybrid Protocols Enhanced IGRP
EIGRP does not use distance vector or link-state logic, but instead it uses a whole new category of routing protocol. This new category has some features similar to link-state protocols, others similar to distance vector protocols, and yet others unlike either of the two. Cisco sometimes categorizes EIGRP as a balanced hybrid protocol, so you should remember the term. The internal workings of EIGRP depend on an algorithm called the Diffusing Update Algorithm (DUAL). DUAL exchanges more topology...
WAN Connections from the Customer Viewpoint
The concepts behind a point-to-point connection are simple. However, to fully understand what the service provider does to build his network to support your point-to-point line, you would need to spend lots of time studying and learning. However, most of what you need to know about WANs for the INTRO exam relates to how WAN connections are implemented between the telephone company and a customer site. Along the way, you will need to learn a little about the terminology used by the provider. In...
Ethernet 10BASET Cabling
The PCs and hub in Figure 3-5 typically use Category 5 UTP cables with RJ-45 connectors, as shown in Figure 3-2. The Ethernet cards in each PC have an RJ-45 connector, as does the hub these connectors are larger versions of the same type of connector used for telephone cords between a phone and the wall plate in the United States. So, connecting the Ethernet cables is as easy as plugging in a new phone at your house. The details behind the specific cable used to connect to the hub are important...
Coaxial Cabling
Coaxial cabling was used for 10BASE2 and 10BASE5 Ethernet in years past. 10BASE5 was referred to as thicknet, and 10BASE2 was referred to as thinnet because 10BASE5 used thicker coaxial cable. Even 10 years ago, newly installed Ethernet networks most often were not 10BASE2 or 10BASE5 networks, but rather were 10BASE-T. I personally have not seen a 10BASE2 or 10BASE5 Ethernet NIC available for sale from a vendor for at least 5 years. So, the relative importance of remembering the details of...
How Spanning Tree Works
The Spanning Tree Algorithm places each bridge or switch port into either a forwarding state or a blocking state. All the ports in the forwarding state are considered to be in the current spanning tree. The collective set of forwarding ports creates a single path over which frames are sent between Ethernet segments. Switches can forward frames out ports and receive frames in ports that are in a forwarding state switches do not forward frames out ports and receive frames in ports that are in a...
Data Link Function 3 Error Detection
Error detection discovers whether bit errors occurred during the transmission of the frame. To do this, most data-link protocols include a frame check sequence (FCS) or cyclical redundancy check (CRC) field in the data-link trailer. This field contains a value that is the result of a mathematical formula applied to the data in the frame. An error is detected when the receiver plugs the contents of the received frame into a mathematical formula. Both the sender and the receiver of the frame use...
How Bridges Learn MAC Addresses
Bridges perform three main functions, as mentioned earlier. One of those key functions is to learn the MAC addresses in the network to build its bridging table. With a full, accurate bridging table, the bridge can make accurate forwarding and filtering decisions. Bridges build the bridge table by listening to incoming frames and examining the source MAC address in the frame. If a frame enters the bridge and the source MAC address is not in the bridge table, the bridge creates an entry in the...
Upgrading a Cisco IOS Software Image into Flash Memory
IOS files typically are stored in Flash memory. Flash memory is rewriteable, permanent storage, which is ideal for storing files that need to be retained when the router loses power. Also, because there are no moving parts, there is a smaller chance of failure as compared with disk drives, which provides better availability. As you will read soon, IOS can be placed on an external TFTP server, but using an external server typically is done for testing in production, practically every Cisco...
Error Recovery Reliability
TCP provides for reliable data transfer, which is also called reliability or error recovery, depending on what document you read. To accomplish reliability, TCP numbers data bytes using the Sequence and Acknowledgment fields in the TCP header. TCP achieves reliability in both directions, using the Sequence Number field of one direction combined with the Acknowledgment field in the opposite direction. Figure 6-5 shows the basic operation. In Figure 6-5, the Acknowledgment field in the TCP header...
Bandwidth Clock Rate and Serial Lines in the
As mentioned back in Chapter 4, Fundamentals of WANs, you can build a WAN link in a lab without using a CSU DSU. The lab network that I used to build the examples in this chapter used three back-to-back serial cables, essentially a DTE and DCE cable pair connected together. To use a back-to-back WAN connection, one router must supply the clocking. Example 13-7 shows an example configuration for Seville, with a couple of important commands related to WAN links. Example 13-7 Seville Router...