Cisco CCENT/CCNA ICND1 100-101: Official Cert
This chapter covers the following exam topics:
Operation of IP Data Networks
Recognize the purpose and functions of various network devices such as Routers,
Switches, Bridges and Hubs.
Select the components required to meet a given network specification.
Predict the data flow between two hosts across a network.
Identify the appropriate media, cables, ports, and connectors to connect Cisco network devices to other network devices and hosts in a LAN
LAN Switching Technologies
Determine the technology and media access control method for Ethernet networks
Fundamentals of Ethernet LANs
Most every enterprise computer network can be separated into two general types of technology: local-area networks (LAN) and wide-area networks (WAN). LANs typically connect nearby devices: devices in the same room, in the same building, or in a campus of buildings.
In contrast, WANs connect devices that are typically relatively far apart. Together, LANs and WANs create a complete enterprise computer network, working together to do the job of a computer network: delivering data from one device to another.
While many types of LANs have existed over the years, today’s networks use two general types of LANs: Ethernet LANs and wireless LANs. Ethernet LANs happen to use cables for the links between nodes, and because many types of cables use copper wires, Ethernet
LANs are often called wired LANs. In comparison, wireless LANs do not use wires or cables, instead using radio waves for the links between nodes.
This chapter introduces Ethernet LANs, with more detailed coverage in Part II (Chapters 6 through 10).
“Do I Know This Already?” Quiz
Use the “Do I Know This Already?” quiz to help decide whether you might want to skim this chapter, or a major section, moving more quickly to the “Exam Preparation Tasks” section near the end of the chapter. You can find the answers at the bottom of the page following the quiz. For thorough explanations, see DVD Appendix C, “Answers to the ‘Do
I Know This Already?’ Quizzes.”
Table 2-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping
Foundation Topics Section
An Overview of LANs
Building Physical Ethernet Networks
Sending Data in Ethernet Networks
44 Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide
In the LAN for a small office, some user devices connect to the LAN using a cable, while others connect using wireless technology (and no cable). Which of the following is true regarding the use of Ethernet in this LAN?
Only the devices that use cables are using Ethernet.
Only the devices that use wireless are using Ethernet.
Both the devices using cables and those using wireless are using Ethernet.
None of the devices are using Ethernet.
Which of the following Ethernet standards defines Gigabit Ethernet over UTP cabling?
None of the other answers is correct.
Which of the following is true about Ethernet crossover cables for Fast Ethernet?
Pins 1 and 2 are reversed on the other end of the cable.
Pins 1 and 2 on one end of the cable connect to pins 3 and 6 on the other end of the cable.
Pins 1 and 2 on one end of the cable connect to pins 3 and 4 on the other end of the cable.
The cable can be up to 1000 meters long to cross over between buildings.
None of the other answers is correct.
Each answer lists two types of devices used in a 100BASE-T network. If these devices were connected with UTP Ethernet cables, which pairs of devices would require a straight-through cable? (Choose three answers.)
PC and router
PC and switch
Hub and switch
Router and hub
Wireless access point (Ethernet port) and switch
Chapter 2: Fundamentals of Ethernet LANs 45
Which of the following is true about the CSMA/CD algorithm?
The algorithm never allows collisions to occur.
Collisions can happen, but the algorithm defines how the computers should notice a collision and how to recover.
The algorithm works with only two devices on the same Ethernet.
None of the other answers is correct.
Which of the following is true about the Ethernet FCS field?
Ethernet uses FCS for error recovery.
It is 2 bytes long.
It resides in the Ethernet trailer, not the Ethernet header.
It is used for encryption.
Which of the following are true about the format of Ethernet addresses? (Choose three answers.)
Each manufacturer puts a unique OUI code into the first 2 bytes of the address.
Each manufacturer puts a unique OUI code into the first 3 bytes of the address.
Each manufacturer puts a unique OUI code into the first half of the address.
The part of the address that holds this manufacturer’s code is called the MAC.
The part of the address that holds this manufacturer’s code is called the OUI.
The part of the address that holds this manufacturer’s code has no specific name.
Which of the following terms describe Ethernet addresses that can be used to send one frame that is delivered to multiple devices on the LAN? (Choose two answers.)
46 Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide
An Overview of LANs
The term Ethernet refers to a family of LAN standards that together define the physical and data link layers of the world’s most popular wired LAN technology. The standards, defined by the Institute of Electrical and Electronics Engineers (IEEE), defines the cabling, the connectors on the ends of the cables, the protocol rules, and everything else required to create an Ethernet LAN.
Typical SOHO LANs
To begin, first think about a Small Office / Home Office (SOHO) LAN today, specifically a LAN that uses only Ethernet LAN technology. First, the LAN needs a device called an
Ethernet LAN switch, which provides many physical ports into which cables can be connected. An Ethernet uses Ethernet cables, which is a general reference to any cable that conforms to any of several Ethernet standards. The LAN uses Ethernet cables to connect different Ethernet devices or nodes to one of the switch’s Ethernet ports.
Figure 2-1 shows a drawing of a SOHO Ethernet LAN. The figure shows a single LAN switch, five cables, and five other Ethernet nodes: three PCs, a printer, and one network device called a router. (The router connects the LAN to the WAN, in this case to the
Typical Small Ethernet-Only SOHO LAN
Although Figure 2-1 shows a simple Ethernet LAN, many SOHO Ethernet LANs today combine the router and switch into a single device. Vendors sell consumer-grade integrated networking devices that work as a router and Ethernet switch, as well as doing other functions. These devices typically have “router” on the packaging, but many models also have four-port or eight-port Ethernet LAN switch ports built into the device.
Answers to the “Do I Know This Already?” quiz:
B, D, and E
B, C, and E
C and D
Chapter 2: Fundamentals of Ethernet LANs 47
Typical SOHO LANs today also support wireless LAN connections. Ethernet defines wired
LAN technology only; in other words, Ethernet LANs use cables. However, you can build one LAN that uses both Ethernet LAN technology as well as wireless LAN technology, which is also defined by the IEEE. Wireless LANs, defined by the IEEE using standards that begin with 802.11, use radio waves to send the bits from one node to the next.
Most wireless LANs rely on yet another networking device: a wireless LAN access point
(AP). The AP acts somewhat like an Ethernet switch, in that all the wireless LAN nodes communicate with the Ethernet switch by sending and receiving data with the wireless AP.
Of course, as a wireless device, the AP does not need Ethernet ports for cables, other than for a single Ethernet link to connect the AP to the Ethernet LAN, as shown in Figure 2-2.
Typical Small Wired and Wireless SOHO LAN
Note that this drawing shows the router, Ethernet switch, and wireless LAN access point as three separate devices so that you can better understand the different roles. However, most
SOHO networks today would use a single device, often labeled as a “wireless router” that does all these functions.
Typical Enterprise LANs
Enterprise networks have similar needs compared to a SOHO network, but on a much larger scale. For example, enterprise Ethernet LANs begin with LAN switches installed in a wiring closet behind a locked door on each floor of a building. The electricians install the Ethernet cabling from that wiring closet to cubicles and conference rooms where devices might need to connect to the LAN. At the same time, most enterprises also support wireless LANs in the same space, to allow people to roam around and still work and to support a growing number of devices that do not have an Ethernet LAN interface.
Figure 2-3 shows a conceptual view of a typical enterprise LAN in a three-story building.
Each floor has an Ethernet LAN switch and a wireless LAN AP. To allow communication between floors, each per-floor switch connects to one centralized distribution switch. For example, PC3 can send data to PC2, but it would first flow through switch SW3 to the first floor to the distribution switch (SWD) and then back up through switch SW2 on the second floor.
48 Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide
To Rest of
Single-Building Enterprise Wired and Wireless LAN
The figure also shows the typical way to connect a LAN to a WAN using a router. LAN switches and wireless access points work to create the LAN itself. Routers connect to both the LAN and the WAN. To connect to the LAN, the router simply uses an Ethernet LAN interface and an Ethernet cable, as shown on the lower right of Figure 2-3.
The rest of this chapter focuses on Ethernet in particular.
The Variety of Ethernet Physical Layer Standards
The term Ethernet refers to an entire family of standards. Some standards define the specifics of how to send data over a particular type of cabling, and at a particular speed. Other standards define protocols, or rules, that the Ethernet nodes must follow to be a part of an
Ethernet LAN. All these Ethernet standards come from the IEEE and include the number
802.3 as the beginning part of the standard name.
Ethernet supports a large variety of options for physical Ethernet links given its long history over the last 40 or so years. Today, Ethernet includes many standards for different kinds of optical and copper cabling, and for speeds from 10 megabits per second (Mbps) up to 100 gigabits per second (100 Gbps). The standards also differ as far as the types of cabling and the allowed length of the cabling.
The most fundamental cabling choice has to do with the materials used inside the cable for the physical transmission of bits: either copper wires or glass fibers. The use of unshielded
twisted-pair (UTP) cabling saves money compared to optical fibers, with Ethernet nodes using the wires inside the cable to send data over electrical circuits. Fiber-optic cabling, the more expensive alternative, allows Ethernet nodes to send light over glass fibers in the center of the cable. Although more expensive, optical cables typically allow longer cabling distances between nodes.
Chapter 2: Fundamentals of Ethernet LANs 49
To be ready to choose the products to purchase for a new Ethernet LAN, a network engineer must know the names and features of the different Ethernet standards supported in
Ethernet products. The IEEE defines Ethernet physical layer standards using a couple of naming conventions. The formal name begins with “802.3” followed by some suffix letters.
The IEEE also uses more meaningful shortcut names that identify the speed, as well as a clue about whether the cabling is UTP (with a suffix that includes “T”) or fiber (with a suffix that includes “X”).
Table 2-2 lists a few Ethernet physical layer standards. First, the table lists enough names so that you get a sense of the IEEE naming conventions. It also lists the four most common standards that use UTP cabling, because this book’s discussion of Ethernet focuses mainly on the UTP options.
Table 2-2 Examples of Types of Ethernet
Speed Common Name Informal IEEE
1000 Mbps Gigabit Ethernet 1000BASE-LX
1000 Mbps Gigabit Ethernet 1000BASE-T
10 Gbps 10 Gig Ethernet 10GBASE-T
Copper, 100 m
Copper, 100 m
Fiber, 5000 m
Copper, 100 m
Copper, 100 m
Fiber-optic cabling contains long thin strands of fiberglass. The attached Ethernet nodes send light over the glass fiber in the cable, encoding the bits as changes in the light.
Consistent Behavior over All Links Using the Ethernet Data Link Layer
Although Ethernet includes many physical layer standards, Ethernet acts like a single LAN technology because it uses the same data link layer standard over all types of Ethernet physical links. That standard defines a common Ethernet header and trailer. (As a reminder, the header and trailer are bytes of overhead data that Ethernet uses to do its job of sending data over a LAN.) No matter whether the data flows over a UTP cable, or any kind of fiber cable, and no matter the speed, the data link header and trailer use the same format.
While the physical layer standards focus on sending bits over a cable, the Ethernet data link protocols focus on sending an Ethernet frame from source to destination Ethernet node.
From a data link perspective, nodes build and forward frames. As first defined in Chapter 1,
“The TCP/IP and OSI Networking Models,” the term frame specifically refers to the header and trailer of a data link protocol, plus the data encapsulated inside that header and trailer.
The various Ethernet nodes simply forward the frame, over all the required links, to deliver the frame to the correct destination.
Figure 2-4 shows an example of the process. In this case, PC1 sends an Ethernet frame to
PC3. The frame travels over a UTP link to Ethernet switch SW1, then over fiber links to
50 Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide
Ethernet switches SW2 and SW3, and finally over another UTP link to PC3. Note that the bits actually travel at four different speeds in this example: 10 Mbps, 1 Gbps, 10 Gbps, and
100 Mbps, respectively.
Eth Data Eth
Eth Data Eth
Ethernet LAN Forwards a Data Link Frame over Many Types of Links
So, what is an Ethernet LAN? It is a combination of user devices, LAN switches, and different kinds of cabling. Each link can use different types of cables, at different speeds.
However, they all work together to deliver Ethernet frames from the one device on the
LAN to some other device.
The rest of this chapter takes these concepts a little deeper, first looking at the details of building the physical Ethernet network, followed by some discussion of the rules for forwarding frames through an Ethernet LAN. Ethernet frame from source to destination
Building Physical Ethernet Networks with UTP
For this second of three major sections of this chapter, I focus on the individual physical links between any two Ethernet nodes. Before the Ethernet network as a whole can send
Ethernet frames between user devices, each node must be ready and able to send data over an individual physical link. This section looks at some of the particulars of how Ethernet sends data over these links.
This section focuses on the three most commonly used Ethernet standards: 10BASE-T
(Ethernet), 100BASE-T (Fast Ethernet, or FE), and 1000BASE-T (Gigabit Ethernet, or GE).
Specifically, this section looks at the details of sending data in both directions over a UTP cable. It then examines the specific wiring of the UTP cables used for 10-Mbps, 100-Mbps, and 1000-Mbps Ethernet.
Transmitting Data Using Twisted Pairs
While it is true that Ethernet sends data over UTP cables, the physical means to send the data uses electricity that flows over the wires inside the UTP cable. To better understand how Ethernet sends data using electricity, break the idea down into two parts: how to create an electrical circuit and then how to make that electrical signal communicate 1s and 0s.
First, to create one electrical circuit, Ethernet defines how to use the two wires inside a single twisted pair of wires, as shown Figure 2-5. The figure does not show a UTP cable between two nodes, but instead shows two individual wires that are inside the UTP cable. An electrical circuit requires a complete loop, so the two nodes, using circuitry on their Ethernet ports, connect the wires in one pair to complete a loop, allowing electricity to flow.
Chapter 2: Fundamentals of Ethernet LANs 51
One Wire in a Pair
Other Wire, Same Pair
Node 1 Node 2
Creating One Electrical Circuit over One Pair to Send in One Direction
To send data, the two devices follow some rules called an encoding scheme. The idea works a lot like when two people talk, using the same language: The speaker says some words in a particular language, and the listener, because she speaks the same language, can understand the spoken words. With an encoding scheme, the transmitting node changes the electrical signal over time, while the other node, the receiver, using the same rules, interprets those changes as either 0s or 1s. (For example, 10BASE-T uses an encoding scheme that encodes a binary 0 as a transition from higher voltage to lower voltage during the middle of a
Note that in an actual UTP cable, the wires will be twisted together and not parallel, as shown in Figure 2-5. The twisting helps solve some important physical transmission issues.
When electrical current passes over any wire, it creates electromagnetic interference (EMI) that interferes with the electrical signals in nearby wires, including the wires in the same cable. (EMI between wire pairs in the same cable is called crosstalk.) Twisting the wire pairs together helps cancel out most of the EMI, so most networking physical links that use copper wires used twisted pairs.
Breaking Down a UTP Ethernet Link
The term Ethernet link refers to any physical cable between two Ethernet nodes. To learn about how a UTP Ethernet link works, it helps to break down the physical link into those basic pieces, as shown in Figure 2-6: the cable itself, the connectors on the ends of the cable, and the matching ports on the devices into which the connectors will be inserted.
Cable with Wires Inside
Basic Components of an Ethernet Link
First, think about the UTP cable itself. The cable holds some copper wires, grouped as twisted pairs. The 10BASE-T and 100BASE-T standards require two pairs of wires, while the
1000BASE-T standard requires four pairs. Each wire has a color-coded plastic coating, with
52 Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide the wires in a pair having a color scheme. For example, for the blue wire pair, one wire’s coating is all blue, while the other wire’s coating is blue-and-white striped.
Many Ethernet UTP cables use an RJ-45 connector on both ends. The RJ-45 connector has eight physical locations into which the eight wires in the cable can be inserted, called pin
positions, or simply pins. These pins create a place where the ends of the copper wires can touch the electronics inside the nodes at the end of the physical link so that electricity can flow.
If available, find a nearby Ethernet UTP cable and examine the connectors closely.
Look for the pin positions and the colors of the wires in the connector.
To complete the physical link, the nodes each need an RJ-45 Ethernet port that matches the RJ-45 connectors on the cable so that the connectors on the ends of the cable can connect to each node. PCs often include this RJ-45 Ethernet port as part of a network interface
card (NIC), which can be an expansion card on the PC or can be built into the system itself.
Switches typically have many RJ-45 ports because switches give user devices a place to connect to the Ethernet LAN.
Figure 2-7 shows photos of the cables, connectors, and ports.
RJ-45 Connectors and Ports (Ethernet NIC © Mark Jansen, LAN Cable ©
The RJ-45 connector is slightly wider, but otherwise similar, to the RJ-11 connectors commonly used for telephone cables in homes in North America.
Chapter 2: Fundamentals of Ethernet LANs 53
The figure shows a connector on the left and ports on the right. The left shows the eight pin positions in the end of the RJ-45 connector. The upper right shows an Ethernet NIC that is not yet installed in a computer. The lower-right part of the figure shows the side of a Cisco
2960 switch, with multiple RJ-45 ports, allowing multiple devices to easily connect to the
Finally, while RJ-45 connectors with UTP cabling can be common, Cisco LAN switches often support other types of connectors as well. When you buy one of the many models of Cisco switches, you need to think about the mix and numbers of each type of physical ports you want on the switch.
To give its customers flexibility as to the type of Ethernet links, even after the customer has bought the switch, Cisco switches include some physical ports whose port hardware can be changed later, after you purchase the switch. One type of port is called a gigabit interface
converter (GBIC), which happened to first arrive on the market around the same time as
Gigabit Ethernet, so it was given the same “gigabit” name. More recently, improved smaller types of removable interfaces, called small form-factor pluggables (SFP), provide the same function of giving users the ability to swap hardware and change the type of physical link.
Figure 2-8 shows a photo of a Cisco switch, with an SFP sitting slightly outside the SFP slot.
Gigabit Fiber SFP Sitting Just Outside a Switch SFP Port
UTP Cabling Pinouts for 10BASE-T and 100BASE-T
So far in this section, you have learned about the equivalent of how to drive a truck on a 1000-acre ranch, but you do not know the equivalent of the local traffic rules. If you worked the ranch, you could drive the truck all over the ranch, any place you wanted to go, and the police would not mind. However, as soon as you get on the public roads, the police want you to behave and follow the rules. Similarly, so far this chapter has discussed the general principles of how to send data, but it has not yet detailed some important rules for
Ethernet cabling: the rules of the road so that all the devices send data using the right wires inside the cable.
This next topic discusses conventions for 10BASE-T and 100BASE-T together, because they use UTP cabling in similar ways (including the use of only two wire pairs). A short comparison of the wiring for 1000BASE-T (Gigabit Ethernet), which uses four pairs, follows.
54 Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide
Straight-Through Cable Pinout
10BASE-T and 100BASE-T use two pair of wires in a UTP cable, one for each direction, as shown in Figure 2-9. The figure shows four wires, all of which sit inside a single UTP cable that connects a PC and a LAN switch. In this example, the PC on the left transmits using the top pair, and the switch on the right transmits using the bottom pair.
One Twisted Pair
One Twisted Pair
Receiver Data Flow Transmitter
Using One Pair for Each Transmission Direction with 10- and 100-Mbps
For correct transmission over the link, the wires in the UTP cable must be connected to the correct pin positions in the RJ-45 connectors. For example, in Figure 2-9, the transmitter on the PC on the left must know the pin positions of the two wires it should use to transmit. Those two wires must be connected to the correct pins in the RJ-45 connector on the switch, so that the switch’s receiver logic can use the correct wires.
To understand the wiring of the cable—which wires need to be in which pin positions on both ends of the cable—you need to first understand how the NICs and switches work. As a rule, Ethernet NIC transmitters use the pair connected to pins 1 and 2; the NIC receivers use a pair of wires at pin positions 3 and 6. LAN switches, knowing those facts about what
Ethernet NICs do, do the opposite: Their receivers use the wire pair at pins 1 and 2, and their transmitters use the wire pair at pins 3 and 6.
To allow a PC NIC to communicate with a switch, the UTP cable must also use a straight-
through cable pinout. The term pinout refers to the wiring of which color wire is placed in each of the eight numbered pin positions in the RJ-45 connector. An Ethernet straightthrough cable connects the wire at pin 1 on one end of the cable to pin 1 at the other end of the cable; the wire at pin 2 needs to connect to pin 2 on the other end of the cable; pin 3 on one end connects to pin 3 on the other, and so on. Also, it uses the wires in one wire pair at pins 1 and 2, and another pair at pins 3 and 6.
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
Chapter 2: Fundamentals of Ethernet LANs 55
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
10BASE-T and 100BASE-T Straight-Through Cable Pinout
Figure 2-11 shows one final perspective on the straight-through cable pinout. In this case,
PC Larry connects to a LAN switch. Note that the figure again does not show the UTP cable, but instead shows the wires that sit inside the cable, to emphasize the idea of wire pairs and pins.
Ethernet Straight-Through Cable Concept
Crossover Cable Pinout
A straight-through cable works correctly when the nodes use opposite pairs for transmitting data. However, when two like devices connect to an Ethernet link, they both transmit on over the same pins. In that case, you then need another type of cabling pinout called a
crossover cable. The crossover cable pinout crosses the pair at the transmit pins on each device to the receive pins on the opposite device.
While that previous sentence is true, this concept is much clearer with a figure such as
Figure 2-12. The figure shows what happens on a link between two switches. The two switches both transmit on the pair at pins 3 and 6, and they both receive on the pair at pins
1 and 2. So, the cable must connect a pair at pins 3 and 6 on each side to pins 1 and 2 on the other side, connecting to the other node’s receiver logic. The top of the figure shows the literal pinouts, and the bottom half shows a conceptual diagram.
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Crossover Ethernet Cable
Choosing the Right Cable Pinouts
For the exam, you should be well prepared to choose which type of cable (straight-through or crossover) is needed in each part of the network. The key is to know whether a device acts like a PC NIC, transmitting at pins 1 and 2, or like a switch, transmitting at pins 3 and 6.
Then, just apply the following logic:
Crossover cable: If the endpoints transmit on the same pin pair
Straight-through cable: If the endpoints transmit on different pin pairs
Table 2-3 lists the devices mentioned in this book and the pin pairs they use, assuming that they use 10BASE-T and 100BASE-TX.
Table 2-3 10BASE-T and 100BASE-TX Pin Pairs Used
Transmits on Pins 1,2
Wireless access point (Ethernet interface)
Transmits on Pins 3,6
For example, Figure 2-13 shows a campus LAN in a single building. In this case, several straight-through cables are used to connect PCs to switches. Additionally, the cables connecting the switches require crossover cables.
Chapter 2: Fundamentals of Ethernet LANs 57
Switch 12 Switch 22
Typical Uses for Straight-Through and Crossover Ethernet Cables
If you have some experience with installing LANs, you might be thinking that you have used the wrong cable before (straight-through or crossover), but the cable worked.
Cisco switches have a feature called auto-mdix that notices when the wrong cable is used and automatically changes its logic to make the link work. However, for the exams, be ready to identify whether the correct cable is shown in the figures.
UTP Cabling Pinouts for 1000BASE-T
1000BASE-T (Gigabit Ethernet) differs from 10BASE-T and 100BASE-T as far as the cabling and pinouts. First, 1000BASE-T requires four wire pairs. Second, it uses more advanced electronics that allow both ends to transmit and receive simultaneously on each wire pair.
However, the wiring pinouts for 1000BASE-T work almost identically to the earlier standards, adding details for the additional two pairs.
The straight-through cable connects each pin with the same numbered pin on the other side, but it does so for all eight pins—pin 1 to pin 1, pin 2 to pin 2, up through pin 8. It keeps one pair at pins 1 and 2 and another at pins 3 and 6, just like in the earlier wiring. It adds a pair at pins 4 and 5 and the final pair at pins 7 and 8 (refer to Figure 2-12).
The Gigabit Ethernet crossover cable crosses the same two-wire pairs as the crossover cable for the other types of Ethernet (the pairs at pins 1,2 and 3,6). It also crosses the two new pairs as well (the pair at pins 4,5 with the pair at pins 7,8).
Sending Data in Ethernet Networks
Although physical layer standards vary quite a bit, other parts of the Ethernet standards work the same way, regardless of the type of physical Ethernet link. Next, this final major section of this chapter looks at several protocols and rules that Ethernet uses regardless of the type of link. In particular, this section examines the details of the Ethernet data link layer protocol, plus how Ethernet nodes, switches, and hubs forward Ethernet frames through an Ethernet LAN.
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Ethernet Data Link Protocols
One of the most significant strengths of the Ethernet family of protocols is that these protocols use the same data link standard. In fact, the core parts of the data link standard date back to the original Ethernet standards.
The Ethernet data link protocol defines the Ethernet frame: an Ethernet header at the front, the encapsulated data in the middle, and an Ethernet trailer at the end. Ethernet actually defines a few alternate formats for the header, with the frame format shown in Figure 2-14 being commonly used today.
Data and Pad
46 – 1500
Commonly Used Ethernet Frame Format
While all the fields in the frame matter, some matter more to the topics discussed in this book. Table 2-4 lists the fields in the header and trailer, and a brief description for reference, with the upcoming pages including more detail about a few of these fields.
Table 2-4 IEEE 802.3 Ethernet Header and Trailer Fields
Field Length in Bytes
Signifies that the next byte begins the Destination MAC
Identifies the intended recipient of this frame
Identifies the sender of this frame
Data and Pad*
Defines the type of protocol listed inside the frame; today, most likely identifies IP version 4 (IPv4) or IP version 6 (IPv6)
Holds data from a higher layer, typically an L3PDU
(usually an IPv4 or IPv6 packet). The sender adds padding to meet the minimum length requirement for this field (46 bytes).
Provides a method for the receiving NIC to determine whether the frame experienced transmission errors
* The IEEE 802.3 specification limits the data portion of the 802.3 frame to a minimum of 46 and a maximum of
1500 bytes. The term maximum transmission unit (MTU) defines the maximum Layer 3 packet that can be sent over a medium. Because the Layer 3 packet rests inside the data portion of an Ethernet frame, 1500 bytes is the largest IP MTU allowed over an Ethernet.
Chapter 2: Fundamentals of Ethernet LANs 59
The source and destination Ethernet address fields play a big role in much of the Ethernet logic included in the CCENT and CCNA certifications. The general idea for each is relatively simple:
The sending node puts its own address in the source address field and the intended Ethernet destination device’s address in the destination address field. The sender transmits the frame, expecting that the Ethernet LAN, as a whole, will deliver the frame to that correct destination.
Ethernet addresses, also called Media Access Control (MAC) addresses, are 6-byte-long
(48-bit-long) binary numbers. For convenience, most computers list MAC addresses as 12-digit hexadecimal numbers. Cisco devices typically add some periods to the number for easier readability as well; for example, a Cisco switch might list a MAC address as 0000.0C12.3456.
Most MAC addresses represent a single NIC or other Ethernet port, so these addresses are often called a unicast Ethernet address. The term unicast is simply a formal way to refer to the fact that the address represents one interface to the Ethernet LAN. (This term also contrasts with two other types of Ethernet addresses, broadcast and multicast, which will be defined later in this section.)
The entire idea of sending data to a destination unicast MAC address works well, but it only works if all the unicast MAC addresses are unique. If two NICs tried to use the same MAC address, there could be confusion. (The problem would be like the confusion caused to the postal service, if you and I both tried to use the same mailing address—would the postal service deliver mail to your house or mine?). If two PCs on the same Ethernet tried to use the same MAC address, to which PC should frames sent to that MAC address be delivered?
Ethernet solves this problem using an administrative process so that, at the time of manufacture, all Ethernet devices are assigned a universally unique MAC address. Before a manufacturer can build Ethernet products, it must ask the IEEE to assign the manufacturer a universally unique 3-byte code, called the organizationally unique identifier (OUI). The manufacturer agrees to give all NICs (and other Ethernet products) a MAC address that begins with its assigned 3-byte OUI. The manufacturer also assigns a unique value for the last 3 bytes, a number that manufacturer has never used with that OUI. As a result, the MAC address of every device in the universe is unique.
The IEEE also calls these universal MAC addresses global MAC addresses.
Figure 2-15 shows the structure of the unicast MAC address, with the OUI.
(NIC Cards, Interfaces)
Size, in bits
Size, in hex digits
6 Hex Digits
6 Hex Digits
00 60 2F 3A 07 BC
Structure of Unicast Ethernet Addresses
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Ethernet addresses go by many names: LAN address, Ethernet address, hardware address, burned-in address, physical address, universal address, or MAC address. For example, the term burned-in address (BIA) refers to the idea that a permanent MAC address has been encoded (burned into) the ROM chip on the NIC. As another example, the IEEE uses the term universal address to emphasize the fact that the address assigned to a NIC by a manufacturer should be unique among all MAC addresses in the universe.
In addition to unicast addresses, Ethernet also uses group addresses. Group addresses identify more than one LAN interface card. A frame sent to a group address might be delivered to a small set of devices on the LAN, or even to all devices on the LAN. In fact, the IEEE defines two general categories of group addresses for Ethernet:
Broadcast address: Frames sent to this address should be delivered to all devices on the
Ethernet LAN. It has a value of FFFF.FFFF.FFFF.
Multicast addresses: Frames sent to a multicast Ethernet address will be copied and forwarded to a subset of the devices on the LAN that volunteers to receive frames sent to a specific multicast address.
Table 2-5 summarizes most of the details about MAC addresses.
Table 2-5 LAN MAC Address Terminology and Features
LAN Addressing Term or Feature
Ethernet address, NIC address, LAN address
Media Access Control. 802.3 (Ethernet) defines the MAC sublayer of IEEE Ethernet.
Other names often used instead of MAC address. These terms describe the 6-byte address of the LAN interface card.
The 6-byte address assigned by the vendor making the card.
A term for a MAC address that represents a single LAN interface.
An address that means “all devices that reside on this LAN right now.”
On Ethernet, a multicast address implies some subset of all devices currently on the Ethernet LAN.
Identifying Network Layer Protocols with the Ethernet Type Field
While the Ethernet header’s address fields play an important and more obvious role in
Ethernet LANs, the Ethernet Type field plays a much less obvious role. The Ethernet Type field, or EtherType, sits in the Ethernet data link layer header, but its purpose is to directly help the network processing on routers and hosts. Basically, the Type field identifies the type of network layer (Layer 3) packet that sits inside the Ethernet frame.
First, think about what sits inside the data part of the Ethernet frame shown earlier in Figure
2-14. Typically, it holds the network layer packet created by the network layer protocol on
Chapter 2: Fundamentals of Ethernet LANs 61 some device in the network. Over the years, those protocols have included IBM Systems
Network Architecture (SNA), Novell NetWare, Digital Equipment Corporation’s DECnet, and Apple Computer’s AppleTalk. Today, the most common network layer protocols are both from TCP/IP: IP version 4 (IPv4) and IP version 6 (IPv6).
The original host has a place to insert a value (a hexadecimal number) to identify the type of packet encapsulated inside the Ethernet frame. However, what number should the sender put in the header to identify an IPv4 packet as the type? Or an IPv6 packet? As it turns out, the IEEE manages a list of EtherType values, so that every network layer protocol that needs a unique EtherType value can have a number. The sender just has to know the list.
(Anyone can view the list; just go to www.ieee.org and search for EtherType.)
For example, a host can send one Ethernet frame with an IPv4 packet and the next Ethernet frame with an IPv6 packet. Each frame would have a different Ethernet Type field value, using the values reserved by the IEEE, as shown in Figure 2-16.
Eth Header IPv4 Eth Trailer
Type = 0800
Eth Header IPv6 Eth Trailer
Type = 86DD
Use of Ethernet Type Field
Error Detection with FCS
Ethernet also defines a way for nodes to find out whether a frame’s bits changed while crossing over an Ethernet link. (Usually, the bits could change because of some kind of electrical interference, or a bad NIC.) Ethernet, like most every other data link protocol covered on the CCNA exams, uses a field in the data link trailer for the purpose of error detection.
The Ethernet Frame Check Sequence (FCS) field in the Ethernet trailer—the only field in the Ethernet trailer—gives the receiving node a way to compare results with the sender, to discover whether errors occurred in the frame. The sender applies a complex math formula to the frame before sending it, storing the result of the formula in the FCS field. The receiver applies the same math formula to the received frame. The receiver then compares its own results with the sender’s results. If the results are the same, the frame did not change; otherwise, an error occurred, and the receiver discards the frame.
Note that error detection does not also mean error recovery. Ethernet defines that the errored frame should be discarded, but Ethernet does not attempt to recover the lost frame.
Other protocols, notably TCP, recover the lost data by noticing that it is lost and sending the data again.
62 Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide
Sending Ethernet Frames with Switches and Hubs
Ethernet LANs behave slightly differently depending on whether the LAN has mostly modern devices, in particular, LAN switches instead of some older LAN devices called LAN hubs. Basically, the use of more modern switches allows the use of full-duplex logic, which is much faster and simpler than half-duplex logic, which is required when using hubs. The final topic in this chapter looks at these basic differences.
Sending in Modern Ethernet LANs Using Full-Duplex
Modern Ethernet LANs use a variety of Ethernet physical standards, but with standard
Ethernet frames that can flow over any of these types of physical links. Each individual link can run at a different speed, but each link allows the attached nodes to send the bits in the frame to the next node. They must work together to deliver the data from the sending
Ethernet node to the destination node.
The process is relatively simple, on purpose; the simplicity lets each device send a large number of frames per second. Figure 2-17 shows an example, in which PC1 sends an
Ethernet frame to PC2.
Eth Data Eth
Source = PC1
Dest = PC2
Example of Sending Data in a Modern Ethernet LAN
Following the steps in the figure:
PC1 builds and sends the original Ethernet frame, using its own MAC address as the source address and PC2’s MAC address as the destination address.
Switch SW1 receives and forwards the Ethernet frame out its G0/1 interface (short for Gigabit interface 0/1) to SW2.
Switch SW2 receives and forwards the Ethernet frame out its F0/2 interface (short for
Fast Ethernet interface 0/2) to PC2.
PC2 receives the frame, recognizes the destination MAC address as its own, and processes the frame.
The Ethernet network in Figure 2-17 uses full-duplex on each link, but the concept might be difficult to see. Full-duplex means that that the NIC or switch port has no half-duplex restrictions. So, to understand full-duplex, you need to understand half-duplex, as follows:
Chapter 2: Fundamentals of Ethernet LANs 63
Half-duplex: Logic in which a port sends data only when it is not also receiving data; in other words, it cannot send and receive at the same time.
Full-duplex: The absence of the half-duplex restriction.
So, with all PCs and LAN switches, and no LAN hubs, all the nodes can use full-duplex. All nodes can send and receive on their port at the same instant in time. For example, in Figure
2-17, PC1 and PC2 could send frames to each other simultaneously, in both directions, without any half-duplex restrictions.
Using Half-Duplex with LAN Hubs
To understand the need for half-duplex logic in some cases, you have to understand a little about an older type of networking device called a LAN hub. When the IEEE first introduced 10BASE-T in 1990, the Ethernet did not yet include LAN switches. Instead of switches, vendors created LAN hubs. The LAN hub provided a number of RJ-45 ports as a place to connect links to PCs, just like a LAN switch, but it used different rules for forwarding data.
LAN hubs forward data using physical layer standards, and are therefore considered to be
Layer 1 devices. When an electrical signal comes in one hub port, the hub repeats that electrical signal out all other ports (except the incoming port). By doing so, the data reaches all the rest of the nodes connected to the hub, so the data hopefully reaches the correct destination. The hub has no concept of Ethernet frames, of addresses, and so on.
The downside of using LAN hubs is that if two or more devices transmitted a signal at the same instant, the electrical signal collides and becomes garbled. The hub repeats all received electrical signals, even if it receives multiple signals at the same time. For example, Figure
2-18 shows the idea, with PCs Archie and Bob sending an electrical signal at the same instant of time (at Steps 1A and 1B) and the hub repeating both electrical signals out toward
Larry on the left (Step 2).
Collision Occurring Because of LAN Hub Behavior
For completeness, note that the hub floods each frame out all other ports (except the incoming port). So, Archie’s frame goes to both Larry and Bob; Bob’s frame goes to
Larry and Archie.
If you replace the hub in Figure 2-18 with a LAN switch, the switch prevents the collision on the left. The switch operates as a Layer 2 device, meaning that it looks at the data link header and trailer. A switch would look at the MAC addresses, and even if the switch needed to forward both frames to Larry on the left, the switch would send one frame and queue the other frame until the first frame was finished.
64 Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide
Now back to the issue created by the hub’s logic: collisions. To prevent these collisions, the Ethernet nodes must use half-duplex logic instead of full-duplex logic. A problem only occurs when two or more devices send at the same time; half-duplex logic tells the nodes that if someone else is sending, wait before sending.
For example, back in Figure 2-18, imagine that Archie began sending his frame early enough so that Bob received the first bits of that frame before Bob tried to send his own frame.
Bob, at Step 1B, would notice that he was receiving a frame from someone else, and using half-duplex logic, would simply wait to send the frame listed at Step 1B.
Nodes that use half-duplex logic actually use a relatively well-known algorithm called
CSMA/CD (carrier sense multiple access with collision detection). The algorithm takes care of the obvious cases but also the cases caused by unfortunate timing. For example, two nodes could check for an incoming frame at the exact same instant, both realize that no other node is sending, and both send their frames at the exact same instant, causing a collision. CSMA/CD covers these cases as well, as follows:
A device with a frame to send listens until the Ethernet is not busy.
When the Ethernet is not busy, the sender begins sending the frame.
The sender listens while sending to discover whether a collision occurs; collisions might be caused by many reasons, including unfortunate timing. If a collision occurs, all currently sending nodes do the following:
They send a jamming signal that tells all nodes that a collision happened.
They independently choose a random time to wait before trying again, to avoid unfortunate timing
The next attempt starts again at Step 1.
Although most modern LANs do not often use hubs, and therefore do not need to use half-duplex, enough old hubs still exist in enterprise networks so that you need to be ready to understand duplex issues. Each NIC and switch port has a duplex setting. For all links between PCs and switches, or between switches, use full-duplex. However, for any link connected to a LAN hub, the connected LAN switch and NIC port should use half-duplex.
Note that the hub itself does not use half-duplex logic, instead just repeating incoming signals out every other port.
Figure 2-19 shows an example, with full-duplex links on the left and a single LAN hub on the right. The hub then requires SW2’s F0/2 interface to use half-duplex logic, along with the PCs connected to the hub.
Full- and Half-Duplex in an Ethernet LAN
Chapter 2: Fundamentals of Ethernet LANs 65
Exam Preparation Tasks
Review All the Key Topics
Review the most important topics from this chapter, noted with the Key Topic icon. Table
2-6 lists these key topics and where each is discussed.
Table 2-6 Key Topics for Chapter 2
Drawing of a typical wired and wireless enterprise LAN
Several types of Ethernet LANs and some details about each
Conceptual drawing of transmitting in one direction each over two different electrical circuits between two Ethernet nodes
10- and 100-Mbps Ethernet straight-through cable pinouts
10- and 100-Mbps Ethernet crossover cable pinouts
List of devices that transmit on wire pair 1,2 and pair 3,6
Example of interfaces using full and half duplex
Format of Ethernet MAC addresses
Definitions of half-duplex and full-duplex
Examples of which interfaces use full-duplex and which interfaces use half-duplex
Complete the Tables and Lists from Memory
Print a copy of DVD Appendix M, “Memory Tables,” or at least the section for this chapter, and complete the tables and lists from memory. DVD Appendix N, “Memory Tables
Answer Key,” includes completed tables and lists for you to check your work.
Definitions of Key Terms
After your first reading of the chapter, try to define these key terms, but do not be concerned about getting them all correct at that time. Chapter 30 directs you in how to use these terms for late-stage preparation for the exam.
Ethernet, IEEE, wired LAN, wireless LAN, Ethernet frame, 10BASE-T, 100BASE-T,
1000BASE-T, Fast Ethernet, Gigabit Ethernet, Ethernet link, RJ-45, Ethernet port, network interface card (NIC), straight-through cable, crossover cable, Ethernet address,
MAC address, unicast address, broadcast address, Frame Check Sequence
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