Chapter 1: Ethernet Basics

Cisco Press

Blueprint topics covered in this chapter:

This chapter covers the following subtopics from the Cisco CCIE Routing and Switching written exam blueprint. Refer to the full blueprint in Table I-1 in the Introduction for more details on the topics covered in each chapter and their context within the blueprint.

  • LAN Switching

  • Ethernet

  • Speed

  • Duplex

  • Unicast Flooding

  • Fast Ethernet

  • Gigabit Ethernet

It's no surprise that the concepts, protocols, and commands related to Ethernet are a key part of the CCIE Routing and Switching written exam. Almost all campus networks today are built using Ethernet technology. Also, Ethernet technology is moving into the WAN with the emergence of metro Ethernet. Even in an IT world, where technology changes rapidly, you can expect that ten years from now, Ethernet will still be an important part of the CCIE Routing and Switching written and lab exams.

For this chapter, if I had to venture a guess, probably 100 percent of you reading this book know a fair amount about Ethernet basics already. I must admit, I was tempted to leave it out. However, I would also venture a guess that at least some of you have forgotten a few facts about Ethernet. So you can read the whole chapter if your Ethernet recollections are a bit fuzzy—or you could just hit the highlights. For exam preparation, it is typically useful to use all the refresher tools: take the "Do I Know This Already?" quiz, complete the definitions of the terms listed at the end of the chapter, print and complete the tables in Appendix E, "Key Tables for CCIE Study," and certainly answer all the CD-ROM questions concerning Ethernet.

"Do I Know This Already?" Quiz

Table 1-1 outlines the major headings in this chapter and the corresponding "Do I Know This Already?" quiz questions.

Table 1-1 "Do I Know This Already?" Foundation Topics Section-to-Question Mapping

Foundation Topics Section

Questions Covered in This Section


Ethernet Layer 1: Wiring, Speed, and Duplex



Ethernet Layer 2: Framing and Addressing



Switching and Bridging Logic



Total Score


In order to best use this pre-chapter assessment, remember to score yourself strictly. You can find the answers in Appendix A, "Answers to the 'Do I Know This Already?' Quizzes."

  1. Which of the following denotes the correct usage of pins on the RJ-45 connectors at the opposite ends of an Ethernet cross-over cable?

    1. 1 to 1

    2. 1 to 2

    3. 1 to 3

    4. 6 to 1

    5. 6 to 2

    6. 6 to 3

  2. Which of the following denotes the correct usage of pins on the RJ-45 connectors at the opposite ends of an Ethernet straight-through cable?

    1. 1 to 1

    2. 1 to 2

    3. 1 to 3

    4. 6 to 1

    5. 6 to 2

    6. 6 to 3

  3. Which of the following commands must be configured on a Cisco IOS switch interface to disable Ethernet auto-negotiation?

    1. no auto-negotiate

    2. no auto

    3. Both speed and duplex

    4. duplex

    5. speed

  4. Consider an Ethernet cross-over cable between two 10/100 ports on Cisco switches. One switch has been configured for 100-Mbps full duplex. Which of the following is true about the other switch?

    1. It will use a speed of 10 Mbps.

    2. It will use a speed of 100 Mbps.

    3. It will use a duplex setting of half duplex.

    4. It will use a duplex setting of full duplex.

  5. Consider an Ethernet cross-over cable between two 10/100/1000 ports on Cisco switches. One switch has been configured for half duplex, and the other for full duplex. The ports successfully negotiate a speed of 1 Gbps. Which of the following could occur as a result of the duplex mismatch?

    1. No frames can be received by the half-duplex switch without it believing an FCS error has occurred.

    2. CDP would detect the mismatch and change the full-duplex switch to half duplex.

    3. CDP would detect the mismatch and issue a log message to that effect.

    4. The half-duplex switch will erroneously believe collisions have occurred.

  6. Which of the following Ethernet header type fields is a 2-byte field?

    1. DSAP

    2. Type (in SNAP header)

    3. Type (in Ethernet V2 header)

    4. LLC Control

  7. Which of the following standards defines a Fast Ethernet standard?

    1. IEEE 802.1Q

    2. IEEE 802.3U

    3. IEEE 802.1X

    4. IEEE 802.3Z

    5. IEEE 802.3AB

    6. IEEE 802.1AD

  8. Suppose a brand-new Cisco IOS–based switch has just been taken out of the box and cabled to several devices. One of the devices sends a frame. For which of the following destinations would a switch flood the frames out all ports (except the port upon which the frame was received)?

    1. Broadcasts

    2. Unknown unicasts

    3. Known unicasts

    4. Multicasts

Foundation Topics

Ethernet Layer 1: Wiring, Speed, and Duplex

Before making an Ethernet LAN functional, end-user devices, routers, and switches must be cabled correctly. To run with fewer transmission errors at higher speeds, and to support longer cable distances, variations of copper and optical cabling can be used. The different Ethernet specifications, cable types, and cable lengths per the various specifications are important for the exam, and are listed in the "Foundation Summary" section.

RJ-45 Pinouts and Category 5 Wiring

You should know the details of cross-over and straight-through Category 5 (Cat 5) or Cat 5e cabling for most any networking job. The EIA/TIA defines the cabling specifications for Ethernet LANs ( and, including the pinouts for the RJ-45 connects, as shown in Figure 1-1.

Figure 1-1

RJ-45 Pinouts with Four-Pair UTP Cabling

The most popular Ethernet standards (10BASE-T and 100BASE-TX) each use two twisted pairs (specifically pairs 2 and 3 shown in Figure 1-1), with one pair used for transmission in each direction. Depending on which pair a device uses to transmit and receive, either a straight-through or cross-over cable is required. Table 1-2 summarizes how the cabling and pinouts work.

Table 1-2 Ethernet Cabling Types

Type of Cable


Key Pins Connected


T568A (both ends) or T568B (both ends)

1 – 1; 2 – 2; 3 – 3; 6 – 6


T568A on one end, T568B on the other

1 – 3; 2 – 6; 3 – 1; 6 – 2

Many Ethernet standards use two twisted pairs, with one pair being used for transmission in each direction. For instance, a PC network interface card (NIC) transmits on pair 1,2 and receives on pair 3,6; switch ports do the opposite. So, a straight-through cable works well, connecting pair 1,2 on the PC (PC transmit pair) to the switch port's pair 1,2, on which the switch receives. When the two devices on the ends of the cable both transmit using the same pins, a cross-over cable is required. For instance, if two connected switches send using the pair at pins 3,6 and receive on pins 1,2, then the cable needs to connect the pair at 3,6 on one end to pins 1,2 at the other end, and vice versa.

Note - Cross-over cables can also be used between a pair of PCs, swapping the transmit pair on one end (1,2) with the receive pins at the other end (3,6).

Cisco also supports a switch feature that lets the switch figure out if the wrong cable is installed: Auto-MDIX (automatic medium-dependent interface crossover) detects the wrong cable and causes the switch to swap the pair it uses for transmitting and receiving, which solves the cabling problem. (As of publication, this feature is not supported on all Cisco switch models.)

Auto-negotiation, Speed, and Duplex

By default, each Cisco switch port uses Ethernet auto-negotiation to determine the speed and duplex setting (half or full). The switches can also set their duplex setting with the duplex interface subcommand, and their speed with—you guessed it—the speed interface subcommand.

Switches can dynamically detect the speed setting on a particular Ethernet segment by using a few different methods. Cisco switches (and many other devices) can sense the speed using theFast Link Pulses (FLP) of the auto-negotiation process. However, if auto-negotiation is disabled on either end of the cable, the switch detects the speed anyway based on the incoming electrical signal. You can force a speed mismatch by statically configuring different speeds on either end of the cable, causing the link to no longer function.

Switches detect duplex settings through auto-negotiation only. If both ends have auto-negotiation enabled, the duplex is negotiated. However, if either device on the cable disables auto-negotiation, the devices without a configured duplex setting must assume a default. Cisco switches use a default duplex setting of half duplex (HDX) (for 10-Mbps and 100-Mbps interfaces) or full duplex (FDX) (for 1000-Mbps interfaces). To disable auto-negotiation on a Cisco switch port, you simply need to statically configure the speed and the duplex settings.

Ethernet devices can use FDX only when collisions cannot occur on the attached cable; a collision-free link can be guaranteed only when a shared hub is not in use. The next few topics review how Ethernet deals with collisions when they do occur, as well as what is different with Ethernet logic in cases where collisions cannot occur and FDX is allowed.


The original Ethernet specifications expected collisions to occur on the LAN. The media was shared, creating a literal electrical bus. Any electrical signal induced onto the wire could collide with a signal induced by another device. When two or more Ethernet frames overlap on the transmission medium at the same instant in time, a collision occurs; the collision results in bit errors and lost frames.

The original Ethernet specifications defined the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) algorithm to deal with the inevitable collisions. CSMA/CD minimizes the number of collisions, but when they occur, CSMA/CD defines how the sending stations can recognize the collisions and retransmit the frame. The following list outlines the steps in the CSMA/CD process:

  1. A device with a frame to send listens until the Ethernet is not busy (in other words, the device cannot sense a carrier signal on the Ethernet segment).

  2. When the Ethernet is not busy, the sender begins sending the frame.

  3. The sender listens to make sure that no collision occurred.

  4. If there was a collision, all stations that sent a frame send a jamming signal to ensure that all stations recognize the collision.

  5. After the jamming is complete, each sender of one of the original collided frames randomizes a timer and waits that long before resending. (Other stations that did not create the collision do not have to wait to send.)

  6. After all timers expire, the original senders can begin again with Step 1.

Collision Domains and Switch Buffering

A collision domain is a set of devices that can send frames that collide with frames sent by another device in that same set of devices. Before the advent of LAN switches, Ethernets were either physically shared (10BASE2 and 10BASE5) or shared by virtue of shared hubs and their Layer 1 "repeat out all other ports" logic. Ethernet switches greatly reduce the number of possible collisions, both through frame buffering and through their more complete Layer 2 logic.

By definition of the term, Ethernet hubs:

  • Operate solely at Ethernet Layer 1

  • Repeat (regenerate) electrical signals to improve cabling distances

  • Forward signals received on a port out all other ports (no buffering)

As a result of a hub's logic, a hub creates a single collision domain. Switches, however, create a different collision domain per switch port, as shown in Figure 1-2.

Figure 1-2

Collision Domains with Hubs and Switches

Switches have the same cabling and signal regeneration benefits as hubs, but switches do a lot more—including sometimes reducing or even eliminating collisions by buffering frames. When switches receive multiple frames on different switch ports, they store the frames in memory buffers to prevent collisions.

For instance, imagine that a switch receives three frames at the same time, entering three different ports, and they all must exit the same switch port. The switch simply stores two of the frames in memory, forwarding the frames sequentially. As a result, in Figure 1-2, the switch prevents any frame sent by Larry from colliding with a frame sent by Archie or Bob—which by definition puts each of the PCs attached to the switch in Figure 1-2 in different collision domains.

When a switch port connects via cable to a single other non-hub device—for instance, like the three PCs in Figure 1-2—no collisions can possibly occur. The only devices that could create a collision are the switch port and the one connected device—and they each have a separate twisted pair on which to transmit. Because collisions cannot occur, such segments can use full-duplex logic.

When a switch port connects to a hub, it needs to operate in HDX mode, because collisions might occur due to the logic used by the hub.

Note - NICs operating in HDX mode use loopback circuitry when transmitting a frame. This circuitry loops the transmitted frame back to the receive side of the NIC, so that when the NIC receives a frame over the cable, the combined looped-back signal and received signal allows the NIC to notice that a collision has occurred.

Basic Switch Port Configuration

The three key configuration elements on a Cisco switch port are auto-negotiation, speed, and duplex. Cisco switches use auto-negotiation by default; it is then disabled if both the speed and duplex are manually configured. You can set the speed using the speed {auto | 10 | 100 | 1000} interface subcommand, assuming the interface supports multiple speeds. You configure the duplex setting using the duplex {auto | half | full} interface subcommand.

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