1 © 2003, Cisco Systems, Inc. All rights reserved. CCNA 1 v3.0 Module 6 Ethernet Fundamentals

222 © 2003, Cisco Systems, Inc. All rights reserved. Objectives

333 © 2003, Cisco Systems, Inc. All rights reserved. Introduction to Ethernet The original idea for Ethernet grew out of the problem of allowing two or more hosts to use the same medium and prevent the signals from interfering with each other. The first Ethernet standard was published in 1980 by a consortium of Digital Equipment Company, Intel, and Xerox (DIX). Ethernet transmitted at up to 10 Mbps over thick coaxial cable up to a distance of two kilometers. This type of coaxial cable was referred to as thicknet and was about the width of a small finger.

444 © 2003, Cisco Systems, Inc. All rights reserved. Introduction to Ethernet In 1985, the Institute of Electrical and Electronics Engineers (IEEE) published standards for LANs. These standards start with the number 802. The standard for Ethernet is Essentially, Ethernet and IEEE are the same standards. In 1995, IEEE announced a standard for a 100-Mbps Ethernet. This was followed by standards for gigabit per second (Gbps, 1 billion bits per second) Ethernet in 1998 and In 2002 IEEE announced the standards for 10 Gigabit Ethernet All the standards are essentially compatible with the original Ethernet standard. An Ethernet frame could leave an older coax 10-Mbps NIC in a PC, be placed onto a 10-Gbps Ethernet fiber link, and end up at a 100-Mbps NIC.

555 © 2003, Cisco Systems, Inc. All rights reserved. IEEE Ethernet Naming Rules 3-Part Names

666 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet and the OSI Model Not Included

777 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet and the OSI Model

888 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet and the OSI Model

999 © 2003, Cisco Systems, Inc. All rights reserved. 10BaseT Half-Duplex Operation 1.What if only PC2 sends a frame 2.What would happen if pc1 and pc2 each send a frame at the same time?

10 © 2003, Cisco Systems, Inc. All rights reserved. 10BaseT Half-Duplex Operation If PC1 and PC2 sent a frame at the same time, a collision would occur. At step 4, the hub would forward both electrical signals, which would cause the overlapping signals to be sent to all the NICs (except for the ones that originated the frames). The collision domain defines the set of devices for which their frames could collide

11 © 2003, Cisco Systems, Inc. All rights reserved. 10BaseT Half-Duplex Operation CSMA/CD CS: Sense if there is a signal coming on the receive pair MA: all stations have access to a shared resource: one bus CD: When a collision happens, detect it How would station 1, 2, 3, and 4 detect a collision???

12 © 2003, Cisco Systems, Inc. All rights reserved. 10BaseT Half-Duplex Operation Stations separated by hubs are within the same collision domain Let’s look at packet tracer to see how hubs work

13 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet and the OSI Model How does Layer 2 addresses the limitations of layer 1

14 © 2003, Cisco Systems, Inc. All rights reserved. Naming MAC addresses are sometimes referred to as burned-in addresses (BIA) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes.

15 © 2003, Cisco Systems, Inc. All rights reserved. Naming

16 © 2003, Cisco Systems, Inc. All rights reserved. Layer 2 Framing The reason for sending frames is to get upper layer data, ultimately the user application data, from the source to the destination. The data package has two parts, the user application data and the encapsulated bytes to be sent to the destination computer. Padding bytes may be added so frames have a minimum length for timing purposes. Generic Frame Format

17 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Frame Structures At the data link layer the frame structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps. However, at the physical layer almost all versions of Ethernet are substantially different from one another with each speed having a distinct set of architecture design rules.

18 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Frame Structures—IEEE Ethernet

19 © 2003, Cisco Systems, Inc. All rights reserved. IEEE Frame

20 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Frame Structures—Ethernet II Use Ethereal to capture Ethernet II frames and analyze them. See if you can find any IEEE802.3 Frames.

21 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Frame Structures

22 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Frame Fields The Ethernet II Type field is incorporated into the current frame definition. The receiving node must determine which higher-layer protocol is present in an incoming frame by examining the Length/Type field. If the two-octet value is equal to or greater than 0x600 (hexadecimal), then the frame is interpreted according to the Ethernet II type code indicated. Else, the value indicates the length (starting from LLC field) and LLC with be used to identify the layer 3 protocol.

23 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Frame Fields The Preamble is an alternating pattern of ones and zeroes used for timing synchronization in the asynchronous 10 Mbps and slower implementations of Ethernet. Faster versions of Ethernet are synchronous, and this timing information is redundant but retained for compatibility. A Start Frame Delimiter consists of a one-octet field that marks the end of the timing information, and contains the bit sequence

24 © 2003, Cisco Systems, Inc. All rights reserved. Media Access Control (MAC) MAC refers to protocols that determine which computer on a shared- medium environment, or collision domain, is allowed to transmit the data. MAC, with LLC, comprises the IEEE version of the OSI Layer 2. MAC and LLC are sublayers of Layer 2. There are two broad categories of Media Access Control, deterministic (taking turns) and non- deterministic (first come, first served).

25 © 2003, Cisco Systems, Inc. All rights reserved. MAC Rules and Collision Detection/Backoff

26 © 2003, Cisco Systems, Inc. All rights reserved. MAC Rules and Collision Detection/Backoff

27 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Timing

28 © 2003, Cisco Systems, Inc. All rights reserved. Interframe Spacing and Backoff

29 © 2003, Cisco Systems, Inc. All rights reserved. Interframe Spacing and Backoff

30 © 2003, Cisco Systems, Inc. All rights reserved. Error Handling

31 © 2003, Cisco Systems, Inc. All rights reserved. Types of Collisions

32 © 2003, Cisco Systems, Inc. All rights reserved. Types of Collisions

33 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Errors

34 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Errors

35 © 2003, Cisco Systems, Inc. All rights reserved. FCS Errors

36 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Auto-Negotiation

37 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Auto Negotiation Auto-negotiation is not 100% reliable, but it does generally work. Links with a duplex mismatch will operate, but will generate large numbers of errors, and can slow down busy networks. For most interfaces, both speed and duplex need to be set to auto for full auto-negotiation to work. Forcing a Catalyst switch port to a specific speed disables auto-negotiation for the duplex setting. Full-duplex mode can be achieved only if both sides of the link are either set to auto- negotiate or manually configured to use full-duplex. Full-duplex will work only if a host is connected directly to a switch or other device, with no repeaters or hubs in-between.

38 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Auto Negotiation Cisco Catalyst Switches Departmental Switches On most multi-module Cisco switches, you can display the current port status with the command: show port mod/port(s) The Duplex and Speed columns of the table will tell you what the current port mode is set to. If the port has been forced to a specific mode, that mode will simply be listed. If the port is set to auto-negotiation, but nothing is plugged into it, the speed and duplex will be reported as auto. If the current mode was the result of a successful auto-negotiation, the labels will be prefixed with "a-" (e.g. a-100 or a-full). To change the mode of a port, you can use the following commands: set port speed mod/port(s) mode, with mode being either 10, 100, or auto. set port duplex mod/port(s) mode, with mode being either full or half.

39 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Auto Negotiation Microsoft Windows 9*/ME/NT/2000 Windows systems can configure their Ethernet controllers from the Network Control Panel. You can find the Network Control Panel in the Control Panel folder under "Settings" in the Start Menu. The "Ethernet Adapter Properties" panel can be found by clicking on the adapter under the "Configuration" tab in Windows 9x, or under the "Adapters" tab in NT 4. Options to set speed and duplex are usually found under an "Advance" tab. You can also get there by right-clicking on the Network Neighborhood or My Network Places and selecting Properties. (In Windows 2000 you would then right-click on Local Area Connection and select Properties again.)

40 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Auto Negotiation Simple Ethernet Config Flash: How an Ethernet switch work Packet Tracer: Collision Domains and Broadcast Domains

41 © 2003, Cisco Systems, Inc. All rights reserved. Ethernet Auto-Negotiation

42 © 2003, Cisco Systems, Inc. All rights reserved. Transmission Priority Rank

43 © 2003, Cisco Systems, Inc. All rights reserved. Summary