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Chapter 5 - Ethernet

5.0 Ethernet

5.0.1 Introduction >5.0.1.1 Introduction

Upon completion of this chapter you will be able to:

  • Describe the operation of the Ethernet sublayer.
  • Identify the major fields of the Ethernet frame.
  • Describe the purpose and characteristics of the Ethernet MAC address.
  • Describe the purpose of ARP.
  • Explain how ARP requests impact network and host performance.
  • Explain basic switching concepts.
  • Compare a fixed configuration and modular switches.
  • Configure a Layer 3 switch.

5.0.1 Introduction >5.0.1.2 Activity – Join My Social Circle!

How are the communications groups identified?

The figure on this page shows a small world map with people from different countries communicating with each other using tablets, video conferencing, cell phones and gaming systems.

5.1 Ethernet Protocol

5.1.1 Ethernet Operation >5.1.1.1 L L C and MAC sublayers

Figure 1 on this page shows the seven layers of the OSI model with the lower two shaded. This figure demonstrates that Ethernet spans layers 1 and 2. 8 0 2.3 covers layer 1, Physical layer and the lower half (MAC) of layer 2, Data Link layer. 8 0 2.2 covers the upper half of layer 2, LLC.

Figure 2 on this page shows a different representation of how the different implementations of Ethernet span the layers. In this figure, we see that Ethernet, Fast Ethernet and Gigabit Ethernet spans layer 1 and the lower half of layer 2. These are the 8 0 2.3 specifications.

5.1.1 Ethernet Operation >5.1.1.2 MAC Sublayer

The Ethernet MAC sublayer has the following two primary responsibilities: Data Encapsulation

  • Frame delimiting
  • Addressing
  • Error Detection

Media Access Control

  • Control of frame placement on and off media
  • Media recovery

The table on this page shows the physical layer and how it connects to the MAC. sublayer of layer 2. There is a highlighted border surrounding the physical layer and he MAC. sublayer. This table also shows how the MAC. sublayer is split into signaling standards as well as physical media standards.

The table is as follows:

DataLink Layer Logical Link Control Sublayer
8 0 2.3 Media Access Control
Physical Layer Physical Signaling Sublayer 10BASE-5(500m) 50 Ohm Coax N-Style 10BASE-2(185m) 50 Ohm Coax BNC 10BASE-T(100m) 100 Ohm UTP RJ-45 100BASE-TX(100m) 100 Ohm UTP RJ-45 1000BASE-CX(25m) 150 Ohm STP mini-DB-9 1000BASE-T(100m) 100 Ohm UTP RJ-45 1000BASE-T(220-550m) 100 Ohm MM Fiber SC 1000BASE-LX (550-500m) MIM or SM Fiber SC
Physical Medium

5.1.1 Ethernet Operation >5.1.1.3 Media Access Control

The image on this page shows three computers in a contention-based access system connected to shared media. Two computers are sending frames. All three computers have a callout that says "I try to send when I am ready."

The table on this page shows that CSMA is usually implemented in conjunction with a method for resolving media contention.

The table is as follows:

Method Characteristics Example
Contention-Based Access
  • Stations can transmit at any time
  • Collisions exist
  • Mechanisms exist to resolve contention problems
    • CSMA/CD for Ethernet networks
    • CSMA/CD 8 0 2.11 wireless networks
  • Ethernet
  • Wireless

5.1.1 Ethernet Operation >5.1.1.4 MAC Address: Ethernet Identity

The table on this page shows how a MAC address is divided into two 24 bit halves. The first 24 bits represent the Organizationally Unique Identifier (OUI) and the second 24 bits represent the unique device (network interface).

The following table shows the Ethernet MAC Address Structure:

Organizationally Unique Identifier (OUI)Vendor Assigned(NIC , Interfaces)
24 Bits24 Bits
6 hex digits6 hex digits
00-60-2F3A-07-BC
Ciscoparticular device

5.1.1 Ethernet Operation >5.1.1.5 Frame Forwarding

The animation on this page shows how a frame is forwarded.

Computer H1, MAC address AA:AA:AA:AA:AA:AA has a callout that says, "I need to send information to computer H3."

There are four computers on shared media. Each has a different MAC address as shown in the following table:

ComputerMAC Address
H1AA:AA:AA:AA:AA:AA
H2BB:BB:BB:BB:BB:BB
H3CC:CC:CC:CC:CC:CC
H4DD:DD:DD:DD:DD:DD

Computer H1 sends a frame to Computer H3 using the correct MAC addresses for source and destination.

The following table shows the frame addressing:

Destination AddressSource AddressData
CC:CC:CC:CC:CC:CCAA:AA:AA:AA:AA:AAEncapsulated data
Frame Addressing 

The frame is forwarded to all of the computers. Computers H2 and H4 ignore the frame since the destination MAC address does not match their own. They have callouts saying, "This is not addressed to me. I shall ignore it." Computer H3 has a callout saying, "This is mine."

5.1.1 Ethernet Operation >5.1.1.6 Activity – MAC and LLC sublayers

Column 1 in the three column table on this page lists the following descriptions of the MAC and LLC sublayers:

  1. Controls the network interface card through software drivers.
  2. Works with the upper layers to add application information for delivery of data to higher level protocols.
  3. Works with hardware to support bandwidth requirements - checks for errors in bits sent and received.
  4. Controls access to the media through signalling and physical media standards requirements.
  5. Supports Ethernet technology by using CSMA/CD or CSMA/CA.
  6. Remains relatively independent of physical equipment.

The other two columns have column headings of:

  • MAC
  • LLC 
 The user is asked to match each description to the correct sublayer.

5.1.2 Ethernet Frame Attributes >5.1.2.1 Ethernet Encapsulation

Figure 1 on this page is a table displaying a timeline with Ethernet standards from 1973 to 2006. As you move the slider closer to 2006, you see the newer standards and their descriptions. At the same time, there is a graphic that represents bandwidth and it grows in size as the slider moves closer to two thousand six.

The following table shows the Ethernet Evolution Timeline:

YearStandardDescription
1973EthernetEthernet invented by Dr Robert Metcalf of Xerox corp.
1980DIX standard
Ethernet II		Digital Equipment Corp, Intel and Xerox (DIX) release a standard for 10 Mb/s Ethernet over coaxial cable
1983IEEE 8 0 2.3
10 BASE-5		10 Mb/s Ethernet over thick coaxial cable
1985IEEE 8 0 2.3a
10BASE-2		10 Mb/s Ethernet over thin coaxial cable
1990IEEE 8 0 2.3i
10 BASE-T		10 Mb/s Ethernet over twisted pair(TP)
1993IEEE 8 0 2.3j
10 BASE-F		10 Mb/s Ethernet over fiber optic
1995IEEE 8 0 2.3u
100BASE-xx		Fast Ethernet: 100 Mb/s Ethernet over twisted pair(TP)and fiber (various standards)
1998IEEE 8 0 2.z
1000 BASE-X		Gigabit Ethernet over fiber optic
1999IEEE 8 0 2.3ab
1000 BASE-T		Gigabit Ethernet over twisted pair
2002IEEE 8 0 2.3ae
10G BASE-xx		10 Gigabit Ethernet over fiber(various standard)
2006IEEE 8 0 2.3an
10G BASE-T		10 Gigabit Ethernet over twisted pair(TP)

Figure 2 on this page shows the structure for the two styles of Ethernet framing, 8 0 2.3 and Ethernet 2.

The following table shows the comparison of 8 0 2.3 and Ethernet 2 frame structure and field size in bytes:

|!IEEE 8 0 2.3|
Field namePreambleStart of frame DelimiterDestination AddressSource AddressLength8 0 2.2 Header and DataFrame Check Sequence
Size7166246 to 15004
|!Ethernet 2
Field namePreambleDestination AddressSource AddressTypeDataFrame Check Sequence
Size866246 to 15004

5.1.2 Ethernet Frame Attributes >5.1.2.2 Ethernet Frame Size

The figure on this page shows how the Ethernet frame was modified to include an extra four bytes for QOS (Quality of Service) and V. LAN ID.

The following table shows the fields in IEEE 8 0 2.3a frame:

Destination AddressSource Address8 0 2.1Q V LAN Tag (4 Bytes)Type/lenDataFrame Check

The following table shows the fields in the 8 0 2.1Q VLAN tag:

Tag Protocol ID
0x8100 (2 Bytes)		User priority (3 Bits)Canonical Format Indicator (1 Bit)

V LAN ID (12 Bits) ||

5.1.2 Ethernet Frame Attributes >5.1.2.3 Introduction to the Ethernet Frame

The figure on this page shows the following descriptions for the field in an 8 0 2.3 Ethernet frame and size:

  • ‘’’Preamble’’’: 7 bytes, Frame Preamble
  • ‘’’Start of Frame Delimiter’’’: 1 byte, Start of Frame Delimiter
  • ‘’’Destination Address’’’: 6 bytes, Destination MAC address
  • ‘’’Source Address’’’: 6 bytes, Source MAC address
  • ‘’’Length’’’: 2 bytes, Frame Length or Encapsulated Protocol Type
  • ‘’’8 0 2.2 Header and Data’’’: 46 to 1500 bytes, data (Encapsulated packet) plus Padding if required
  • ‘’’Frame check sequence’’’: 4 bytes, Frame check Sequence (CRC checksum)

5.1.2 Ethernet Frame Attributes >5.1.2.4 Activity – Ethernet Frame Fields

Figure 1 on this page lists the following Ethernet frame field names:

  • 8 0 2.2 Header and Data
  • Frame Check sequence
  • Type
  • Start of Frame Delimiter
  • Destination Address
  • Preamble
  • Source Address

Figure 1 also lists the following 8 0 2.3 Ethernet frame field descriptions:

  • Uses Pad to increase this frame field to at least 64 bytes
  • Describe which higher-level protocol has been used
  • The frame’s originating NIC or interface MAC address
  • Assists a host in determining if the frame received is addressed to it
  • Notifies destinations to get ready for a new frame
  • Synchronizes sending and receiving devices for frame delivery
  • Detects errors in an Ethernet Frame

The user is asked to match the Ethernet frame fields with their descriptions.

Figure 2 on this page lists the following Ethernet frame field names:

  • Source Address
  • Frame Check Sequence
  • Start of Frame Delimiter
  • Preamble
  • 8 0 2.2 Header and Data
  • Length
  • Destination Address

Figure 2 also lists the following field sizes in bytes:

  • 7
  • 1
  • 6
  • 6
  • 2
  • 46 - 1500
  • 4

The user is asked to match the Ethernet frame fields with their size.

5.1.3 Ethernet MAC >5.1.3.1 MAC Addresses and Hexadecimal

Table 1 on this page displays Decimal values between 0 and 15 with the Binary and Hexadecimal equivalents. This table demonstrates why Hexadecimal has letters A through F along with numbers zero through nine.

The following table lists the values:

DecimalBinaryHexadecimal
000000
100011
200102
300113
401004
501015
601106
701117
810008
910019
101010A
111011B
121100C
131101D
141110E
151111F

Table 2 on this page displays Decimal values between 0 and 255 with the Binary and Hexadecimal equivalents. This table only shows nineteen examples:

The following table lists the examples:

DecimalBinaryHexadecimal
00000 000000
10000 000101
20000 001002
30000 001103
40000 010004
50000 010105
60000 011006
70000 011107
80000 100008
100000 10100A
150000 11110F
160001 000010
320010 000020
640100 000040
1281000 000080
1921100 0000C0
2021100 1010CA
2401111 0000F0
2551111 1111FF

5.1.3 Ethernet MAC >5.1.3.2 MAC Address Representations

Figure 1 on this page shows the output of i p config/all command on a PC with the Physical Address is highlighted in orange. The physical address is the MAC address.

The output of the i p config/all command is:

C:\>ipconfig/all
Physical Address.........: 00-18-DE-C7-F3-F8

Figure 2 on this page shows that a MAC address can be represented with dashes, colons or periods.

The following table shows the different representations:

With dashes00-60-2F-3A-07-BC
With Colons00:60:2F:3A:07:BC
With Periods0060.2F3A.07BC

5.1.3 Ethernet MAC >5.1.3.3 Unicast MAC Address

The animation on this page shows several computers connected to a switch. Host 1 wants to send a packet to a server (a unicast). The diagram shows the MAC address and IP address of both host 1 and the server. The graphic also shows the frame with source and destination MAC address and the encapsulated packet with source and destination IP address.

When the animation plays, Host 1 has a callout that says, "I need to send this frame to the server". The frame travels from host 1 to the server since the switch forwards the frame based on MAC address and knows where the server is located.

The following table lists the IP and MAC address for the Source Host and the Server:

DeviceIP AddressMAC Address
Host 1192.168.1.500-07-E9-63-CE-53
Server192.168.1.20000-07-E9-42-AC-28

The Ethernet frame is displayed in the following table:

Dest MACSource MACSource IPDest IP  
00-07-E9-42-AC-2800-07-E9-63-CE-53192.168.1.5192.168.1.200User DataTrailer

The Source IP,the Dest IP, and the User Data make up the encapsulated IP Packet.

The description for this animation is, "Unicast IP and MAC destination addresses are used by the source to forward a packet."

5.1.3 Ethernet MAC >5.1.3.4 Broadcast MAC Address

The animation on this page shows several computers connected to a switch. Host 1 wants to send a packet to all other hosts (a broadcast). The diagram shows the MAC address and IP address of host 1. The graphic also shows the frame with source and destination MA.C address and the encapsulated packet with source and destination IP address. The destination MAC address is formed by all Fs (the broadcast MAC address).

When the animation plays, Host 1 has a callout that says, "I need to send data to all hosts on the network". The frame travels from host 1 to all other hosts connected to the switch.

The following table lists the IP and MAC address for the Source Host and the Server:

DeviceIP AddressMAC Address
Host 1192.168.1.500-07-E9-63-CE-53
Destination Host Group192.168.1.255FF-FF-FF-FF-FF-FF

The Ethernet frame is displayed in the following table:

Dest MACSource MACSource IPDest IP  
FF-FF-FF-FF-FF-FF00-07-E9-63-CE-53192.168.1.5192.168.1.255User DataTrailer

The description for this animation is, "Broadcast IP and broadcast MAC destination addresses are used by the source to forward a packet to all hosts on the network."

5.1.3 Ethernet MAC >5.1.3.5 Multicast MAC Address

The animation on this page shows several computers connected to a switch. Host one wants to send a packet to a group of computers (a multicast). The diagram shows the MAC address and IP address of host 1. The graphic also shows the frame with source and destination MAC address and the encapsulated packet with source and destination IP address. The destination IP address is a multicast address (224 dot 0 dot 0 dot 200) and the destination MAC address is special. It starts with 0 1 dash 0 0 dash 5E and ends with the hexadecimal equivalent of the last three octets of the IP multicast address, 0 0 dash 0 0 dash C8.

When the animation plays, Host 1 has a callout that says, "I need to send to a group of hosts on the network". the frame travels from host 1 to the selected hosts that are listening to the multicast address.

The following table lists the IP and MAC address for the Source Host and the Server:

DeviceIP AddressMAC Address
Host 1192.168.1.500-07-E9-63-CE-53
Destination Host Group224.0.0.20001-00-5E-00-00-C8

The Ethernet frame is displayed in the following table:

Dest MACSource MACSource IPDest IP  
01-00-5E-00-00-C800-07-E9-63-CE-53192.168.1.5224.0.0.200User dataTrailer

IThe description for this animation is, "Multicast IP and MAC destination addresses deliver packet/frame to specific group of member hosts."

5.1.3 Ethernet MAC >5.1.3.6 Lab – Viewing Network Device MAC addresses

See Lab Descriptions.

5.1.4 MAC and IP >5.1.4.1 MAC and IP

This interactive figure shows a map of North America. There are four buttons at the bottom for North America, Canada, Nova Scotia and Halifax. When you click the buttons, the map zooms in to the specific region. This demonstrates how IP addresses are hierarchical.

5.1.4 MAC and IP >5.1.4.2 End-to-End Connectivity, MAC and IP

Animation 1 on this page shows how the Ethernet Frame header and trailer are wrapped around the IP packet. A switch examines destination and source MAC addresses in the header. A router examines source and destination IP addresses in the IP packet. This process is called encapsulation.

The following table shows the IP packet encapsulated in an Ethernet frame:

HeaderIP Packet  
Destination MAC Address
BB:BB:BB:BB:BB:BB		Source MAC address
AA:AA:AA:AA:AA:AA		Source IP Address
10.0.0.		Destination IP address
192.168.1.		DataTrailer

Animation 2 on this page shows that an IP packet can move across different types of media. Since the MAC sublayer of the data link layer is responsible for placing the frame on the wire, IP is media independent. This figure demonstrates how frames are encapsulated based on the technology of the link.

A host PC is sending a frame from a PC in Paris to a laptop in Japan. The frame goes from the router at Paris through satellites to reach the destination laptop.

At each hop along the path, an intermediary device accepts frames from one medium, de-encapsulates the frame and then forwards the packets in a new frame. The headers of each frame are formatted for the specific medium that it will cross.

Data link layer protocols govern how to format a frame for use on different media. Different protocols may be in use for different media.

5.1.4 MAC and IP.>5.1.4.3 Lab – Using Wireshark to examine Ethernet frames.

See Lab Descriptions.

5.1.4 MAC and IP >5.1.4.4 Packet Tracer – Identify MAC and IP Addresses

Objectives:

Part 1: Gather PDU Information
Part 2: Reflection Questions

5.2 Address Resolution Protocol

5.2.1 ARP >5.2.1.1 Introduction to ARP

The figure on this page shows four computers connected to a switch. Each computer is labelled with its IP address.

The following table lists the IP addresses:

ComputerIP Address
H1192.168.1.5
H2192.168.1.6
H3192.168.1.8
H4192.168.1.7

Host 1 has a callout saying, "I need to send information to 192.168.1.7, But I only have the IP address. I don't know the MAC address of the device that has that IP".

5.2.1 ARP >5.2.1.2 ARP Functions

The animation on this page shows how a host makes a broadcast on the LAN when it needs to find the MAC address for the destination. The destination host will respond to the broadcast but the other hosts will not.

The ARP process is:

  1. H1 has a callout saying, "I must send out an ARP request to learn the MAC address of the host with the IP address of 192.168.1.7.". H1 sends the ARP as broadcast to all the computers.
  2. H2 and H4 have callouts saying, "This isn't me.".
  3. H4 has a callout saying, "This is me. I will send back my MAC address.".
  4. H4 sends its MAC address to H1.
  5. H1 receives the MAC address from H4.
  6. H1 now has a callout saying, "Now I have the MAC so I can forward my information.".
  7. H1 sends the information to H4.
  8. H4 has a callout saying, "Thanks, I got it.".

5.2.1 ARP >5.2.1.3 ARP Operation

Figure 1 on this page shows four hosts and a router on a LAN segment. Each is labelled with its IP address and MAC address. The figure also shows the ARP Cache for Host A which has no ARP entry.

The following table lists the MAC addresses and IP addresses of the devices:

DeviceIP AddressMAC Address
Host A10.10.0.100-0d-88-c7-9a-24
Host B10.10.0.200-08-a3-b6-ce-04
Host C10.10.0.300-0d-56-09-fb-d1
Host D10.10.0.00-12-3f-d4-6d-1b
R1 Interface G0/010.10.0.25400-10-7b-e7-fa-ef

Host A wants to send data to Host C, but does not know its MAC address.

Figure 2 on this page illustrates how Host A sends out a broadcast asking for the MAC address associated with IP 10.10.0.3. This process is called ARP (Address Resolution Protocol). The request reaches all the other hosts and the router.

Figure 3 on this page shows that the request from Host A reaches all the other hosts and the router. Host C with IP address 10.10.0.3, responds with anARP reply that includes its MAC address and now host A knows both the IP and MAC address for the destination.

Figure 4 on this page shows that Host A now adds the MAC to IP address map to its ARP cache.

The following table represents the ARP cache:

Host A – ARP Cache
10.10.0.300-0d-56-09-fb-d1

Figure 5 on this page shows that Host A can forward data directly to Host C via MAC address because it can now populate the layer 3 and layer 2 source and destination addresses.

5.2.1 ARP >5.2.1.4 ARP role in remote communication

Figure 1 on this page is the same as figure 1 in the previous section, four hosts and a router on a LAN segment. Each is labelled with its IP address and MAC address. The figure also shows the ARP Cache for Host A which has the ARP entry for Host C from the previous section.

The following table lists the MAC addresses and IP addresses of the devices:

DeviceIP AddressMAC Address
Host A10.10.0.100-0d-88-c7-9a-24
Host B10.10.0.200-08-a3-b6-ce-04
Host C10.10.0.300-0d-56-09-fb-d1
Host D10.10.0.00-12-3f-d4-6d-1b
R1 Interface G0/010.10.0.25400-10-7b-e7-fa-ef

Host A wants to send data to a host on a different LAN with IP address 176.10.10.50 but has no ARP entry for the default gateway, IP address 10.10.0.254. The host sends an ARP request for its gateway, the router.

Figure 2 on this page illustrates how Host A sends an ARP request looking for the MAC address associated with IP 10.10.0.254. All of the other hosts receive the request.

[Figure 3 on this page shows that the router, R1, with IP address 10.10.0.254, responds with a unicast ARP reply, to Host A, that includes the MAC address for the G0/0 interface.

Figure 4 on this page shows that Host A adds the MAC to IP address map to its ARP cache.

The following table represents the ARP cache:

Host A – ARP Cache
10.10.0.300-0d-56-09-fb-d1
10.10.0.25400-10-7b-e7-fa-ef

Figure 5 on this page shows that Host A forwards the data destined for 176.10.10.50 to the default gateway for further processing. The router has a callout saying, "I will forward the packet based on the information in my routing table.".

5.2.1 ARP >5.2.1.5 Removing Entries from an ARP Table

The figure on this page is the same as figure 1 in the previous section, four hosts and a router on a LAN segment. Each is labelled with its IP address and MAC address. Host C has a red X over it. This figure demonstrates that Host C is being removed from the network. It is critical that ARP cache entries be purged after some time has passed, otherwise data will be forwarded incorrectly.

If host C's IP and MAC address are not removed from Host A's ARP cache,Host A may still try to communicate with Host C.

5.2.1 ARP >5.2.1.6 ARP Tables on networking devices

Figure 1 on this page shows that on a Cisco router the show i p a r p command is used to display the ARP table. This lists the Protocol, IP Address, Age, Hardware Address, Type and Interface.

Figure 2 on this page shows that on a Windows 7 PC the a r p - a command is used to display the ARP table. This lists destinations learned and grouped by interface. The output includes the Internet IP address, Physical Address and Type.

5.2.1 ARP >5.2.1.7 Packet Tracer – Examine the ARP table

Objectives

Part 1: Examine an ARP Request
Part 2: Examine a Switch MAC Address Table
Part 3: Examine the ARP Process in Remote Communication

5.2.1.8 Lab-Observing ARP with the windows CLI, IOS CLI, and Wireshark

See Lab Descriptions.

5.2.2 ARP Issues >5.2.2.1 How ARP can create problems

The figure on this page shows seven computers connected to a multi-access media. This figure demonstrates that ARP broadcasts can flood the local media.

ARP Issues are:

  • Broadcasts, overhead on the media
  • Security

A false ARP message can provide an incorrect MAC address that will then hijack frames using that address (called a spoof)

5.2.2 ARP Issues >5.2.2.2 Mitigating ARP Problems

The figure on this page shows sixteen computers connected to a switch. This figure shows how a switch creates a separate collision domain for each connected device. This prevents the ARP reply from being broadcast, rather it is a unicast back to the origination.

Each computer has its own collision domain.

5.3 LAN Switches

5.3.1 Switching >5.3.1.1 Switch Port Fundamentals

The interactive activity on this page shows how a switch uses a MAC address table to forward frames. This table maps a MAC address to a port on the switch. The figure shows a central switch to which eight hosts are connected.

The following table represents the MAC table:

MAC Table
fa0/1fa0/2fa0/3fa0/4
206d.8c01.0000206d.8c01.1111206d.8c01.2222206d.8c01.3333
fa0/5fa0/6fa0/7fa0/8
206d.8c01.4444206d.8c01.5555206d.8c01.6666206d.8c01.7777

The user is asked to first click a source PC then a destination PC and then the "'Send'" button. The source and destination MAC address in the table which is associated with a specific port are highlighted.

5.3.1 Switching >5.3.1.2 Switch MAC Address Table

Figure 1 on this page shows a switch with PC1 connected to port 1, PC2 connected to port 3, and port 2 going somewhere else. PC1 generates a frame and sends it to the switch.

Figure 2 on this page shows that the switch receives the frame and enters the MAC address for PC1 into the MAC address table and maps it to port 1.

Figure 3 on this page shows that since the switch does not have an entry for the destination MAC address, it broadcasts the frame through the remaining ports.

Figure 4 on this page shows that PC2 responds to the broadcast and forwards a frame back to the switch.

Figure 5 on this page shows that the switch receives the frame and enters the MAC address for PC2 into the MAC address table and maps it to port 3.

Figure 6 on this page shows that the frame leaves the switch through port 1 and arrives at PC1.

The process that a switch uses to build its MAC Address Table is decribed in the page notes.

5.3.1 Switching >5.3.1.3 Duplex Settings

The figure on this page shows two diagrams.

The first diagram shows a server connected to a hub which is connected to a switch. In this diagram, data can only flow in one direction at a time between the server and the switch. This is known as half duplex.

Half Duplex (CSMA/CD):

  • Unidirectional data flow
  • Higher potential for collision
  • Hub connectivity

The second diagram replaces the hub with another switch. In this diagram, data can flow in both directions simultaneously. This is known as full duplex.

Full Duplex:

  • Point-to-point only
  • Attached to dedicated switched port
  • Requires full duplex support on both ends
  • Collision-free
  • Collision detect circuit disabled

5.3.1 Switching >5.3.1.4 Auto-MDIX

Figure shows 4 connections:

  • switch to switch
  • switch to router
  • switch to PC
  • router to PC

MDIX auto detects the type of connection required and configures the interface accordingly.

5.3.1 Switching >5.3.1.5 Frame Forwarding Methods on Cisco Switches

Figure 1 on this page shows 2 switches. One is set to store and forward and the other is set to cut through.

The following table compares the two switches:

Store and forwardCut through
A store and forward switch receives the entire frame, and computes the CRC. If the CRC is valid, the switch looks up the destination address, which determines the outgoing interface. The frame is then forwarded out the correct port.A cut through switch forwards the frame before it is entirely received. At a minimum, the destination address of the frame must be read before the frame can be forwarded.

Figure 2 on this page is an animation that shows a switch that receives an entire frame and checks the CRC value before forwarding the frame. This is known as store and forward switching.

The source sends a frame to the switch.

The following table represents the frame::

Destination
Address		Source
Address		DataCRC

The store and forward switch receives the entire frame and computes the CRC. If the CRC is valid, the switch looks up the destination address, which determines the outgoing interface. The frame is then forwarded out the correct port.

5.3.1 Switching >5.3.1.6 Cut through Switching

The figure on this page is an animation that shows a switch that starts forwarding the frame before it receives the entire frame. This is known as cut through switching.

A cut through switch forwards the frame before it is entirely received. At a minimum, the destination address of the frame must be read before the frame can be forwarded.

5.3.1 Switching >5.3.1.7 Activity – Frame Forwarding Methods

Figure 1 on this page lists the following six description:

  1. Buffers frames until the full frame has been received by the switch.
  2. Checks the frame for errors before releasing it out of its switch ports – if the full frame was not received, the switch discards it.
  3. No error checking on frames for is performed by the switch before releasing the frame out of its ports.
  4. A great method to use to conserve bandwidth on your network.
  5. The destination network interface card (NIC) discards any incomplete frames using this frame forwarding method.
  6. The faster switching method, but may produce more errors in data integrity – therefore, more bandwidth may be consumed.

The user is asked to match the descriptions to their correct frame forwarding method, either Store and Forward or Cut Through.

Figure 2 on this page shows the following topology:

  • 3 PC's are connected to switch S1.
  • PC1 is connected to Port1.
  • PC2 is connected to Port2.
  • PC3 is connected to Port3.
  • S1 is a brand new switch. PC1 is sending data to PC2.
  • S1 is using full duplex, MDIX, and fast forward as a frame switching method.

Read the scenario based on the topology shown. Identify how the frames will be processed by dragging your answers to the appropriate fields provided in the table. All answers will not be used.

The scenario is as follows:

  • Cabling used in this topology will be Blank.
  • To find where PC2 is located, PC1 will send out a Blank data frame.
  • PC2 will respond back to PC1 by sending back a Blank message.
  • If PC2 receives only half of the data in the frame, it will Blank.
  • If PC2 receives many damaged frames on port 2, S1 likely will change back to Blank switching.

The supplied answers are:

  • Straight through
  • Cut through
  • Unicast
  • Store and forward
  • Broadcast
  • Discard it
  • Reply back to PC1

5.3.1 Switching >5.3.1.8 Memory Buffering on switches

The table on this page compares Port Based and Shared memory buffering as follows:

Port based memoryIn port-based memory buffering, frames are stored in queues that are linked to specific incoming and outgoing ports
Shared memoryShared memory buffering deposits all frames into a common memory buffer, which all the ports on the switch share

5.3.1 Switching >5.3.1.9 Activity - Switch It!

The figure on this page shows the following topology of four computers and one hub connected to a Catalyst 2950 series switch topology:

The following table represents the switch:

Switch PortsFa1Fa2Fa3Fa4Fa5Fa6Fa7Fa8Fa9Fa10Fa11Fa12
Device MAC AddressPC1
OA		 PC2
OB		 PC3
OC		 PC4
OD		 Hub
0E		   

The hub has two PC's connected to it with MAC addresses 0E and 0F.

Determine how the switch forwards a frame based on the source MAC and destination MAC address and information in the switch MAC table.

An example of the frame is represented in the following table:

PreambleDestination
MAC		Source
MAC		Length TypeEncapsulated
Data		End of frame
 FF0E   

An example of the MAC Table is as follows:

Fa1 Fa2 Fa3 Fa4 Fa5 Fa6 Fa7 Fa8 Fa9 Fa10 Fa11 Fa12
0A 0C 0E

Answer the questions below using the information provided.

Question 1: Where will the switch forward the frame?

Fa1, Fa2, Fa3, Fa4, Fa5, Fa6, Fa7, Fa8, Fa9, Fa10, Fa11, Fa12

Question 2: When the switch forwards the frame, which statement(s) are true?

  • Switch adds the source MAC address to the MAC table
  • Frame is a broadcast frame and will be forwarded to all ports
  • Frame is a unicast frame and will be sent to specific port only
  • Frame is a unicast frame and will be flooded to all ports
  • Frame is a unicast frame but it will be dropped at the switch

The learner can generate new problems as they desire.

5.3.1 Switching >5.3.1.10 Lab – Viewing the Switch MAC Address Table

See Lab Descriptions.

5.3.2 Fixed or Modular >5.3.2.1 Fixed versus Modular Configuration

Figure 1 on this page shows a switch connected to an IP phone and a wireless access port. Both devices can receive their power from the switch using Power Over Ethernet or POE.

PoE ports look the same as any switch port. Check the model of the switch and the wireless access point to determine if the ports support PoE.

Figure 2 on this page shows different form factors for switches as follows:

Fixed Configuration SwitchesFeatures and options are limited to those that originally come with the switch.
Modular Configuration SwitchesThe chassis accepts line accepts that contain the ports.
Stackable Configuration SwitchesStackable switches, connected by a special cable, effectively operate as one large switch.

5.3.2 Fixed or Modular >5.3.2.2 Module Options for Cisco Switch Slots

the Figure on this page shows three SFP modules as follows:

  • Cisco Optical Gigabit Ethernet SFP
  • Cisco 1000 base T copper SFP
  • Cisco 2 channel 1000 base BX Optical SFP

5.3.3 Layer 3 Switching >5.3.3.1 Layer 3 versus layer 3 Switching

Figure 1 on this page shows 4 computers connected to a standard LAN switch (layer 2 switch) connected to a router. The router is connected to the Internet.

Figure 2 on this page shows 4 computers connected to a Multi Layer switch (layer 2 - 3 switch) connected to the Internet. This diagram shows that you can eliminate a device in some cases.

5.3.3 Layer 3 Switching >5.3.3.2 Cisco Express Forwarding

The figure on this page shows a logical representation of how a layer 3 switch works. The interfaces on a layer 3 switch interact with the internal Route Processor utilising Cisco Express Forwarding (CEF).

The Route Processor incorporates the Routing Table, the FB Table, and the Adjacency Table.

5.3.3 Layer 3 Switching >5.3.3.3 Types of Layer 3 Interfaces

The figure on this page shows a Lan consisting of a layer 3 switch. There are 2 standard layer 2 switches attached to the layer 3 switch. Each layer 2 switch has 2 PC's attached. There are 2 V Lans, Faculty (v lan 10) and Student (v lan 20). Each layer 2 switch is connected to the layer 3 switch using a Trunk. The layer 3 switch has two switched virtual interfaces (SVI's) used to route between the two V Lans.

5.3.3 Layer 3 Switching >5.3.3.4 Configuring a Routed Port on a Layer 3 Switch

The figure on this page shows the CLI of a layer 3 switch. The output shows how to configure an interface to act as a routed interface using the no switchport command followed by a standard interface configuration sequence to set IP address and subnet mask. Then the show IP interface brief command is issued to show the interface is configured with IP address and it is up:

S1 (config)#interface f0/6
S1 (config-if)#no switchport
S1 (config-if)#ip address 192.168.200.1 255.255.255.0
S1 (config-if)#no shutdown
S1 (config-if)#end
S1#
*Mar 1 00:15:40.115 %SYS-5-CONFIG_I: Configured from console by console
S1#show ip interface brief
Interface           IP_Address       OK?   Method   Status               Protocol
V Lan1             Unassigned     YES   unset   administratively down   down
FastEthernet0/1     Unassigned     YES   unset   down               down
FastEthernet0/2     Unassigned     YES   unset   down               down
FastEthernet0/3     Unassigned     YES   unset   down               down
FastEthernet0/4     Unassigned     YES   unset   down               down
FastEthernet0/5     Unassigned     YES   unset   down               down
FastEthernet0/6   192.168.200.1   YES   manual     up                 up
FastEthernet0/7     Unassigned     YES   manual   up                 up
FastEthernet0/8     Unassigned     YES   manual   up                 up
<output omitted>

5.3.3 Layer 3 Switching >5.3.3.5 Packet Tracer - Configure Layer 3 Switches

Objectives

Part1: Document the Current Network Configurations
Part 2: Configure, Deploy, and Test the New Multilayer Switch

5.4 Summary

5.4.1 Summary >5.4.1.1 Activity – MAC and Choose Instructions

Figure shows an NIC and a stack of fixed configuration switches.

Ethernet uses end and intermediary devices to identify and deliver frames through networks.

Video link to discuss the activity :http://tv.netevents.org/the-history-of-ethernet-with-bob-metcalfe-inventor-of-ethernet/

5.4.1 Summary >5.4.1.2 Summary

The figure on this page shows 4 computers connected to shared media. Each has a MAC address as follows:

DeviceMAC Address
PC1AA:AA:AA:AA:AA:AA
PC2BB:BB:BB:BB:BB:BB
PC3CC:CC:CC:CC:CC:CC
PC4DD:DD:DD:DD:DD:DD

The figure also shows a portion of the frame header including the source and destination MAC addresses:

Frame Addressing 
Destination AddressSource AddressData
CC:CC:CC:CC:CC:CCAA:AA:AA:AA:AA:AAEncapsulated data
  • The PC2 and PC4 both have callouts saying, "This is not addressed to me. I shall ignore it.".
  • The PC3 has a callout saying, "This is this is mine.".

End of Chapter 5: Ethernet.

Next - Chapter 6: Network Layer.

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Page last modified on July 31, 2014, at 11:50 AM