ETHERNETChapter 5

Intro to Routing & Switching

OBJECTIVES Upon completion of this chapter, you should

be able to: Describe the operation of the Ethernet sublayers. 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 fixed configuration and modular

switches. Configure a Layer 3 switch.

ETHERNET OPERATION

5.1.1

ETHERNET OPERATION Most widely used LAN technology

What 2 layers does it operate at?Data link & physical

What are the 2 sublayers of the data link?LLC & MAC

LLC IEEE 802.2 Helps communicate with the upper

network layer Adds control info implemented it in software

MAC SUBLAYER Encapsulates the data

Adds MAC addressesAdds error detection to frame

Media Access ControlPlacing the frames on the media

Ethernet is a logical bus; physical starSignal passes to allCan send wheneverCan be collisions

CSMA Ethernet is contention-based/non-deterministic

Data contends or shares for a spot on the media Doesn’t know when it’ll get access to it

Listens for signal on media No signal = transmit data Transmit at same time= collision

Devices do not keep track of whose turn it is

More collisions= less throughput SOLUTION: CSMA/CD & CSMA/CA

CSMA/CD ON ETHERNET NETWORK Listens for silence

Silence= transmit data If devices transmit at same time,

collisionBackoff random time, listen for silence,

retransmit

Collision detection not a problem much anymoreUsing switches & full-duplex, this is not a

problem anymore

CURRENT SWITCHED NETWORK

CSMA/CA Wireless

Listen for silence Sends Ready to Send message to AP Gets a Clear to Send message from AP Sends data

MAC ADDRESSES Used to identify frame Each host has a unique address

Burned into NIC hardware 48-bits / 12 hex digits / 6 bytes 1st 24 bits OUI; 2nd 24 bits serial #

FORWARDING FRAMES

ACTIVITY

REVIEW How many bits is a MAC address?

48 bits What is the OUI in this MAC?

A2:07:CC:F6:AD:32A2:07:CC

What does a wireless network use to avoid collisions?CSMA/CA

What sublayer places the frames on the media?MAC

End of Day One

REVIEW The OUI is how many bytes?

3 Ethernet shares access to the media. It

contends for the media and does not take turns transmitting. Not taking turns means the network is…Nondeterministic

Is the MAC address found in hardware or software?Software

ABOUT THE ETHERNET FRAMES

5.1.2

FRAME SIZE Min. frame size is 64 bytes; max is 1522 bytes

Data is 46-1500 bytes

64 bytes is considered a collision fragment & will be droppedPREAMBLE- used timing/synchronization of the frame between send & receiver. Basically saying., “Here comes a frame!”

ACTIVITY5.1.2.4

REVIEW What important addresses are

encapsulated into a frame?Source & destination MAC

What does the preamble used for?Timing/synchronization

What is the minimum frame size?64 bytes

What happens if a frame is less than 64 bytes?Considered a fragment & dropped

REVIEW How many bits in a MAC address?

48 bits How many hex digits?

12 The first 6 hex digits are what?

The OUI The OUI would then be the 1st ____ bytes.

3 What sublayer is used to communicate

with the upper layers?LLC

ETHERNET MAC

5.1.3

BINARY/DECIMAL CONVERSION IP Address: 32 bits, 4 octets

8 bits in each octet11111111.10101010.11001100.00100101

Written in decimal192.101.28.36

Value in each octet from 0-255That’s a total of 256 numbers.

BINARY/DECIMAL CONVERSIONS Add up the values of the binary 1’s

156

11100101229

128 64 32 16 8 4 2 1

1 0 0 1 1 1 0 0

MAC ADDRESSING Ipconfig /all Hexadecimal (Base 16) 0-9, A-F (10-15)

16 total #’s

BINARY/DEC/HEX CONVERSIONS

CONVERSION PRACTICE Handouts

HOW MAC ADDRESSES ARE SHOWN Begin with a 0x

0xA4

UNICAST MAC ADDRESS One to one communication

BROADCAST MAC ADDRESS One to all in a network Dest. MAC address will be all F’s DHCP & ARP use broadcasts

MULTICAST MAC ADDRESS One to a group in a network Remote gaming or video conference Dest. IP will be 224.0.0.0 -

239.255.255.255 Dest. MAC will begin with 01-00-5E

LAB 5.1.3.6

REVIEW What kind of message gets sent from

one PC to a group within a network?Multicast

Identify each as unicast, multicast, or broadcast:

MAC & IP

5.1.4

MAC & IP MAC address

Burned into NIC (DOES NOT CHANGE) Similar to the name of a person Physical address

IP address Similar to the address of a person Based on where the host is actually located Logical address

Both the physical MAC & logical IP addresses are required for a computer to communicate just like both the name and address of a person are required to send a letter

MAC & IP- TO OTHER NETWORKS Destination IP NEVER changes! Source & Destination MAC changes at

each router interface

LAB 5.1.4.3 Wireshark

REVIEW How do you change your MAC address?

Get a new NIC T or F. The destination IP address

changes during transmission.False

What happens to the source & destination MAC addresses as you go from router to router across the Internet?They change (router port substituted (mac))

REVIEW What is IEEE 802.2?

LLC sublayer What is IEEE 802.3?

Ethernet/MAC sublayer What is a layer 2 address?

MAC address What is a layer 3 address?

IP address

ARP

5.2.1

HOW ARP WORKS ARP table of IP/MACs

Added from communicationAdded from ARP requests

You have the dest. IP, not the MAC ARP request is all F’s where?

Destination MAC How is an ARP request sent?

Broadcast Who replies to the ARP request?

Only one with matching dest. IP

HOW ARP WORKS- LOCAL

HOW PROXY ARP WORKS- REMOTE

VIEWING THE ARP TABLE Windows-

Arp –a

Cisco routerRouter# show ip arp

REVIEW If you want to access a remote server,

and you don’t have the destination MAC, what gets substituted for it?The default gateway’s MAC address

A router has ports with MAC addresses. How do you view the router’s ARP table?Router#sh ip arp

What does ARP find? What do you know?Finds the dest. MAC; you know the dest. IP

ARP ISSUES

5.2.2

PROBLEMS WITH ARP Broadcasts

It’s a broadcast. If many devices started at same time, there’d be a flood of ARP requests which would cause a reduction in performance for a short period of time.

SecurityARP poisoning (or spoofing)Attacker forges MAC address to have frames

delivered to different computer

Solution: Use a switch

USING A SWITCH Segments network into smaller collision

domains Replies go to one device only Implement security too

SWITCHING

5.3.1

HOW A SWITCH WORKS

BUILDING A SWITCH TABLE

SWITCH DUPLEX SETTINGS Full duplex, Half duplex, Auto

Must match setting of deviceHalf duplex uses CSMA/CD to avoid

collisions

WHAT CABLE SHOULD IT BE?

MDIX AUTOWhen the auto-MDIX feature is enabled, the switch detects the required cable type for copper Ethernet connections and configures the interfaces accordingly

STORE-AND-FORWARD SWITCHING

when the switch receives the frame, it stores the data in buffers until the complete frame has been received.

During the storage process, the switch analyzes the frame for information about its destination.

In this process, the switch also performs an error check using the Cyclic Redundancy Check (CRC) trailer portion of the Ethernet frame.

CUT-THROUGH SWITCHING The switch buffers just enough of the

frame to read the destination MAC address so that it can determine to which port to forward the 

The destination MAC address is located in the first 6 bytes of the frame following the preamble. The switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards 

FORWARDING THE MESSAGE Cisco uses cut-through switching

As soon as destination MAC is read, it forwards the frame

Fast-forward Lowest latency; in and out

Fragment-free Store first 64 bytes before forwarding Most errors & collision happen there (runts) If it makes it through, should be error-free

ACTIVITY

ACTIVITY Complete 5.3.1.7

WHAT WILL HAPPEN?

REVIEW 2 switches connect to each other. One port

is 100Mbps and the other is 1000Mbps. What speed will that connection operate at? 100Mbps

What feature will allow you to use a straight-through cable to connect two switches together? Auto-MDIX

Your switch port is connected to a hub with 3 computers on it. How many MAC addresses will be in the table for that port? 3

ACTIVITY & LAB Switch it!

5.3.1.9Do it at least 4 timesDifferent scenario each time

Lab together

Draw network from MAC address table

TYPES OF CISCO SWITCHES

5.3.2

SWITCH FEATURES Fixed Configuration Switch

As is; no add-ons or changing A 24 port switch will always be 24 ports

Modular Add-in cards/ports

Stackable Connected by special cable to act as one switch Fixed may be stackable

PoE (Power over Ethernet) Delivers power to a device, like an IP phone We use this in school

More $$, faster forwarding rate

WHICH IS WHICH?

REVIEW Which type of switch can you add a card to

add more Ethernet ports or add fiber ports? Modular

You bought a 24 port switch and cannot add more ports to it. What kind of switch did you buy? Fixed

You have the switch above and need more ports. You then buy more 24 port switches and connect them with a special cable. What kind of switch do you now have? Fixed & stackable

LAYER 3 SWITCHING

5.3.3

LAYER 3 SWITCH Looks like a switch

Combined with router functions Adds in router functions

Knows which IP addresses are out each port too

Fast

ROUTED PORT Make an Ethernet port a routed port

To connect to the ISPConfigure a port

What have we configured with an IP on a switch? IP for remote managementThis is similar

CONFIGURE ROUTED PORT

F0/6

LAB 5.3.3.5 Configuring a Layer 3 Switch

REVIEW What address(es) does a layer 2 switch read?

MAC addresses What address(es) does a layer 3 switch read?

IP & MAC You want to connect your Layer 3 switch to

your ISP instead of using a regular router. What must you configure one of the ports as? Routed port

Which command enables the routing function on a switch port? No switchport

REVIEW & STUDY Complete the study guide handout

Take the quiz on netacad.com

Jeopardy review

SUMMARYIn this chapter, you learned: Ethernet is the most widely used LAN technology

used today. Ethernet standards define both the Layer 2

protocols and the Layer 1 technologies. The Ethernet frame structure adds headers and

trailers around the Layer 3 PDU to encapsulate the message being sent.

As an implementation of the IEEE 802.2/3 standards, the Ethernet frame provides MAC addressing and error checking.

Using switches in the local network has reduced the probability of frame collisions in half-duplex links.

SUMMARY The Layer 2 addressing provided by Ethernet

supports unicast, multicast, and broadcast communications.

Ethernet uses the ARP to determine the MAC addresses of destinations and map them against known IP addresses.

Each node on a network has both a MAC address and an IP address.

The ARP protocol resolves IPv4 addresses to MAC addresses and maintains a table of mappings.

A Layer 2 switch builds a MAC address table that it uses to make forwarding decisions.

SUMMARY Layer 3 switches are also capable of

performing Layer 3 routing functions, reducing the need for dedicated routers on a LAN.

Layer 3 switches have specialized switching hardware so they can typically route data as quickly as they can switch.

ETHERNETChapter 5

Intro to Routing & Switching