Ethernet

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE I Chapter 6 1

Ethernet

Network Fundamentals – Chapter 9

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 2

Objectives Identify Ethernet

Identify the function and characteristics of the media access control method used by Ethernet protocol.

Identify different types of layer 2 addressing and impacts network operation and performance.

Identify the application and benefits of using Ethernet switches in a LAN as opposed to using hubs.

Identify basic switch operations

Identify the ARP process.


What is Ethernet?

A family of LAN products described by the IEEE standards

Developed in the early 1980s and first published by the consortium Digital equipment, Intel and Xerox (DIX) as an open standard

To maintain compatibility with the ISO the IEEE publish the 802.3 standard in 1985

Many technologies have challenged Ethernets dominance. However Ethernet has continued to be the dominate LAN product


Physical and Data Link Features of Ethernet Standards and Implementation for Ethernet include

802.2 and 802.3 both defined by the IEEE


Physical and Data Link Features of Ethernet

Ethernet operates across two layers of the OSI model



Logic Link Control – Connecting the Upper Layers



Media Access Control (MAC)


Ethernet – media changes Ethernet went from Bus type Coaxial based cable to

Star based UTP/STP cablesCoaxial

Thicknet, (10Base5)

Thinet(10Base2)

CSMA then CSMA/CD

Low Bandwidth

Difficult to scale

UTP (10Base-T)

UTP (100Base-TX)

Fiber(100Base –FX)

CSMA/CD

Higher Bandwidth

Scalable


Ethernet Hubs versus Switches in a LAN. Legacy Ethernet – Using Hubs suffered multiple

collisions Hubs not intelligent devices, Bandwidth is shared, not dedicated per host


Ethernet – media changes The movement of Hub based LANs to switched based

Ethernet LANs reduced the size of collision domains and improved network availability

Multi-port Bridge

Intelligent device- forwards frames based

on MAC address

Reduce the size of collision domains

Increased Bandwidth

Communication channel inside switch is

dedicated

Can support Virtual LANs


Ethernet Switches versus Hubs in a LAN. Ethernet – Using Switches


Ethernet Switches versus Hubs in a LAN. A switch can eliminate collisions, backoffs and re-

transmissions, the leading factors in reduced throughput on a hub-based Ethernet network


Switch operations Switches forward frames based on their destination MAC

addresses. To accomplish their purpose switches use:

Learning (Populate MAC address table with MAC addresses of connected hosts)

Aging (Removes old MAC addresses from table, 5 mins)

Flooding (If a destination entry is not found in the MAC address table that matches the destination in the received frame, the switch floods the frame out all its ports except the port on which the frame entered the switch)

Selective forwarding (Forwarding a frame based on its destination MAC address)

Filtering (Un-forwarded frames based on failed CRC and Port security)



Physical Implementations of the Ethernet


Ethernet – media changes High bandwidth Ethernet cabling is used to connect

MANS and WAN


Ethernet Media Access Control Method CSMA/CD


Carrier Sense Multiple Access with Collision Detection

Listen before sending – if a device has data to send it must listen for the carrier before sending, if the carrier is detected it must wait a specified time before transmitting

Detecting a collision - If two devices or more fail to detect the carrier they will transmit at the same time. Their data will propagate across the media until a collision occurs, destroying the data. Any device detecting a collision will then transmit a Jam signal

Jam and Random Backoff - Other devices detect the Jam signal and invoke a back off signal for a random amount of time allowing the original collision to subside


Ethernet Delay (Latency, Bit time, Slot time)

Ethernet data bits don’t move in zero time

Data passing through devices experience delay, can result in collision because the receiving device transmits because it doesn’t see the frame

Bit Time = time for 1 bit to be sensed and placed on the media

Slot time determines Min size of Frame and Max media length


Ethernet and IEEE frame fields The IEEE included the extra field (Start of Frame) to allow for extra

frame size to accommodate the frames used in switching technology i.e. frames that carry a VLAN field


Layer 2 addressing is used to identify hosts The Ethernet MAC Address


Layer 2 addressing uses Hexadecimal MAC addresses use Hexadecimal Numbering


Layer 2 MAC addressing MAC addressing is used in LANs


MAC addresses 48 bits or 12 Hex digits Ethernet Unicast, Multicast and Broadcast

First 6 hex digits OUI + 6 hex digits serial number

Organizational Unique Identifier (OUI)


MAC Address communication types

Unicast – A unique frame sent form a single source to a single destination

Multicast – sent from a single host to a group (or set) of host a LAN segment. Multicast MAC addresses begin with 01-00-5E (Consumes Bandwidth)

Broadcast – sent from a single host to all hosts on the local segment or LAN on Ethernet networks the broadcast address is FF-FF-FF-FF-FF-FF in hexadecimal or 48 1’s in binary (Consumes more Bandwidth)


Address Resolution Protocol (ARP) process.

Mapping IP to MAC Addresses required to build all frames before frame is converted to bits and placed onto media


Address Resolution Protocol (ARP) process. ARP – Destinations Outside the Local Network


Address Resolution Protocol (ARP) process. ARP – Removing Address Mappings

When hosts are no longer available mappings have to aged out


Address Resolution Protocol (ARP) process. ARP Broadcasts are issued if mapping is not in table


ARP - process

ARP table maps Destination IP to MAC addresses

Can be populated Dynamically by sensing devices on network or via ARP request (Broadcasts)

Can be populated Statically not recommended as entries don’t age out

To build a Frame the table is first consulted if an entry is not found the source issues an ARP broadcast.

If the destination IP is on a different network an ARP request is sent to determine the MAC address of the gateway


Labs

9.8.1- Address Resolution Protocol- ARP (ARP command using Wireshark to examine ARP exchanges)

Ethernet

Education

Transcript of Ethernet