1. 09 Ethernet CCNA Exploration Chapter 9
31/03/2017
Ethernet
CCNA Exploration
Chapter 9
S Ward Abingdon and Witney College

2. Jane’s educated guess as to why the word ‘Ethernet’?
“Aristotle was a Greek philosopher born in 384 BC. He was one of the greatest thinkers of the world and his written works encompassed all major areas of thought. Aristotle mistakenly believed that the Earth was at the center of the universe and made up of only four elements: earth, water, air, and fire. He also thought that celestial bodies such as the sun, moon, and stars, were perfect and divine, and made of a fifth element called ETHER.” Source:

3. Ethernet Ethernet OSI Model Layers 1 (physical) and 2 (data link)
TCP/IP Model Network Access layer
Application
Presentation
Session
Transport
Network
Data link
Physical
Application
Transport
Internet
Network Access
Ethernet

4. Ethernet Most common LAN technology today
Star Topology (Physical)
Point-to-Point Topology (Logical) see p. 323
Different media (copper cable, optical fibre)
Different bandwidths
100Mbps - Fast Ethernet
1000Mbps - Gigabit Ethernet
Same addressing scheme – mac/physical
Same basic frame format

5. Ethernet History 802.2 Ethernet 802.3
First LAN was Ethernet, designed at Xerox
1980 Ethernet standard published by DIX (Digital, Intel, Xerox)
1985 IEEE modified Ethernet standard and published as 802.3
Ethernet
802.3
802.2
MAC
LLC

6. Sublayers
Logical Link Control sublayer links to upper layers; is independent of equipment
Media Access Control sublayer provides addressing; frame format, error detection, CSMA/CD
Physical Layer handles bits; puts signals on the medium, detects signals
MAC
LLC

7. Advantages of Ethernet
Simplicity and ease of maintenance
Ability to incorporate new technologies
(e.g. fiber optic, higher bandwidths)
Reliability
Low cost of installation and upgrade
100BaseT (Fast Ethernet, UTP)
1000BaseT(Gigabit Ethernet, UTP)
1000BaseX (Gigabit Ethernet, Fiber)

8. Shared Medium
Physical bus topology 10Base5 (thick coaxial cable, distance 500m) and 10Base2 (thin coaxial cable, distance 185m)
Physical star topology 10BaseT (UTP cable, distance 100m, hubs)
Collisions happen – but managed with CSMA/CD

9. Hubs and Switches “Legacy Ethernet” Collisions are managed by CSMA/CD
10Base5, 10Base2 or 10BaseT (1990) with hubs; designed to work with collisions; devices transmit at the same time
Collisions are managed by CSMA/CD
Poor performance if a lot of traffic and therefore a lot of collisions
Collisions avoided by using switches and full duplex operation

10. Hubs and Switches
Switch forwards frames only to the intended destination (known address)
- Dedicated ports
Hub forwards frames through all ports (except incoming port)
- Floods the network

11. Half Duplex Transmission
Hubs (dumb hub)
One-way traffic, i.e. walkie talkie
Necessary on a shared medium
If PC1 is transmitting, but also detects incoming signals, then there is a collision

12. Full Duplex Transmission
Switches (smart/intelligent ‘hub’)
Two way traffic, i.e. telephone
PC can transmit and receive at same time
Not on shared mediums – full bandwidth used
Switches minimize possibility of collisions
No collisions – 99.9% free

13. Review of Hubs and Switches
Shared medium
Shared bandwidth
Collisions
Point to point links
Dedicated bandwidth
Use full duplex – no collisions
Hub
Switch

14. Fast and Gigabit Ethernet
Moving from hubs to switches came higher bandwidth: 100 Mbps - Fast Ethernet (1995)
only 2 pairs of wires needed to operate, Cat5 or 5e
distance is still 100 metres
Later came 1000 Mbps - Gigabit Ethernet (1999)
all 4 pairs of wires needed to operate, Cat5e, 100m
i.e. Voice over IP (VoIP) and multimedia services
Gigabit Ethernet requires fully switched (no hubs) and full duplex operation (send and receive)

15. LAN, MAN, and WAN
Ethernet was developed for local area networks (LANs) confined to a single building or group of buildings on one site
Using fiber optics and gigabit speeds, Ethernet can be used for Metropolitan Area Networks (MANs) throughout a town or city
Ethernet can even be used over larger areas so distinction between LAN and WAN is no longer clear

16. An Ethernet Frame – 7 fields
Packet from Network layer is encapsulated
Packet
Frame header
Packet
Trailer
Preamble
Destination address
Start of frame delimiter
Source address
Length /type 7 1 6 2
Packet Data
Frame Check Seq. 4
Field sizes in bytes. Preamble and StartFD are not counted in frame size. Frame size is 64 to 1518 bytes (VLAN’s 1522b).

17. Frame Fields – see pgs.325-326 Preamble and start of frame delimiter:
acts as a wake-up call, helps synchronization, shows where frame starts
Destination Address:
MAC address of destination, 6 bytes hold 12 hexadecimal digits; switches use this address to forward frames
Source Address:
MAC address of sender, 6 bytes hold 12 hexadecimal digits; switches use this address to add entries in their lookup tables

18. Frame Fields (continued)
Length or type field:
2 bytes define exact length of data field
length or type values used
used later in CRC process
upper-layer protocol type is added
Ethernet II is frame format used in TCP/IP networks – 802.3

19. Frame Fields (continued)
Data and Pad fields
contains Layer 3 PDU = an IP packet
if packet is less than 64 bytes, then field length is made up to 64 bytes with a “pad” of zeros
Frame Check Sequence field
used for CRC (cyclic redundancy check) to detect corrupt frames
Sender=results of CRC
Receiver=generates a CRC
If calculations match – no errors
If calculations do not match – frame is dropped

20. Ethernet MAC Address Unique identification for a device (or NIC)
Burned into ROM -- copied to RAM
First 3 bytes identify manufacturer (Organizationally Unique Identifier-OUI)
Nic (device) reads destination MAC address to see if it should process frame
Switch reads destination MAC address to see where it should forward frame

21. Writing/Reading a MAC Address
Hex digits are written in different ways:
A-3C-78-00
00:05:9A:3C:78:00
0005.9A3C.7800
All of these are the same mac address
A = manufacturer’s ID, assigned by IEEE and 3C = assigned by manufacturer
(1st - ipconfig/all to get mac address)

22. Different Addresses
MAC addresses are used to identify devices within a network (switches)
MAC addresses are Layer 2 addresses in frame header
IP addresses are used to pass data between networks (routers)
IP addresses are Layer 3 addresses in packet header
The addresses identify the network and device

23. Packets on a long journey…
Packet header with IP addresses is created by source host and stays the same throughout the journey
Frame header is stripped off and replaced by each router, so MAC addresses are different for every hop of the journey (routers’ macs)
If parts of the journey are not over Ethernet, then there will be a different addressing system used (i.e. LocalTalk or IPX/SPX protocols)

24. Unicast, Multicast, Broadcast
Unicast: message sent to one particular host
it must contain the destination host’s IP address and MAC address
Broadcast: message for all hosts on a network “Host” part of IP address is all binary 1s.
i.e MAC address is all binary 1s, FF:FF:FF:FF:FF:FF in hex
Multicast: message for a group of devices using IP address range to

25. More on Collisions Ethernet originally used shared coaxial cable
If hosts transmitted at the same time, there was a collision
Later networks used hubs and UTP cable, but medium is still shared and collisions occurred

26. Hubs and Collision Domains
Collision domain – area where collisions occur
Add more hubs and PCs – collision domain gets bigger = more traffic, more collisions
Hosts connected by hubs share bandwidth
Only one PC can send

27. CSMA/CD
Carrier Sense: ‘Listen’ to see if there are signals on the cable
Multiple Access: Hosts share the same cable and all have access to it
Collision Detection: Detect and manage any collisions of signals when they occur
This is the ‘first come, first served’ method of letting hosts put signals on the medium

28. Listen for signals
Are there signals on the cable?
Yes.

29. Wait if there are signals
Wait until there are no more signals

30. Listen for signals
Are there signals on the cable now?
No.

31. Put signals on cable
Put my signals on the cable.

32. Listen for collisions: no
No collision.
All is well.
My message was sent.

33. Listen for collisions: yes
There is a collision.
Stop sending signals.
Send jamming signal.
My message is lost.

34. Listen again No signals now. Wait for a random length of time.
Send message again.

35. CSMA/CD
Collisions happen if a host transmits when there is a signal on the cable but the host does not yet know about it
Latency is the time a signal takes to travel to the far end of a cable
The longer the cable and the more intermediate devices, then more latency
All clear

36. CSMA/CD
If a host detects a collision while it is sending the first 64 bits of a frame, then CSMA/CD works and the frame will get resent later
If the host has sent 64 bits and then detects a collision, it is too late; it will not resend
Latency must be small enough so that all collisions are detected in time
This limits cable length and the number of intermediate devices

37. Some Definitions
Latency or propagation delay: the time it takes for a signal to pass from source to destination
Bit time: the time it takes for a device to put one bit on the cable (Or for the receiving device to read it)
Slot time: the time for a signal to travel to the far end of the largest allowed network; maximum time required to detect a collision

38. Interframe Spacing
The time between the end of one frame and the start of the next frame
Gives the medium a chance to stabilize
Gives devices time to process the frame
Devices wait a minimum of 96 bit times after a frame has arrived before they can send
9.6 microseconds for 10 Mbps Ethernet
0.96 microseconds for 100 Mbps Ethernet

39. How Switch Tables Work
Switch builds a switching (lookup) table matching its port numbers to the MAC addresses of devices connected to it
When a frame arrives, it reads the destination MAC address, looks it up in the table, finds the right port and forwards the frame

40. Switch Does Flooding
If the switch does not find the destination address in its table, then it floods the frame through all ports except the incoming port to find the destination address (floods the network)
Broadcast messages also get flooded in networks, i.e. address resolution protocol
IP to MAC address mapping, arp requests and
arp replies

41. Switch Learns Addresses
switch learns addresses by looking at the source MAC address of an incoming frame
then matches the address to the port where the frame came in and puts the information in its table (RAM table)
entries are time stamped and removed from the table when time runs out (“aging”)
entries can be refreshed when another frame comes in from the same host
Check out

42. Address Resolution Protocol (ARP) Table – Layer 2 protocol
A host PC wants to send a message
It knows the destination IP address and puts it in the packet header
It looks in its own ARP table and finds the corresponding MAC address
It puts the MAC address in the frame header

43. Address Resolution Protocol
A host wants to send a message
It knows the destination IP address
The destination MAC address is not in its ARP table
Host broadcasts “Calling , what is your MAC address?”
replies “My MAC address is…”
Host sends message and updates ARP table

44. Remote Addresses
Host can see that destination IP address is on another network
It finds the IP address of the default gateway
It sends an ARP request for the matching MAC address of the default gateway
Default gateway router replies and gives its own MAC address
Host sends message via router and updates its ARP table

45. Proxy ARP See http://www.visualland.net/view.php?cid=862
If a host cannot tell that the destination IP address is on another network, it will send an ARP request asking for the matching MAC address
The router will reply, giving its own MAC address
Router: "send it to me, and I'll get it to where it needs to go"
The host will send the message via the router

46. ARP Broadcasts arp is a protocol of IPv4 protocol suite
IPv6 LANs use NDP (neighbor discovery protocol) to translate 128-bit IPv6 (logical) addresses into 48-bit hardware (physical) addresses
Open command prompt window
U:\>arp/?
U:\>arp –a [look at your command output]
Interface = ??
Internet Addresses = ??

47. The End Complete Packet Tracer Labs in Chapter 9
Open cisco netacad; launch chapter 9; type in lab #’s