Transcript Chapter 1

Chapter 5:
Ethernet Part II
Assistant Professor Pongpisit Wuttidittachotti, Ph.D.
Faculty of Information Technology
King Mongkut's University of Technology North Bangkok (KMUTNB)
CCNA1-1
Chapter 9-2
Note for Instructors
• These presentations are the result of a collaboration among
the instructors at St. Clair College in Windsor, Ontario.
• Thanks must go out to Rick Graziani of Cabrillo College. His
material and additional information was used as a reference
in their creation.
• If anyone finds any errors or omissions, please let me know
at:
• [email protected].
CCNA1-2
Chapter 9-2
Another Layer of Addressing
CCNA1-3
Chapter 9-2
Ethernet
Ethernet MAC
CSMA/CD
CCNA1-4
Chapter 9-2
Ethernet MAC method
• In a shared media environment, all devices have guaranteed
access to the medium but they have no prioritized claim on it.
• If more than one device transmits simultaneously
• The physical signals collide.
• The network must recover.
• Collisions are the cost that Ethernet pays to get the low
overhead associated with each transmission.
• Ethernet uses Carrier Sense Multiple Access with Collision
Detection (CSMA/CD) to detect and handle collisions and
manage the resumption of communications.
CCNA1-5
Chapter 9-2
CSMA/CD: The Process
• To transmit, each host will listen on the media.
• If a signal from another device is present, it will wait for a
specific amount of time and listen again.
• If no signal is present, it will transmit.
CCNA1-6
Chapter 9-2
CSMA/CD: The Process
• It can happen that two devices will determine that it is safe to
transmit at exactly the same time.
• In that case, both will transmit their frame.
Collision!
CCNA1-7
Chapter 9-2
CSMA/CD: The Process
• Both devices detect the collision and send out a jamming
signal.
• The jamming signal is detected by all devices and all
devices now know that a collision has occurred on the
network.
CCNA1-8
Chapter 9-2
CSMA/CD: The Process
• The jamming signal causes each device to invoke a backoff
algorithm.
• Devices wait a random amount of time before returning to
listening mode.
• The random time ensures that the original devices that
caused the collision won’t repeat it.
CCNA1-9
Chapter 9-2
CSMA/CD: The Process
CCNA1-10
Chapter 9-2
Ethernet Timing
• Latency:
• Each transmission encounters a certain amount of delay
before reaching the destination.
• Every network device encountered in the path adds to the
delay or increases the latency of the transmission.
• Increases the chance of collisions.
CCNA1-11
Chapter 9-2
Ethernet Timing
• Timing and Synchronization:
• The 8 byte (64 bit) preamble is transmitted at the start of
the frame.
CCNA1-12
Chapter 9-2
Ethernet Timing
• Jam Signal:
• As soon as a collision is detected, the sending devices
transmit a 32-bit "jam" signal - simply a repeating
1, 0, 1, 0 pattern.
• Less than 64 bytes (runt).
• Avoids detection of the jam signal as a frame.
CCNA1-13
Chapter 9-2
Ethernet Timing
• Backoff Timing:
• If a collision occurs, all devices wait a random amount of
time before listening again.
• If media congestion results in the MAC layer unable to
send the frame after 16 attempts, it gives up and
generates an error to the Network layer.
CCNA1-14
Chapter 9-2
Ethernet
Ethernet Physical Layer
CCNA1-15
Chapter 9-2
Ethernet Physical Layer
• The differences between standard Ethernet, Fast Ethernet,
Gigabit Ethernet, and 10 Gigabit Ethernet occur at the
Physical layer.
• Ethernet is covered by the IEEE 802.3 standards.
• Four data rates over fiber and twisted pair:
• 10 Mbps - 10Base-T Ethernet
• 100 Mbps - Fast Ethernet
• 1000 Mbps - Gigabit Ethernet
• 10 Gbps - 10 Gigabit Ethernet
CCNA1-16
Chapter 9-2
Ethernet Physical Layer
CCNA1-17
Ethernet Type
Bandwidth
Cable Type
Maximum
Distance (m)
10BASE5
10 Mbps
Thick Coax
500
10BASE2
10 Mbps
Thin Coax
185
10BASE-T
10 Mbps
Cat3/Cat5 UTP
100
100BASE-TX
100 Mbps
Cat5 UTP
100
100BASE-FX
100 Mbps
MM or SM Fiber
400/2000
1000BASE-T
1 Gbps
Cat5e UTP
100
1000BASE-TX
1 Gbps
Cat6 UTP
100
1000BASE-SX
1 Gbps
MM Fiber
550
1000BASE-LX
1 Gbps
SM Fiber
2000
10GBASE-T
10 Gbps
Cat6a/Cat7 UTP
100
10GBASE-LX4
10 Gbps
MM Fiber
300
10GBASE-LX4
10Gbps
SM Fiber
10,000
Chapter 9-2
10 Mbps Ethernet
• 10BASE5 using Thicknet coaxial cable.
• 10BASE2 using Thinnet coaxial cable.
• Early implementations and no longer supported under
802.3 standard.
CCNA1-18
Chapter 9-2
10 Mbps Ethernet
• 10BASE-T using Cat3/Cat5 unshielded twisted-pair cable.
• Considered to be classic Ethernet.
• Physical star topology.
• Up to 100 meters in length.
• 10BASE-T uses two pairs of a four-pair cable terminated
with an RJ-45 jack.
• Pins 1 and 2 – Transmit
Pins 3 and 6 - Receive
• Generally not chosen for new LAN installations.
• There are still many 10BASE-T Ethernet networks in
existence today.
• Links connected to a switch can support either halfduplex or full-duplex operation.
CCNA1-19
Chapter 9-2
100 Mbps: Fast Ethernet
• 100 Mbps Ethernet implemented using twisted-pair copper
wire or fiber media.
• 100BASE-TX using Cat5 or later UTP
• 100BASE-FX using fiber-optic cable
• Because the higher frequency signals used in Fast
Ethernet are more susceptible to noise, two separate
encoding steps are used by 100-Mbps Ethernet to
enhance signal integrity.
CCNA1-20
Chapter 9-2
100 Mbps: Fast Ethernet
• 100BASE-TX:
• Either two pairs of Category 5 UTP copper wire or two
strands of optical fiber.
• Uses the same two pairs of UTP as 10BASE-T.
• Pins 1 and 2 – Transmit
Pins 3 and 6 – Receive
• Requires Category 5 or later UTP.
• Physical star topology.
• 100BASE-TX networks typically use a switch at the
center of the star instead of a hub.
CCNA1-21
Chapter 9-2
100 Mbps: Fast Ethernet
• 100BASE-FX:
• Uses the same signaling procedure as 100BASE-TX.
• Uses optical fiber media rather than UTP copper.
• 100BASE-FX uses Low Cost Fiber Interface Connectors
(commonly called the duplex SC connector).
• Fiber implementations are point-to-point connections:
• Two computers
• A computer and a switch
• Between two switches.
CCNA1-22
Chapter 9-2
1000 Mbps: Gigabit Ethernet
• The development of Gigabit Ethernet standards resulted in
specifications for UTP copper, single-mode fiber, and
multimode fiber.
• With signals occurring in less time, the bits become more
susceptible to noise, and therefore timing is critical.
• Gigabit Ethernet uses two separate encoding steps.
• More efficient to use codes that represent the binary bit
stream.
• Synchronization.
• Efficient usage of bandwidth.
• Improved tolerance to noise.
CCNA1-23
Chapter 9-2
1000 Mbps: Gigabit Ethernet
• 1000BASE-T Ethernet:
• Full-duplex transmission using all four pairs in Category 5
or later UTP cable.
• Gigabit Ethernet over copper wire enables an increase
from 100 Mbps per wire pair to 125 Mbps per wire pair.
• 500 Mbps for the four pairs.
• Each wire pair signals in full duplex, doubling the 500
Mbps to 1000 Mbps.
CCNA1-24
Chapter 9-2
1000 Mbps: Gigabit Ethernet
• 1000BASE-T Ethernet:
• Allows the transmission and reception of data in both
directions - on the same wire and at the same time.
• This traffic flow creates permanent collisions on the wire
pairs.
• The hybrid circuits detecting the signals use sophisticated
techniques such as:
• Echo cancellation.
• Layer 1 Forward Error Correction (FEC).
• Selection of varying voltage levels.
CCNA1-25
Chapter 9-2
1000 Mbps: Gigabit Ethernet
• 1000BASE-SX and 1000BASE-LX:
• Advantages over UTP:
• Noise immunity, small physical size and increased
unrepeated distances and bandwidth.
• Support full-duplex binary transmission at 1250 Mbps
over two strands of optical fiber.
CCNA1-26
Chapter 9-2
Ethernet
Hubs and Switches
CCNA1-27
Chapter 9-2
Hubs and Switches
• Collision Domain:
• The area of a network where collisions can occur.
• Includes a hub and all connected devices.
• Each port on a switch is considered a separate collision
domain even if there is only one device attached to the
port.
• Broadcast Domain:
• The area of a network where connected devices can
receive a broadcast.
• Usually includes Layer 1 and 2 devices.
• A collection of collision domains.
• A router (Layer 3 device) is the usual boundary since
routers block broadcasts.
Chapter 9-2
CCNA1-28
Legacy Ethernet: Using Hubs
CCNA1-29
Chapter 9-2
Legacy Ethernet: Using Hubs
• So, what does a
hub do when it
receives
information?
• Remember, a hub
is nothing more
than a multiport
repeater.
CCNA1-30
Chapter 9-2
Legacy Ethernet: Using Hubs
• The hub will flood it out all
ports except for the incoming
port.
• A hub is a Layer 1 device and
does NOT look at Layer 2
addresses, so it is fast in
transmitting data.
• A hub or series of hubs is
called a single Collision
Domain.
CCNA1-31
Chapter 9-2
Legacy Ethernet: Using Hubs
Wasted
Bandwidth
CCNA1-32
• Disadvantages:
• Collision Domains
• Wasted Bandwidth
• All ports of the hub share
the total available
bandwidth.
• Limited Scalability
• Increased Latency
Chapter 9-2
Legacy Ethernet: Using Hubs
Where is the
collision
domain?
CCNA1-33
Chapter 9-2
Legacy Ethernet: Using Hubs
• When a node wishes to
communicate with ALL hosts
on the network, it sends a
BROADCAST frame with a
destination MAC address of
0xFFFFFFFFFFFF.
• All nodes on the network
recognize that they should look
at the contents of the frame.
• Broadcasts are used in the
Address Resolution Protocol
(ARP), the Dynamic Host
Configuration Protocol (DHCP)
and other areas. (More Later!)
CCNA1-34
Chapter 9-2
Legacy Ethernet: Using Hubs
• Broadcasts cannot be entirely avoided if you are to have a
dynamic network.
• However, too many broadcasts can create a lot of
unnecessary traffic on a network and they should be
minimized as much as possible.
• Layer 1 and Layer 2 devices (repeaters, hubs, bridges and
switches) must forward a broadcast.
• Layer 1 devices have no choice because they do not look
at MAC addresses.
• Layer 2 devices have no other choice because they
cannot learn the 0xFFFFFFFFFFFF address.
CCNA1-35
Chapter 9-2
Legacy Ethernet: Using Hubs
• Host 1111 sends a broadcast
to all nodes on the network.
• All hosts recognize the MAC
broadcast address and act
on the information in the
frame.
• Where is the Broadcast
Domain?
CCNA1-36
Chapter 9-2
Ethernet: Using Switches
CCNA1-37
Chapter 9-2
Ethernet: Using Switches
• Switches are also
known as learning
bridges or learning
switches.
• A switch has a source
address table in cache
(RAM) where it stores
source MAC addresses
for each port.
CCNA1-38
Chapter 9-2
Ethernet: Using Switches
• Switch receives an
Ethernet frame.
• Searches the source
address table for the
destination MAC
address.
CCNA1-39
Chapter 9-2
Ethernet: Using Switches
• If it finds a match, it
forwards the frame by
only sending it out that
port (selective
forwarding).
• If the destination
address is not in the
table, it floods it out all
ports.
CCNA1-40
Chapter 9-2
How does a switch learn an address?
• First, the switch will see if
the SA (1111) is in it’s
table.
• If it is, it resets a timer.
• If it is NOT in the table it
adds it, with the port
number.
• Next the switch will flood
the frame out all other
ports, because the DA is
not in the source address
table.
CCNA1-41
Chapter 9-2
How does a switch learn an address?
• Most communications
involve some sort of
client-server relationship
or exchange of
information.
• Now 3333 sends data
back to 1111.
• The switch sees if it has
the SA stored. It does
NOT so it adds it.
• Next, it checks the DA
and sends it out port 1.
CCNA1-42
Chapter 9-2
How does a switch learn an address?
• Now, with both MAC
addresses in the table,
any information between
1111 and 3333 can be
sent (selectively
forwarded) out the
appropriate port.
CCNA1-43
Chapter 9-2
Multiple Transmissions - No Collisions
• Unlike a hub, a collision
does NOT occur, which
would cause the two PCs
to have to retransmit the
frames.
• The switch buffers the
frames and sends them
out port #6 one at a time.
• The sending PCs have
no idea that there was
another PC wanting to
send to the same
destination.
CCNA1-44
Chapter 9-2
Collision Domains
• When there is only one
device on a switch port,
the collision domain is
only between the PC
and the switch.
• With a full-duplex PC
and switch port, there
will be no collision,
since the devices and
the medium can send
and receive at the
same time.
CCNA1-45
Chapter 9-2
What happens here?
• Notice the Source
Address Table
has multiple
entries for port 1.
• The switch
selectively
forwards the
frame out port #1.
CCNA1-46
Chapter 9-2
What happens here?
• But the hub is
only a layer 1
device, so a hub
floods it out all
ports.
How many
collision
domains exist?
CCNA1-47
Chapter 9-2
What happens here?
• Notice the Source
Address Table
has multiple
entries for port 1.
• Does that matter
in this case?
CCNA1-48
Chapter 9-2
What happens here?
Broadcast
Domain
CCNA1-49
• The switch is a
Layer 2 device so
the broadcast
frame is sent out
all of the ports.
• The hubs are
Layer 1 devices
and also forward
the frame.
Chapter 9-2
Switches: Five Basic Operations
• Learning:
• Adds source MAC address/port to the table.
• Aging:
• Addresses will be cleared after a specific length of
inactivity.
• Flooding:
• Sends a frame out all ports if the SA is not in the table.
• Forwarding:
• Sends a frame out the proper port on a match.
• Filtering:
• Will not forward a frame out the received port.
• Drops corrupt frames.
CCNA1-50
Chapter 9-2
Ethernet
Address Resolution Protocol (ARP)
CCNA1-51
Chapter 9-2
Resolving IPv4 addresses to MAC Addresses
• Two address types:
• MAC address:
• Physical address of
the host
• Burned in to the NIC
• Layer 2 address
• Network Address:
• Logical address of the
host
• Assigned by network
administrator
• Layer 3 address
CCNA1-52
Chapter 9-2
Resolving IPv4 addresses to MAC Addresses
• Physical (MAC):
• The physical address uniquely identifies the host from all
other hosts on all other networks at Layer 2.
• This is the address that is absolutely necessary to get the
information into the host. The IP address by itself won't
accomplish that.
CCNA1-53
Chapter 9-2
Resolving IPv4 addresses to MAC Addresses
• Logical (IP):
• The logical address uniquely identifies the host and the
network to which it belongs at Layer 3.
• Routers base their decisions on the IP address when
determining the best path for the packet.
CCNA1-54
Chapter 9-2
Resolving IPv4 addresses to MAC Addresses
• So…..How do we obtain both addresses to build the packets
and frames?
• MAC Address: ARP (Address Resolution Protocol)
• IP Address: Static and Dynamic
CCNA1-55
Chapter 9-2
Resolving IPv4 addresses to MAC Addresses
• Address Resolution Protocol (ARP):
• Binds an IP address to a MAC address.
• Devices, (hosts, routers, servers, etc.) use IP addresses
to reach other devices within their own network/subnet or
across different networks/subnets.
• The Layer 3 IP addresses in the packet consist of both
the original source and the final destination address.
• Once the packet is sent, these addresses do not change.
CCNA1-56
Chapter 9-2
Resolving IPv4 addresses to MAC Addresses
• Address Resolution Protocol (ARP):
• Data Link layer addresses, such as Ethernet MAC
addresses are used to get the IP packet from one hop to
the next.
• If the sender and the receiver are on different networks
(or subnets) the data link address in the data link frame
will be modified to reflect the new data link address
source and destination.
• Again,
• The IP addresses in the Layer 3 packet do not change.
CCNA1-57
Chapter 9-2
Resolving IPv4 addresses to MAC Addresses
• Address Resolution Protocol (ARP):
• Why do devices need to map a MAC address to an IP
address?
• There is no built-in connection or relationship between
the MAC (physical) address and the assigned IP
(logical) address.
• IP hosts and routers use Address Resolution Protocol
(ARP) to resolve a known IP address to the
corresponding MAC address.
CCNA1-58
Chapter 9-2
The ARP Process: The Same Subnet
CCNA1-59
Chapter 9-2
The ARP Process: Different Subnet
CCNA1-60
Chapter 9-2
The ARP Process: Removing Mappings
CCNA1-61
Chapter 9-2
The ARP Process: Issues
• Overhead on the Media:
• As a broadcast frame, an ARP request is received and
processed by every device on the local network.
• Usually minimal but can be significant if all users and
devices were to power up and start using network
services at the same time.
CCNA1-62
Chapter 9-2
The ARP Process: Issues
• Security:
• ARP spoofing, or ARP poisoning, is a technique used by
an attacker to inject the wrong MAC address association
into a network by issuing fake ARP requests.
• An attacker forges the MAC address of a device and then
frames can be sent to the wrong destination.
CCNA1-63
Chapter 9-2
The ARP Process: One Final Note
• Ethernet encapsulates an ARP packet in the same manner
as an IP packet as it travels on the physical network from one
computer to another.
• ARP is a separate protocol residing at Layer 3 of the OSI
Model and does not use the services of IP since ARP
requests are never routed.
CCNA1-64
Chapter 9-2