Cisco CCNA 3.0

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Cisco CCNA 3.0

• Semester 1 Chapter 6
• Part 1 Ethernet Fundamentals
• Karl Wick SUNY Ulster

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Equal Access For All

• Understanding how network devices gain access to
  the network media is essential for understanding
  and troubleshooting the operation of the entire
  network.
• Ethernet LANS use shared bandwidth.
• Data collisions are dealt with using a process
  called CSMA-CD.

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Timeline

• Early 1970s Univ. of Hawaii Alohanet
• 1980 Ethernet by DEC, Intel and Xerox
• 1985 IEEE 802.3 Standard for LANS
• 1995 100Mbps Ethernet
• 1998/9 Gigabit Ethernet
• All are backward compatible.

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Layer 2 is the Data Link layer

• The data link layer provides transit of data
  across a physical link.
• The data link layer works with source and
  destination MAC (Media Access Control) addresses.
• The data link layer creates frames from packets
  by adding headers and trailers.

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Layer 2, the data link layer

• Communicates with upper layers using Logical Link
  Control.
• Decides which computer can use the media at a
  given time. (CSMA-CD)
• Uses MAC addresses to get data to its intended
  destination.

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The IEEE

• Defines network standards.
• These standards involve layer 1 and 2
• IEEE divides layer 2 into two parts
• Logical Link Control - communicates up
• Media Access Control - control of layer 1

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OSI Model vs IEEE Standard
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The IEEE Standard (LLC)

• Defined by the IEEE 802.2 standard
• Functions independently from the technologies of
  the physical layer.
• Communicates between logical layer 3 and physical
  layer 1
• Enables multiple upper level protocols to share a
  physical link.

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The IEEE Standard (MAC)

• Deals with the protocols that a host must follow
  to access the physical media. (Which can be
  ethernet , token ring, FDDI, etc.)
• Deals with the actual physical technology.
• Creates orderly access to the medium.

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IEEE Specifications (Services)
Layer 2 LLC Layer 2 MAC Layer 1
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Layer 1 Technologies
Layer 2 LLC Layer 2 MAC Layer 1
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Sub-layers of Layer 2

• Data link sub-layers contribute significantly to
  technology compatibility and computer
  communication.
• The MAC sub-layer is concerned with the physical
  components that will be used to communicate the
  information.
• The Logical Link Control (LLC) sub-layer remains
  relatively independent of the physical equipment
  that will be used for the communication process.

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A Expanded Seven Layer Model
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Important Layer 2 concepts

• Communicates with upper layers through LLC.
• Uses a Flat Addressing scheme (unique addresses)
• Uses framing to organize data.
• Uses Media Access Control to handle contention
  for shared media (the network hardware).
• Think LLC, Naming, Framing, MAC

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MAC Addressing
Vendor Code Serial Number
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MAC Address

• Each MAC address in the world is supposed to be
  unique.
• MAC addresses are usually written as hexidecimal
  numbers.
• The first six digits identify the vendor
• The last six digits are assigned by the vendor.

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MAC Address

• The NIC uses the MAC address to assess whether an
  incoming message should be passed onto the upper
  layers of the OSI model.
• The NIC makes this assessment without using CPU
  processing time, enabling better communication
  times on an Ethernet network.
• An address matching either its own OR the
  broadcast address will pass upward.

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Broadcast MAC Address

• FF FF FF FF FF FF
• (48 binary ones)
• Processed by all NICs.

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Media Access Control (MAC)

• Deterministic taking turns (Token Ring, FDDI)
• A special data token is passed around the hosts
• When a host holds the token it can send data
• Non-deterministic first come, first served
• Carrier sense, multiple access, collision detect
• CSMA/CD uses jam signal, variable backoff

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Ethernet and 802.3 LANs

• Are broadcast media. Every station sees every
  frame.
• Each station must examine each frame.
• Frames for the station get passed upward to the
  next layer.
• Frames for other stations are ignored.
• Every station reads broadcast frames (with
  special address of FF FF FF FF FF FF).

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Flat Addressing

• Each station has a unique fixed address.
• This does not scale well to large networks

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Framing - Extra Information Carried

• Source and Destination Addresses
• When communication starts and ends
• Whose turn it is to talk
• Error notices
• A frame is the layer 2 protocol data unit.

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Generic Frame Format
MAC Address
Optional
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Calculating the FCS

• Cyclic Redundancy Check (CRC) performs
  calculations on the data.
• Two-dimensional parity adds an 9th bit that
  makes an 8 bit sequence have an odd or even
  number of binary 1s.
• Internet checksum adds the values of all of the
  data bits to arrive at a sum.

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802.3 and Ethernet Frames
(All sizes in bytes)
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Fields permitted or required in an 802.3 Frame

• Preamble
• Start Frame Delimiter
• Destination Address
• Source Address
• Length/Type
• Data and Pad
• FCS
• Extension

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Preamble and Start

• The Preamble is an alternating pattern of ones
  and zeroes used for timing synchronization in the
  asynchronous 10 Mbps and slower implementations
  of Ethernet. Faster versions of Ethernet are
  synchronous, and this timing information is
  redundant but retained for compatibility.
• A Start Frame Delimiter consists of a one-octet
  field that marks the end of the timing
  information, and contains the bit sequence
  10101011.

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Addresses

• The Destination Address field contains the MAC
  destination address. The destination address can
  be unicast, multicast (group), or broadcast (all
  nodes).
• The Source Address field contains the MAC source
  address. The source address is generally the
  unicast address of the transmitting Ethernet
  node.
• There are, however, an increasing number of
  virtual protocols in use that use and sometimes
  share a specific source MAC address to identify
  the virtual entity.

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Length or Type

• If the value is less than 1536 decimal, (060016)
  then the value indicates length. If the value is
  equal to or greater than 1536 decimal (0600H),
  the value indicates that the type and contents of
  the Data field are decoded per the protocol
  indicated.
• The length interpretation is used where the LLC
  Layer provides the protocol identification and
  indicates the number of bytes of data that
  follows this field.
• The type value specifies the upper-layer protocol
  to receive the data after Ethernet processing is
  completed.

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Data and Padding

• The Data and Pad field may be of any length that
  does not cause the frame to exceed the maximum
  frame size.
• The maximum transmission unit (MTU) for Ethernet
  is 1500 octets, so the data should not exceed
  that size.
• The content of this field is unspecified. An
  unspecified pad is inserted immediately after the
  user data when there is not enough user data for
  the frame to meet the minimum frame length.
• Ethernet requires that the frame be not less than
  46 octets or more than 1518 octets.

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Frame Check Sequence

• A FCS contains a four byte CRC value that is
  created by the sending device and is recalculated
  by the receiving device to check for damaged
  frames.
• Since the corruption of a single bit anywhere
  from the beginning of the Destination Address
  through the end of the FCS field will cause the
  checksum to be different, the coverage of the FCS
  includes itself.
• It is not possible to distinguish between
  corruption of the FCS itself and corruption of
  any preceding field used in the calculation.

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Part 2

• Ethernet Operations

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CSMA-CDOperation
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CSMA-CD Flowchart
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Propagation and Timing

• The speed of a single bit traveling along a CAT 5
  cable is about 20cm or 8 inches per nanosecond.
• It therefore takes about 500 nanoseconds to
  traverse a 100 meter long cable.
• Transmit time (next slide) can be far shorter
  than this.
• For Half duplex, collisions must be detected
  before a full frame is sent.

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Transmit Time

• The time it takes for the NIC to put a single bit
  onto the wire.
• To allow 1000-Mbps Ethernet to operate in half
  duplex, an extension field was added when sending
  small frames, purely to keep the transmitter busy
  long enough for a collision fragment to return.

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Transmit Time

• A minimum sized 46 octet frame has 46 8 370
  bits. For gigabit 370 Bits _at_ 1nsec 370 ns.
  This is shorter than the potential propagation
  delay on a 100 meter line. (500nS)
• The receiving station ignores the extension.
• Half duplex is not allowed on 10Gbps connections.

Half Duplex not allowed
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Interframe Spacing

• The minimum spacing between two non-colliding
  frames is also called the interframe spacing.
• This is measured from the last bit of the FCS
  field of the first frame to the first bit of the
  preamble of the second frame.
• After a frame has been sent, all stations are
  required to wait a minimum of 96 bit-times (0.96
  microseconds for 100-Mbps Ethernet ) before any
  station may legally transmit the next frame.

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Collision!

• The first station detecting a collision
  immediately sends a jam signal. All other
  involved stations repeat this signal for a short
  time and data transmission stops.
• The line is then cleared and the two (or more)
  stations directly involved in the collision wait
  for a pseudo-random amount of time before trying
  to send again.

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Backoff Time

• After a collision occurs and all stations wait
  the full interframe spacing before attempting to
  use the line.
• The stations that collided must wait an
  additional and progressively longer period of
  time before attempting to retransmit the collided
  frame.
• This is the Backoff time and is a random number
  with a maximum possible value that increases if
  the stations collide again.

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Worst Case Scenario

• If the MAC layer is unable to send the frame
  after sixteen attempts, it gives up and generates
  an error to the network layer.
• Such an occurrence is fairly rare and would
  happen only under extremely heavy network loads,
  or when a physical problem exists on the network.

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Types of Collisions
can
(Full duplex) Or when abnormally high voltage
levels occur (half duplex)
Or switch
Late collisions are not noticed by the NIC and
resending of missing data must be handled by
upper layers in the model.
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Ethernet Errors

• Collision (caused) runt Simultaneous
  transmission occurring before slot time has
  elapsed Undersized frame.
• Late collision Simultaneous transmission
  occurring after slot time has elapsed
• Jabber, long frame and range errors Excessively
  or illegally long transmission  - AKA Giant
• Short frame, collision fragment or runt
  Illegally short transmission
• FCS error Corrupted transmission
• Alignment error Insufficient or excessive
  number of bits transmitted
• Range error Actual and reported number of
  octets in frame do not match
• Ghost or jabber Unusually long Preamble or Jam
  event

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Solution to 6.2.7 Activity
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More About Errors

• Alignment Error Frame does not end on an octet
  boundary. Collision or bad drivers.
• Range Error Number of octets in the data field
  does not match the number specified in the length
  field.
• FCS Error At least one bit of transmission has
  changed.
• Ghosting Error Noise on the line. Ground loops
  and wiring problems.

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Ethernet auto-negotiation

• This process defines how two link partners may
  automatically negotiate a configuration offering
  the best common performance level.
• Normal Link Pulse A pulse sent about every 16
  milliseconds, whenever a station is not engaged
  in transmitting a message.
• Auto-Negotiation is accomplished by transmitting
  a burst of 10BASE-T Link Pulses from each of the
  two link partners
• Called FLP (Fast link pulse).

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Ethernet auto-negotiation

• The burst communicates the capabilities of the
  transmitting station to its link partner.
• After both stations have interpreted what the
  other partner is offering, both switch to the
  highest performance common configuration and
  establish a link at that speed.
• If anything interrupts communications and the
  link is lost, the two link partners first attempt
  to link again at the last negotiated speed.
• If that fails, or if it has been too long since
  the link was lost, the Auto-Negotiation process
  starts over.

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Link establishment and full and half duplex

• Auto-Negotiation is optional for most Ethernet
  implementations.
• Speed and duplex setting may be forced by the
  administrator.
• In the event that link partners are capable of
  sharing more than one common technology, refer to
  the list at the right for order of preference.

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Bonus Stuff
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Quiz Question 3

• Which of the following is an example of
  non-deterministic LAN technology?
• Ethernet
• FDDI
• IEEE 802.5
• Token Ring
• By extension The other three ARE deterministic
  technologies.

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General Info

• When Cisco prints a number like
• 0X600 or
• 0X2102
• Just ignore the x. It signifies a hexadecimal
  value instead of a decimal value.

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Layer Two Components
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Common Topologies
star, of course
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The End