CCNA 1 Module 8

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CCNA 1 Module 8

• Ethernet Switching
• By Larry Twigg
• 11/21/05

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8.1.1 Layer 2 Bridging

• Bridging Table is empty at start up
• Bridge adds source MAC address and the port it
  came to the bridge on
• Destination MAC address is checked
• In the table sent out that port
• Not in the table sent out all ports
• In table but from receiving port so it is not
  forwarded.
• See Diagram in curriculum for ex.

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8.1.2 Layer 2 Switching

• A switch is a multiport bridge
• Bridge and switches create collision domains
• Bridges and switches do not affect broadcast
  domains
• The CAM (content addressable memory) holds the
  switching table.

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8.1.3 Switch operation

• Switch operates like a bridge just more ports.
• Microsegmentation - switch is used to divide
  devices into their own collision domains
• Switches enable full duplex
• In theory bandwidth can be doubled

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8.1.3 cont.

• CAM is a hardware technology causing faster
  delivery of data
• CAM is an ASIC (Application specific integrated
  circuit)

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8.1.4 Latency

• Latency is the delay between the time a frame
  begins to leave the source device and when the
  first part of the frame reaches its destination.

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Causes of Latency

• Media delays - finite speeds
• Circuit delays - electronics
• Software delays switching decisions
• Frame structure must read at least the
  destination address

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8.1.5 Two Switching Methods
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Store-and-forward
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Cut-through
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Frame Transmission Modes
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Switch Modes

• Synchronous switching
• The source port and destination port must be
  operating at the same bit rate
• Asynchronous switching
• The bit rates are not the same
• The frame must be stored at one bit rate before
  it is sent out at the other bit rate
• Store-and-forward must be used  

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8.1.6 STP

• Switching loops can lead to broadcast storms that
  will overwhelm a network.
• To counteract loops, switches are provided with
  the Spanning-Tree Protocol (STP)
• Switches in a LAN using STP
• Send Bridge Protocol Data Units (BPDUs) out all
  its ports
• Lets other switches know of its existence
• Elect a root bridge (switch) for the network
• Switches use the Spanning-Tree Algorithm (STA) to
  resolve and shut down the redundant paths

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STP

• Each port using Spanning-Tree Protocol is in one
  of the following five states
• Blocking
• Listening
• Learning
• Forwarding
• Disabled

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STP

• A port moves through five states as follows
• From initialization to blocking
• From blocking to listening or to disabled
• From listening to learning or to disabled
• From learning to forwarding or to disabled
• From forwarding to disabled
• Resolving and eliminating loops creates a logical
  hierarchical tree with no loops
• The alternate paths are available if needed

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8.2.1 Shared Media Environment

• Shared media environment
• multiple hosts have access to the same medium
• Extended shared media environment
• a networking device extends the environment to
  accommodate multiple access or longer cable
  distances
• Point-to-point network environment
• one device is connected to only one other device
  (ex. dialup network connections)

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8.2.2 Collision Domains

• Collision Domains
• Connected physical network segments where
  collisions can occur
• Collisions cause
• The network to be inefficient
• Transmissions to stops for a period of time

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Collision Domains

• The types of devices that interconnect the media
  segments define collision domains
• Classified as OSI Layer 1, 2 or 3 devices
• Layer 1 devices do not break up collision domains
• Layer 2 and Layer 3 devices break up collision
  domains
• Increasing the number of collision domains is
  known as segmentation

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Repeater Rule

• Four repeater rule
• No more than four repeaters between any two
  computers
• Contributing Factors
• Repeater latency
• Propagation delay
• NIC latency
• Late collision frames add delay that is referred
  to as consumption delay

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8.2.3 Layer 1 Devices

• Layer 1 devices
• repeaters and hubs
• Extend collision domains
• Primary function is extending cable segments
• Additional hosts increase the amount of traffic
• More traffic greater chances of collisions
• This results in diminished performance

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Layer 2 Devices

• Layer 2 devices
• Bridges and Switches
• Segments collision domains
• Controls frame propagation using the MAC address
• Tracks the MAC addresses and segment they are on

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Layer 3 Devices

• Layer 3 devices
• Routers
• Do not forward collisions
• Breaks up collision domains
• Broadcast domains are controlled

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8.2.4 Layer 2 Broadcasts

• Ethernet Broadcasts
• When a node needs to communicate with all hosts
  on the network
• A broadcast frame with a destination MAC address
  0xFFFFFFFFFFFF is sent
• The network interface card (NIC) of every host
  must respond

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Layer 2 Broadcasts

• Layer 2 devices must flood all broadcast and
  multicast traffic
• Broadcast Radiation
• The accumulation of broadcast and multicast
  traffic from each device
• Broadcast storm
• Circulation of broadcast radiation that saturates
  the network
• There is no bandwidth left for application data

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Layer 2 Broadcasts

• The three sources of broadcasts and multicasts
• Workstations
• Routers
• Multicast Applications

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8.2.5 Broadcast Domain

• Broadcast Domain
• A grouping of collision domains
• All the nodes that are a part of that network
  segment bounded by a layer three device
• Broadcasts have to be controlled at Layer 3
  devices
• Layer 2 and Layer 1 devices do not control
  broadcasts

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8.2.6 Data Flow

• Collision and Broadcast Domains
• How data frames propagate through a network
• The movement of data through Layer 1, 2 and 3
  devices
• How data must be encapsulated

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Data Flow

• Layer 2 devices filter data frames based on the
  destination MAC address
• A Layer 2 device will forward the frame unless
  something prevents it from doing so
• Layer 3 devices filter data packets based on IP
  destination address
• A Layer 3 device will not forward the frame
  unless it has to
• Layer 3 device creates multiple collision and
  broadcast domains