LAN Hardware

1
LAN Hardware

• Brad Guenther
• EECE 542

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Terminology

• Collision Domain An area of a network that
  allows only one device to talk at a time.
• Broadcast A message that uses all ones in the
  destination address field (layer two and or layer
  three). That is received by any station that can
  see it.
• Broadcast Domain An area of a network containing
  nodes that can send and receive broadcast
  messages to/from one another.
• Managed vs. Unmanaged Device Can the devices
  configuration be modified remotely

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Layer 1 (Physical) Devices

• Repeater, Hub,MAU (Token Ring),Concentrators
  (FDDI)
• Active vs. Passive
• Strengthen and retime signals (active only)
• No forwarding decisions
• Add additional nodes
• Transceiver
• Cables

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Layer 1 Devices (cont)

• Extends max. cable length (to a point)
• Extend both collision and broadcast domains
• Half Duplex Only

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Layer 2 (Data Link) Devices

• Bridge
• Typically 2 ports
• Make forwarding decisions based on layer 2 MAC
  addresses
• Builds a bridging table by examining incoming
  frames
• Switch
• Multiport Bridge
• NIC

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Layer 2 Devices (cont)

• Break up collision domains (segmentation,mircosegm
  entation)
• Extend broadcast domains
• Are used more and more to replace hubs
• Can translate between layer two
  protocols/encapsulations
• Half or full duplex operation
• Builds virtual point to point connections

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Layer 2 Devices (cont)

• Build and maintain tables based on layer two
  addresses
• 5 States blocking, listening, learning,
  forwarding, disabled
• Addresses represent actual end nodes
• Tables are typically built on the fly and are
  not built manually or stored in any sort of
  permanent/non-volatile memory
• Addresses age out of memory potential problem!
• Cut through vs. Store Forward

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Loops and Redundant Paths

• Problem of loops Redundant links are nice, but
  bridging / switching loops can cause frames to
  circulate indefinitely.
• Simple example two bridges providing redundant
  links between 2 network segments

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Solution Spanning Tree

• Spanning tree algorithm allows redundant links
  without bridging or switching loops
• Dynamically blocks redundant links until they are
  needed.
• Leaves only one active path between layer two
  devices.
• Uses special frames called BPDUs

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Spanning Tree Algorithm

• Step 1 Identify the root bridge.
• Only one root bridge per tree (broadcast domain)
• Usually based on the numerically smallest number
  bridge/switch port. Arbitrary ID.
• Upon receiving a BPDU, a bridge compares the
  source ID with its own.
• If its ID is smaller than the senders, it
  discards BPDU
• If its ID is larger, increment root path cost
  and forward frame and stop sending its own BPDU

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Spanning Tree (cont)

• Step 2 Find Root port
• After root bridge is established bridges examine
  the root cost of every BPDU for every port.
• Root port is the one with the lowest cost
• Bridge should have one and only one root port
• Root bridge/switch does not have a root port

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Spanning Tree (cont)

• Step 3 Choose Designated Bridges
• After root port is identified, all bridges
  attached to a given segment must determine which
  one provides the path to that segment
• The port that provides that connection is known
  as the designated port and the bridge that it is
  on is the designated bridge for that segment

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Spanning Tree (cont)

• If a bridge or switch is connected to multiple
  segments, it can have multiple designated ports
• Ports that are not designated are put in blocking
  mode (disabled until needed).

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Spanning Tree (cont)

• 5 states Blocking, Listening, Learning,
  Forwarding, Disabled
• Handles topology changes using TCN TCAs
• Disable spanning tree on ports connected directly
  to workstations! (Why?)
• Other variants exist RSTP (Rapid Spanning Tree
  Protocol), RPR (Resilient Packet Ring), and
  proprietary extensions

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More Complex Example

• 5 segments, 4 bridges, 1 switch
• 1.) ID Root Bridge/Switch
• 2.) ID Root ports
• 3.) ID Designated ports
• 4.) Set everything else to blocking
• What happens when a redundant link goes down?

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Layer 3 (Network) Devices

• Routers (Firewalls)
• Make forwarding decisions based on layer three
  addresses
• Addresses refer to the network of the destination
  rather than the destination node itself
• Tables may be maintained dynamically or
  statically (Benefits Drawbacks To Both)
• Static maps often stored in non-volatile memory

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Layer 3 Devices (cont)

• Deal with loops in more complicated fashion
• Typically much smarter than switches
• Use routing protocols to move data in routed
  protocols
• Break up broadcast domains