Local Area Network LAN Switching

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Spanning-Tree Protocol

• The main function of the Spanning-Tree Protocol
  is to allow duplicate switched/bridged paths
  without incurring the latency effects of loops in
  the network.
• Builds loop-free tree from a point known as a
  root
• Redundant paths allowed, but only one active path

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Spanning Tree Algorithm (STA)

• Part of 802.1d standard
• Simple principle Build a loop-free tree from
  some identified point known as the root.
• Redundant paths allowed, but only one active
  path.

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SPANNING TREE

• Developed by Digital Equipment Corporation
• IEEE revised standard released 802.1d
• Cisco Switches use IEEE 802.1d
• Continually probes network automatically
  reconfigures ports.
• New Standard IEEE 802.1w has recently been
  developed.

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Introducing Spanning-Tree Protocol

• The main function of the Spanning Tree Protocol
  (STP) is to allow redundant switched/bridged
  paths without suffering the effects of loops in
  the network.

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• The STA is used to calculate a loop-free path.
• Spanning-tree frames called bridge protocol data
  units (BPDUs) are sent and received by all
  switches in the network at regular intervals and
  are used to determine the spanning tree topology.
• A separate instance of STP runs within each
  configured VLAN.

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Understanding STP States

• Blocking - No frames forwarded, BPDUs heard
• Listening - No frames forwarded, listening for
  frames
• Learning - No frames forwarded, learning
  addresses
• Forwarding - Frames forwarded, learning addresses
  
• Disabled - No frames forwarded, no BPDUs heard

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Bridging without Spanning Tree
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Even worseBroadcast Frames
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Spanning Tree takes care of this
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Spanning Tree Process

• Elect a Root Bridge on the LAN
• All switches send out Configuration Bridge
  Protocol Data Units (Configuration BPDUs)
• BPDUs are sent out all interfaces every two
  seconds (by default - can change)

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bridge protocol data units are sent and received
by all switches in the network at regular
intervals and are used to determine the spanning
tree topology.
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Path Cost
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STP Timers
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IEEE 802.1w (RSTP)

• Designed to speed recalcualtion process
• Converges faster than STP
• Provides rapid connectivity following switch
  failure
• Enhanced Software Image (EI) must be installed
  on your switch

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RSTP STP Comparisions