Spanning Tree Protocol

Loop protection
  • Connect two switches to each other
    • They'll send traffic back and forth forever
    • There's no "counting" mechanism at the MAC layer
  • This is an easy way to bring down a network
    • And somewhat difficult to troubleshoot
    • Relatively easy to resolve
  • IEEE standard 802.1D to prevent loops in bridged (switched) networks (1990)
Switch operation
  • Forwarding decisions made by MAC address
    • Keeps a big table of MAC address that have been seen
    • All forwarding decisions are filtered through this list
  • If the destination MAC is unknown, the frame is flooded
    • Sent to every switch port in the local subnet/VLAN
    • Hopefully the destination station will respond
  • Flooding is hopefully a temporary process
    • Directed traffic resumes when the MAC is seen
STP port states
  • Blocking - Not forwarding to prevent a loop
  • Listening - Not forwarding and cleaning the MAC table
  • Learning - Not forwarding and adding to the MAC table
  • Forwarding - Data passes through and is fully operational
  • Disabled - Administrator has turned off the port
RSTP (802.1w)
  • Rapid Spanning Tree Protocol (802.1w)
    • A much-needed updated of STP
    • This is the latest standard
  • Faster convergence
    • From 30 to 50 seconds to 6 seconds
  • Backwards-compatible with 802.1D STP
    • You can mix both in your network
  • Very similar process
    • An update, not a wholesale change

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