Spanning Tree Protocol (STP)

Because you can connect switches together in any fashion you can create redundant connections called bridging loops or switching loops.

• In the early days of switches a frame could get caught in a loop and bring the network down.
• STP uses special frames called BPDUs to communicate with other switches
  • BPDU: Bridge Protocol Data Unit
• Configuration BPDUs elect one switch as root bridge to act as center of STP.
• There can be redundant links for fault tolerance but certain ports will be placed in a blocking state and will not send/receive frames.
• If a link or device goes down a special BPDU called a TCN will be sent out allowing blocking ports to move to a forward state if they’re needed.
  • TCN: Topology Change Notification
• RSTP is currently in use, STP was retired in 2001.
  • RSTP: Rapid Spanning Tree Protocol
  • Allows rapid convergence time following some kind of network change.