• Differences from legacy STP
    • Rapid convergence due to sync process.
    • Simplifies port states.
    • Additional port roles.
  • Port States:
    • Discarding
      • Drops frames
    • Learning
      • Dropping frames but builds CAM table
    • Forwarding
      • Normal Forwarding
  • Roles:
    • Root/Designated same as legacy STP
    • New:
      • Alternate
        • Same as UplinkFast in regular STP.
        • Upstream interface that’s less desirable to root.
        • No need to configure UplinkFast, just part of RSTP.
        • Immediately begins forwarding upon root port failure.
      • Backup
        • Backup downstream facing interfaces.
        • Activates if primary designated port fails.
        • Normal state is discarding.
        • Only seen in shared segments – ie. hubs.
      • Edge
        • Same as STP PortFast ports.
        • No Topology Changes, immediately begins forwarding.
  • Link Types:
    • Point to Point – Full Duplex
    • Shared – Half Duplex
    • Sync process only occurs on point to point Designated ports.
  • RSTP Sync Process:
    • Process of non-Root Bridge to sync root port with rest of topology.
    • After root port selected, all non-edge ports are considered designated.
    • Bridge then sends out proposals with root bridge info – port costs, etc.
    • Downstream bridges review and either agree or send alternate info.
      • Sending additional info means downstream switch thinks it has better path to root bridge.
      • Enabling Edge ports is important because it decreases the amount of churn when sync process is occurring. Edge ports do not partake in Syncronization.
  • Failure Detection:
    • In STP, BPDUs are only sent out by Root Bridge
    • In RSTP, BPDUs are generated by every RSTP speaker via Hellos – 2 second default.
    • Reconvergence begins after 3 hellos are missed.
    • MaxAge is used for backwards compatiblity with STP.
  • Convergence:
    • When root port failure occurs, the Alternate port takes over.
      • New root is synced down to all bridges.
    • If no alternate port is available, the bridge declares itself as root bridge, synchronizes, then adapts if there is better info.
    • Slow convergence can happen with large meshy topologies.
  • TCN:
    • Generated after link goes into forwarding state.
    • Originates from switch that detects event.
    • Floods via reverse path forwarding.
    • Causes unicast traffic flooding.

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