Rapid Spanning-Tree Protocol

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.
- SYNC PROCESS MAKES SURE ALL RSTP SPEAKERS AGREE ON ROOT PORTS AND DESIGNATED PORTS.
  - 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.

Rapid Spanning-Tree Protocol – 802.1D-2004