Posts Tagged ‘rstp’
Fast Convergence for Designated Ports
RSTP protocol’s fast convergence depends on the use of point-to-point links connecting switches. In order to quickly transition a designated port into non-discarding state, the upstream switch needs to make sure that the downstream neighbor agrees with that idea. This constitutes the process known as handshake (or proposal/agreement):
- Upstream bridge sends a proposal out of a designated port. As a matter of fact, it just sets the proposal bit in outgoing configuration BPDUs.
- Downstream bridge receives the proposal, and if it agrees with the upstream port role, it starts the process known as synchronization.
- Synchronization implies the downstream bridge blocking all non-edge designated ports, prior to sending an agreement to the upstream bridge.
- Synchronization is needed to make sure there are no loops in the topology, after the upstream bridge unblocks its designated port.
- If the downstream bridge does not agree with the proposal, it will continues sending it’s own configuration BPDUs with the proposal bit set. Eventually one of the bridges will accept the superior information and send an agreement.
Fast Convergence for Other Port Types
See more detailed overview at: http://blog.ine.com/wp-content/uploads/2010/04/understanding-stp-rstp-convergence.pdf