Posts Tagged ‘convergence’


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):

  1. 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.
  2. Downstream bridge receives the proposal, and if it agrees with the upstream port role, it starts the process known as synchronization.
  3. Synchronization implies the downstream bridge blocking all non-edge designated ports, prior to sending an agreement to the upstream bridge.
  4. Synchronization is needed to make sure there are no loops in the topology, after the upstream bridge unblocks its designated port.
  5. 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

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We are going to discuss Cisco’s proprietary extensions to STP algorithm, namely UplinkFast and BackBone Fast. Those two features aim to reduce the time it takes STP to re-activate topology after a link failure. While UplinkFast seems pretty intuitive, BB Fast is more complicated.

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In this post we are going to look into STP convergence process. Many people have perfect understanding of STP, but yet face difficulties when they see questions like “How many seconds will it take for STP to recover connectivity if a given link fails?”. The post will follow the outline below:

1) General overview of STP convergence process
2) How STP converges if a directly connected link fails
3) How STP converges when it detects indirect link failure
4) Topology changes and their effect

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