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CCNP Enterprise Core (350-401) Cisco Datalink Layer

Establishment of an RSTP (802.1W) Topology

When it comes to RSTP establishing its topology, this occurs faster with the use of a special rapid spanning tree handshake over regular spanning tree protocol. RSTP uses a synchronisation process to add a switch to its topology by carrying out the following process:

  1. Two switches are connected to each other, they verify the capabilities of their connection are full-duplex meaning it is a point to point link with no other devices present.
  2. The two switches establish a handshake with each other to advertise a proposal within configuration bridge protocol data units (BPDUs) that they should be set up as designated ports.
  3. As there can only be one designated port in a link, each switch needs to identify whether it is the higher priority switch by using the same process as regular spanning tree protocol (802.1D).
  4. After the process has completed, the lower priority switch makes its local port as the root port and moves all non-edge ports into a discarding state.
  5. The lower priority switch sends a configuration BPDU to the root bridge to signal that synchronisation is occurring on the lower priority switch.
  6. The lower priority switch moves its root port into a forwarding state. The higher priority switch moves its port to the lower priority switch into a forwarding state too (the designated port).
  7. The lower priority switch repeats this process for any switches downstream to it.

If during the downstream process any switches fail to acknowledge the handshake, the switches revert to 802.1D state to prevent a forwarding loop occurring.

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