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Rapid STP (RSTP) Notes

Rapid STP (RSTP)

 – 802.1W

 – STP plus fast convergence features

 – No forward-delay or max-age-time

     -> Theoretical reconvergence time is up to 6 seconds

New Port Types

 – Access ports —> Edge port

 – Trunk port —> Point-to-Point port

 – Switch to Hub —> Shared port

 – Multiple connections to a Hub —> All ports except one will become Backup ports

     -> Backup ports are ports that receive our own BPDUs

Failover does not have to wait for reconvergence


 – Please make this port a root port

 – Receiving switch

     -> Disconnect from rest of topology

     -> Decide

     -> Send agreement

     -> Make port a root port

     -> Reconnect to the rest of topology

 – Disagreement

     -> Both switches don’t want to change port to a root port

 – The root bridge never sends an agreement

Proposal and Agreement Procedure

     1.  In RSTP once the root bridge is elected, it sends a Proposal message out all designated ports

     2.  The Proposal is a request to make the receiving port a root port

     3.  Whenever a switch receives a Proposal, it disconnects itself from the rest of the topology to decide to agree or disagree

     4.  If agree, an Agreement message is sent back and the sending port becomes a root port

     5.  Once the root port is decided, the switch sends Proposal messages further out potential designated ports

Configuring RSTP

SW1(config)#  spanning-tree mode rapid-pvst

Convergence Methods

 – Only two

     -> STP 

          -> Slower

     -> RSTP

          -> Faster

          -> Used by MST for convergence

Topological Design

  – STP to VLAN mapping

     -> Common Spanning Tree (CST)

     -> Per-VLAN Spanning Tree (PVST)

     -> Per-VLAN Spanning Tree + (PVST+)

     -> Multiple Spanning Tree (MST)

Common Spanning Tree (CST)

 – One STP instance for all VLANs

 – Only one root election

 – Limited to 100 VLANs

 – No load balancing between links or switches

 – Only supports dot1q

 – Only supports STP for convergence

Per-VLAN Spanning Tree (PVST)

 – One STP instance per VLAN

 – 100 VLANs means

     -> 100 STP instances

     -> 100 root bridges

     -> 100 BPDUs

 – Only supports ISL

 – Only supports STP for convergence

Per-VLAN Spanning Tree + (PVST+)

 – Supports both ISL and dot1q

 – Supports both STP and RSTP for convergence

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