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Currently studying Spanning Tree, I'm not sure about the behaviour when we add a new switch to an existing STP topology (with cisco pvst+).

Assuming we plug this new bridge called S4 with only one link, to the existing infrastructure (S3 bridge). This new S4 switch has already the VLAN configured.

Could you confirm to me the following statements with these 2 hypothesis ?

Hypothesis 1: new root bridge with higher priority

  • The new ports up in S4 (new switch) and S3 (existing switch) will be immmediately put in listening state
  • The new switch will send BPDU to announce itself as root bridge
  • Existing S3 switch will ignore the S4 BPDU bridge which announcing itself as root
  • Existing switch S3 will forward to S4 the BPDU of the existing root bridge
  • S4 will stop to anounce itself as root, and its port will get the role of root port
  • After S4 put this port as root port, the listening timer of this port continue and don't start again from 0

Hypothesis 2: new root bridge with lower/better priority

  • The new ports up in S4 (new switch) and S3 (existing switch) will be immmediately put in listening state
  • The new switch will send BPDU to announce itself as root bridge
  • Existing S3 switch will ignore the S4 BPDU during the max_age timer (default 20s) before to announce also S4 as the new root bridge
  • New convergence process start

And as complementary question, in the main lines, what will occur regarding this two hypothesis but with a RSTP ?

Ports immediately go to Learning state, and with higher priority nothing happens and with better priority a new proposal-agreement process start from S4 and S3, and continue on all the infrastructure ?

Many thanks in advance, your help will be very appreciated !

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