01-27-2018 05:41 AM - edited 03-08-2019 01:35 PM
Hello community,
Regarding my Spanning Tree studies, I'm not sure about the behaviour on a specific use case, and specifically when we add a new switch to an existing STP topology (with cisco pvst+).
In this purpose I will expose to you my understanding of the situation, and maybe you could help me to confirm or correct me if I'm wrong.
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
Hypothesis 2: new root bridge with lower/better priority
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 !
Solved! Go to Solution.
01-28-2018 04:31 PM
Hello Benoit,
Hypothesis 1 is correct on all accounts.
Hypothesis 2 is not correct; what really happens is this:
With RSTP, things are somewhat different. In Scenario 1 (Hypothesis 1), both switches will send a Proposal to each other; this is because every RSTP port in Designated Discarding or Designated Learning role/state combination sends out Proposals. However, S3 will ignore S4's Proposal because the BPDU will be inferior to S3's own BPDUs. When S4 receives S3's Proposal, it will learn about the proper root bridge, elect the Root port toward S3, block all its non-edge Designated ports, and send back an Agreement, then unblock the Root port. Upon the Agreement, S3 will unblock its port toward S4. No other processing, apart from generating and flooding a Topology Change event, will happen.
In Scenario 2, it starts the same - both switches will send a Proposal to each other. This time, S4 will ignore S3's Proposal because it is inferior. S3, upon receiving S4's Proposal, will learn about a new root switch, elect the port toward S4 as a new Root port, reevaluate roles of all its remaining ports, block all of its non-edge Designated Ports, and send back an Agreement which will enable both S3 and S4 to unblock their ports on this new link. Further, as S3 has moved its non-edge Designated ports into Discarding state, this allows these ports to send out Proposals, so the Proposal wave has now effectively moved to S3, and this process will repeat in a cascade throughout the network.
Feel welcome to ask further!
Best regards,
Peter
01-28-2018 04:31 PM
Hello Benoit,
Hypothesis 1 is correct on all accounts.
Hypothesis 2 is not correct; what really happens is this:
With RSTP, things are somewhat different. In Scenario 1 (Hypothesis 1), both switches will send a Proposal to each other; this is because every RSTP port in Designated Discarding or Designated Learning role/state combination sends out Proposals. However, S3 will ignore S4's Proposal because the BPDU will be inferior to S3's own BPDUs. When S4 receives S3's Proposal, it will learn about the proper root bridge, elect the Root port toward S3, block all its non-edge Designated ports, and send back an Agreement, then unblock the Root port. Upon the Agreement, S3 will unblock its port toward S4. No other processing, apart from generating and flooding a Topology Change event, will happen.
In Scenario 2, it starts the same - both switches will send a Proposal to each other. This time, S4 will ignore S3's Proposal because it is inferior. S3, upon receiving S4's Proposal, will learn about a new root switch, elect the port toward S4 as a new Root port, reevaluate roles of all its remaining ports, block all of its non-edge Designated Ports, and send back an Agreement which will enable both S3 and S4 to unblock their ports on this new link. Further, as S3 has moved its non-edge Designated ports into Discarding state, this allows these ports to send out Proposals, so the Proposal wave has now effectively moved to S3, and this process will repeat in a cascade throughout the network.
Feel welcome to ask further!
Best regards,
Peter
02-07-2018 02:51 PM - edited 02-07-2018 02:55 PM
Thanks for this very clear answer.
05-05-2022 01:36 AM
hi @Peter Paluch regarding your answer
@Peter Paluch wrote:With RSTP, things are somewhat different. In Scenario 1 (Hypothesis 1), both switches will send a Proposal to each other; this is because every RSTP port in Designated Discarding or Designated Learning role/state combination sends out Proposals. However, S3 will ignore S4's Proposal because the BPDU will be inferior to S3's own BPDUs. When S4 receives S3's Proposal, it will learn about the proper root bridge, elect the Root port toward S3, block all its non-edge Designated ports, and send back an Agreement, then unblock the Root port. Upon the Agreement, S3 will unblock its port toward S4. No other processing, apart from generating and flooding a Topology Change event, will happen.
does that mean that no Network outage will happen when a new switch (with higher STP Priority than current Root-Bridge) is connected to a running network?
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