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New Member

Need some help in Understanding RSTP Convergence Process

Hi Experts,

I would need some help in undestanding the RSTP Convergence Process.  I have taken the below sample topology for my discussion and have summarized my understanding of the RSTP Convergence process and few questions marked in Purple.. Would need your help to clearify and validate my understanding.

Thanks a lot for your time to look at this thread and for replying.

RSTP_Convergence1.png

1. On the above topology lets assume all the switch are started simultaneouly. The port Fa1/0 on SW2 towards SW1 comes up first. so the following should happen.

     a. SW2 sends out a proposal message to SW1 for becoming a root.

     b. SW1 rejects the proposal since it has a better bridge ID (BID) and sends out the alternative better info.

     c. Now SW2 Fa1/0 will now become the Root Port and SW1 Fa1/0 will be designated port.

     d. While working on Proposal/Agreement, the ports Fa1/1 on both SW1 and SW2 are kept at Blocked State - Need validation here

2. Now lets assume that the port Fa1/0 on SW4 comes up on the segment between SW2 and SW4.

     a. Now SW4 will send out the proposal (saying himself as the root).

     b. SW2 will receive this proposal (will it on the blocked port if 1d is valid) on Fa1/1 and rejects the proposal since it knows about the better root (BID) cached on its port Fa1/0 and sends that superior information to SW4.

     c. Now SW4 receives that infomration, marks its port Fa1/0 as the root Port and SW2 will have Fa1/1 as Designated port.

     d. Switch4 will have its port Fa1/1 as Blocked.

3. Now its the turn of the  link from SW3 - SW4 or SW3 - SW1 to come up next.

i. Assuming SW3-SW4 comes up first:

      a. C sends out proposal (saying himself as the root).

      b. SW4 receives this proposal and since BID of superior, it accepts the proposal and sets its port Fa1/1 as Root Port. This would also set SW3's Fa1/0 as the Designated Port.

      c. Now SW4 has to some how translate this towards SW1. how does it do that ? does it do that automatically? if so, does SW4, SW2 and SW1 change the Port Roles accordingly?

      d. What will be the status of port Fa1/1 SW4 at this point in time? will it be blocked?

ii. Assuming SW3 - SW1 comes up now:

       lets assume SW1 already knows about Root Bridge as SW3. now it has got a better path to the Root. so Root port selection has to happen again.. What steps does SW1 take now? and what happens after that?

      Assuming that SW1 doesnot know about Root Bridge yet, at some point the message from SW3 towards SW4 and SW3 towards SW1 as part of 3(i) should clash. At that point what would happen?

Also, does it mean that the total convergence happens in less than a second? also what would happen to the traffic till the convergence is happening?

Please clarify

Thanks

-Vijay Swaminathan.

Message was edited by: Vijay Swaminathan

Everyone's tags (1)
7 REPLIES
Cisco Employee

Re: Need some help in Understanding RSTP Convergence Process

Hello Vijay,

Good questions! Let's move through them individually.

Step 1:

d. While working on Proposal/Agreement, the ports Fa1/1 on both SW1 and SW2 are kept at Blocked State - Need validation here

Yes, that is correct. During the Proposal/Agreement negotiation, the state is kept at Discarding.

Step 2:

b. SW2 will receive this proposal (will it on the blocked port if 1d is valid) on Fa1/1 and rejects the proposal since it knows about the better root (BID) cached on its port Fa1/0 and sends that superior information to SW4.

Yes, that is correct. Please note that "rejecting the proposal" simply means ignoring it. Also, "sending superior information" simply means sending the superior BPDU that was always being sent out that port.

Step 3:

b. SW4 receives this proposal and since BID of superior, it accepts the proposal and sets its port Fa1/1 as Root Port. This would also set SW3's Fa1/0 as the Designated Port.

True; however, note that the SW3's Fa1/0 role is Designated from the very beginning - it is the starting (default) role of each newly activated port in RSTP.

c. Now SW4 has to some how translate this towards SW1. how does it do that ? does it do that automatically? if so, does SW4, SW2 and SW1 change the Port Roles accordingly?

When SW4 learns about a new root port (leading even to a new root bridge), it basically means a "restart" of the RSTP on SW4, meaning that all remaining ports will be reinitialized and put into Designated/Discarding role/state. Now ,by definition, each non-edge Designated/Discarding or Designated/Learning port sends Proposals. That is how the information about the new root bridge SW3 reachable via SW4 propagates to SW2. Do not forget that Proposals are BPDUs like any other, containing the information about the root switch as known by the sending switch, and all other usual parameters, and in addition, this BPDU has the Proposal bit set.

The situation will then repeat on SW2 - after it receives a Proposal from SW4, it finds out that a new root switch has emerged in the network and that the root port has changed. It will therefore set F1/1 as the new Root/Discarding port role/state, all other non-edge ports will be moved to Designated/Discarding, and after sending an Agreement back to SW4, the F1/1 on SW2 will be set to Root/Forwarding (upon receiving the Agreement, SW4 will move the F1/0 from Designated/Discarding to Designated/Forwarding, making the link bidirectionally forwarding rapidly).

Proposals sent from the non-edge Designated/Discarding ports on SW2 also reach SW1. When SW1 receives a Proposal that contains information about a better root switch than SW1 itself, SW1 ceases to be the root switch, selects the F1/0 as the new Root/Discarding port, sets all non-edge Designated ports to Discarding, send an Agreement back to SW2 and sets the F1/0 to Root/Forwarding. SW2, upon receiving this Agreement, will also immediately move the F1/0 to Designated/Forwarding.

d. What will be the status of port Fa1/1 SW4 at this point in time? will it be blocked?

When F1/1 on SW4 first comes up, it is Designated/Discarding. After receiving the Proposal from SW3, it moves to Root/Discarding, and after performing the Sync operation on the remaining non-edge designated ports on SW4, F1/1 sends out an Agreement and moves to Root/Forwarding.

ii. Assuming SW3 - SW1 comes up now:

       lets assume SW1 already knows about Root Bridge as SW3. now it has got a better path to the Root. so Root port selection has to happen again.. What steps does SW1 take now? and what happens after that?

After the F1/1 link between SW1 and SW3 comes up, both ports are put into Designated/Discarding role/state and start sending Proposals. Obviously, SW1's Proposal is inferior to SW3's, therefore SW3 will simply ignore the Proposal from SW1 and continue sending its own. Upon receiving a Proposal from SW3, SW1 will notice that a new root port is available. Therefore, it will select F1/1 as the new Root/Discarding port, reset all remaining non-edge ports to Designated/Discarding and send an Agreement back to SW3. Then it can put the F1/1 to Root/Forwarding, and SW3 can put the F1/1 into Designated/Forwarding.

Because on SW1, the F1/0 has become Designated/Discarding, it sends out Proposals. On SW2, these Proposals make the F1/0 the new root port (because while the total root path cost is identical both via SW1 and SW4, the path via SW1 is preferred by SW2 because of SW1's lower MAC address). Hence, SW2 will select F1/0 as the new Root/Discarding, put all remaining non-edge ports to Designated/Discarding, then send an Agreement from F1/0 back to SW1 and put it into Root/Forwarding. Then, SW2 will start sending out Proposals on the Designated/Discarding ports.

Now, an interesting situation arises on the link between SW2 and SW4. SW2 is sending Proposals to SW4 but to SW4, they are irrelevant: they do not make SW4 reconsider its root port, and because the root path cost as indicated in the Proposals from SW2 is higher than SW4's own root path cost, this Proposal is inferior to the BPDU sent out from SW4 F1/0. Therefore, this Proposal is ignored, and SW4 continues to send its own BPDUs (without the Proposal bit set as the F1/0 is not currently in Designated/Discarding or Designated/Learning). Upon receiving SW4's BPDU, SW2 will note that its F1/1 is neither the root port nor the designated port on this link. Therefore, SW4 will move this port into Alternate/Discarding and cease sending BPDUs altogether.

Assuming that SW1 doesnot know about Root Bridge yet, at some point the message from SW3 towards SW4 and SW3 towards SW1 as part of 3(i) should clash. At that point what would happen?

I do not quite understand this question.

Also, does it mean that the total convergence happens in less than a second? also what would happen to the traffic till the convergence is happening?

Indeed, the convergence in RSTP is driven by the Proposal/Agreement mechanism, and in a sensibly designed network, the RSTP convergence should indeed be sub-second. The traffic during the convergence is either dropped or flooded - this depends on whether a MAC address points out a port that is put into Discarding state, or whether the MAC address is not known at all.

Feel welcome to ask further!

Best regards,

Peter

New Member

Re: Need some help in Understanding RSTP Convergence Process

Hi Peter,

You are just awesome :-) you had answers for all the basic silly doubts that I had in my mind. I was just wondering how did you manage it to keep so simple but to the point. I truely amazed.

I would need some help more. I have just decided to put this discussion as a document so that it might help people like me to understand the RSTP convergence process.

I have added a little more details that I know off along with your wonderful write up. just need your help to validate those..

I again thank you for all the pain and time that you took to make it so simple.

Thanks again..

-Vijay

Cisco Employee

Re: Need some help in Understanding RSTP Convergence Process

Hello Vijay,

This is just to assure you that I am keeping an eye on this thread but unfortunately, I am right now quite busy and don't have enough time to participate here at CSC as much as I would like to. Hopefully, the workload will subside in a few days and I will be more than happy to return here and continue the discussion about RSTP.

Thank you for your patience!

Best regards,

Peter

New Member

Re: Need some help in Understanding RSTP Convergence Process

Sure Peter Not a problem at all. I just consolidated my understanding in a document so that it can be used on a longer run for beginners like me

Take your time . not in a hurry at all. I got all my doubts cleared in RSTP with your help :-)

Regards,

-Vijay

New Member

Re: Need some help in Understanding RSTP Convergence Process

Cant believe how nicely the RSTP is broken into bits by Peter...Got all my doubts cleared within 10 minutes...

Thanks to  Peter and Vijay

New Member

Proposals from SW2 is higher

Proposals from SW2 is higher than SW4's own root path cost, this Proposal is inferior to the BPDU sent out from SW4 F1/0. Therefore, this Proposal is ignored, and SW4 continues to send its own BPDUs (without the Proposal bit set as the F1/0 is not currently in Designated/Discarding or Designated/Learning). Upon receiving SW4's BPDU, SW2 will note that its F1/1 is neither the root port nor the designated port on this link. Therefore, SW4 will move this port into Alternate/Discarding and cease sending BPDUs altogether

--> here why the proposal bit is not set? could you please elaborate more on this.

I am not able to quite get it.Is it because both the bpdus are talking about same root bridge?

Thanks

Ravi Pande

New Member

-->this Proposal is ignored,

-->this Proposal is ignored, and SW4 continues to send its own BPDUs (without the Proposal bit set as the F1/0 is not currently in Designated/Discarding or Designated/Learning). Upon receiving SW4's BPDU, SW2 will note that its F1/1 is neither the root port nor the designated port on this link. Therefore, SW4 will move this port into Alternate/Discarding and cease sending BPDUs altogether.

I dont understand why the proposal bit is not set here.

Is it because like both the switch know that sw3 is the root bridge?

Could you please elaborate more on this I am confused.

Thanks

Ravi

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