Rapid Spanning Tree Protocol

[skazeem96]

I have a query.

Sw-A connected to Sw-B and SW-B is connected to Sw-C and Sw-D.

Step: When SW-B receive proposal packet from SW-A, SW-B starts Sync process on non-edge ports  ( moves non-edge ports to designated /discarding).

Now my question is : with respective to above step, SW-B sending proposal flag in BBDU and send it to SW-C and SW-D and receive agreement flag set in BPDU from Sw-C and Sw-D and then send an agreement to SW-A, ( or )  first SW-B send the agreement to the  SW-A and then send proposal to Sw-C and Sw-D.

Which will happen first ?

[mlund · Accepted solution]

Hi

First send agreement to sw-A, then send proposal to sw-C an D.

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli/CLIConfigurationGuide/RPVSpanningTree.html#10743

/Mikael

[skazeem96]

Hi Mikael,

Thanks for sharing me the good information to  understanding on Proposal / Agreement bit set using RSTP. 

I will be in sync with you if get some more questions on RSTP, RVST and MST.

Thanks.

[skazeem96]

I had read multiple documents to understand the RSTP concepts but still I have multiple questions to get clarity.  If you answer the below questions for me, which I can develop good confidence on RSTP.

1.Let's say I have only one switch.I enabled RSTP in my switch. by default, all ports are on edge ports.

• Will edge port forward BPDUs ?
• If  yes", will they continuously forward BPDUs? How long did they forward ?

2. Let's say Switch A and Switch B is connected. Switch A is Root Bridge.

Case:1 Switch A (D/F)------------------------(R/F) Switch B
                                                                               |
                                                                           (A/D)

Case:2 Switch A (D/F)------------------------(R/F) Switch B
                                                                               |
                                                                           (D/F)

Case:3 Switch A (D/F)-----------HUB--------------(R/F)Switch B
                                                                                     |
                                                                                (A/D)

When the connection between  S-A and S-B  fails, then S-B thins ks itself as a Root Bridge and move its non -edge ports to (Designated /Discarding) and send proposal  bit set in BPDU.In this process, if S-B receives better BPDU with proposal bit set, S-B  sends an Agreement and move its port (Alternate/Discarding) to  (Root / forwarding).

• How much time did it take to (A/D) Alternate / Discarding port to move to (Root / Forwarding) ?
• How much time did it take to (D/F) Designated / Forwarding  port to move to (Root / Forwarding) ?
• Connection fails between S-A and HUB.The link between HUB and Switch -B is up.How much time did it take to (A/D) Alternate / Discarding port to move to (Root / Forwarding) ?

3. In STP only Root bridge will generate but In RSTP, Is all the Switches will generate BPDU periodically (2sec) what is the benefit?

4. In RSTP, link failure is considered as a topology change Only non-edge interfaces (Point to Point links Switch to Switches) that move to the forwarding state are considered as a topology change. Link failure at Edge port is not considered as topology change.

   I want  to understand the below points. Once a switch detects a topology change,

• It will start a "topology change while timer" (4sec). what is the benefit of this
• If Switch receives BPDUs with TC bit set, It clears the MAC addresses learned on all its ports, except the one that receives the topology change.why?

- Azeem