Doubts on PVSTP, RPVSTP and MST

Hi all,
Still have many generic doubts about STP behavior. Hope you can Help me:

  • During first syncronization, every switch things it is the root, so everyone sends bpdus. So, no port still begin in blocking state but in listening state, does it right?

  • In a stable topology bpdus are sent only from root bridge every 2 seconds? Right? If yes this bpdus does arrive to all the switches on the topology? If yes does every switch replicate the root bpdus to the neighbors?

  • In pvstp, does every vlan create their own bpdu? So the vlan bpdus are sent tagged? If yes what is the main goal of the vlan 1 bpdus?

  • During tcn, a switch sends a tcn bpdu to the root bridge, the neighbors switch that at first receives this tcn bpdu, does reduces immediately his Max age timer or it wait for root bpdu?

  • In RSTP starting topology how does a designated port goes into forwarding immediately if it starts as blocked port so it can’t send any bpdu but only receive?

  • in RSTP every switch produce and send its own bpdu every 2 seconds, does it right?

  • In mst how can IST performs calculation on single bpdu sent tagged from a pvstp switch? How ist is able to understand pvstp topology only looking at vlan 1 bpdus?

Many thanks!

Hello Valentina

Many of the answers to your questions are found within the STP lessons on the site, however, I will attempt to answer your questions specifically, or refer to you the appropriate lesson…

Yes. All interfaces that are up begin sending BPDUs and are in the listening state to listen for BPDUs from other devices. Only if a superior BPDU is received, only then will it considere itself not the root bridge.

For normal STP, BPDUs are generated by the root bridge every hello interval, which by default is 2 seconds, and each downstream switch that receives a BPDU on its root port will relay it to downstream switches. However, RSTP works differently, as all switches generate BPDUs.

In PVSTP, each VLAN has its own STP topology, and each one has its own root bridge. The root bridge of each VLAN sends out BPDUs. Assuming VLAN 1 is the native VAN on all trunks, then VLAN1 PVST+ BPDU is sent untagged while other VLANs PVST+ BPDUs are sent tagged with their appropriate VLAN.

Take a look at this post: Spanning Tree Topology Change Notification (TCN) - #24 by andrew

Take a look at the Discarding state in the following lesson:


In the MST and PVST Interoperability lesson, it states:

Deciding the port role on the boundary interface for all VLANs is risky…after all, it means that we assume that all VLANs use the same root bridge, root ports, etc. We don’t know if the PVST+ domain agrees on the port role for all VLANs that we select on our MST boundary interface. There are, however, some simple checks that MST can use to figure out if PVST+ agrees on the port roles. Let’s take a look at each possible port role.

Those checks can be further understood from the lesson itself.

I hope this has been helpful!