May I know in which phase(learning, listening) is the root switch elected ? and the root port elected ?
in another lecture you also mentioned that only the root port and designated port will be in listening state. how are the these elected as we are in blocking state ?
In RSTP there is a mention of CST not accepting inferior BPDU, can I get some more clarification on that ? Like what exactly does this mean and where does CST block such BPDU ?
Not to worry, that’s why we’re here, to answer your questions!
The phases you are referring to have to do with the process through which switch ports go to determine their port role (i.e. root port, designated port, blocking etc…) These phases take place after the root bridge has already been discovered because the location of the route bridge affects that process.
When does the root bridge election take place? As soon as a switch is turned on, it considers itself the root bridge until it receives a BPDU that tells it otherwise. The connected ports start out in the blocking state, and then go to listening and learning, and all ports will send and receive BPDUs from neighboring switches. The switch will compare the BPDUs with its own, and if it has the best priority, it will remain the root, and all ports will become designated. If another switch sends “better” BPDUs, then the local switch will relinquish its hold on the root designation, and make the port from which it received that “better” BPDU the root port. The rest of the ports will go through the process of determining their roles using the listening learning forwarding/blocking process.
Only a root or designated port will move to the listening state. The non-designated port will stay in the blocking state.
To clarify, what Rene is saying is that if a port that is just connected is in a blocking state, and the BPDUs it receives tell it to remain in a blocking state, it will not change state. (Blocked ports still receive BPDUs). If however the port has a potential of becoming a root or a designated port, only then will it transition to listening and learning.
Rapid spanning tree will accept inferior BPDUs. The classic spanning tree ignores them. Does this ring a bell? This is pretty much the backbone fast feature of classic spanning tree.
Rene is mentioning here that RSTP which is a more advanced version of STP behaves differently from classic 802.1D STP in that it will accept and process inferior BPDUs while classic STP will simply ignore/discard them. Now you can configure classic STP to accept and process them by employing the STP BackboneFast feature. If you check out that lesson, you will see why this is useful. However, RSTP employs this behavior by default. Does that make sense?