RSTP Discarding State BPDUs

[Rem Markov]

Hey! So I'm struggling to understand the concept of Discarding in RSTP. 

In STP, There are two state , Blocking and Listening. In blocking the ports will only listen to BPDU and in Listening they will also send BPDUs and then a Root Bridge will be elected.

In RSTP there is no Listening state, but I don't understand how can port in Discarding State send their BPDUs to decide who will be the Root Bridge, The port in Discarding also only listen to BPDUs no? 

In conclusion I don't understand how can RSTP decide who will be the Root Bridge in Discarding State if Ports in Discarding State don't send BPDUs 

[balaji.bandi]

In STP - Only the Root bridge sends BPDU

in RSTP - all Bridges can forward BPDU

The below document explain how they process :

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html#anc4

[Rem Markov]

But when I'm setting my first RSTP how ports roles are assigned ? how can I know who will be the Root Bridge? Imagine I'm Setting a RSTP with 4 Switches all of them will be discarding, but how will they know who will be the Root Bridge if they don't send BPDUs

[MHM Cisco World · Accepted solution]

friend the traffic forward is divide into two 
1- BPDU <<- this will not effect by port role/state
2- other traffic (data traffic) <<- this effect by port role/state, here the discarding meaning discard the data traffic not BPDU.

[Rem Markov]

BPDUs are being sent and recieved by all switches no matter the state in RSTP? or in all the variation of STP?

[MHM Cisco World]

simple lab 
IOU1 is root all interface are DP 
IOU2 have one DP and one RP
IOU3 have one RP and one BLK 

you can see in show spanning tree interface detail that 
DP always send BPDU 
RP and BLK always receive ONLY

here I want to mention that after Root Election all SW NOT generate it BPDU, only root SW generate BPDU and other SW modify and forward it.

[Rem Markov]

Thank you so much!