For the next downstream switch, the upper BPDU is transferred to the designated and recurrent states of its non-rejectionants, completing the proposal/agreement process. The SWB will restore all of its ports and send its own BPDU proposal to its neighbours downstream, if it receives a response from its neighbours downstream, it will redirect its base port and send a BPDU agreement to SWA, set at 1, and the notice of the proposal will be set at 0. SWA ports will switch to redirection. I`ll use the topology below to go through the synchronization process. 2.) Unlike animation, the proposal/agreement process between S2 and S3 does not take place at all. It is true that S2 can send a proposal to S3. However, S3 and S2 have already chosen another root port, and the arrival of BPDU on both sides will not change this selection. This means that the connection between S2 and S3 must be blocked, otherwise a loop would be formed to cross these two switches. On the other hand, a proposal/agreement procedure is used to quickly establish a link to a transit state.
Therefore, even if s2 or S3 sends a proposal to its neighbor, no agreement can be sent, because the connection between S2 and S3 must remain blocked. On the connection between S4 and S2, the F0/2 of the S4 is the designated port, while the Fa0/2 of the S2 is the root port, so the S2 should not send suggestions for this link. The right sequence of events is therefore that the S4 sends a proposal to the S2 and responds with an agreement on S4 after blocking all its non-edge ports. There is only one thing the switches need to do. If a port switches to redirect state in RSTP, it means there is a change in topology. Note that in 802.1w (RSTP), this is the only thing that triggers the topology change process now. In the former 802.1D-Spanning-Tree, a TCN was created when a port switched to redirect mode or switched from learning or transmitting to blocking mode. However, in RSTP, the topology change process is only generated when a port goes into redirect mode. In addition, the BPDU TCN is no longer used if the network only operates 802.1w. RSTP now uses only one type of BPDU, the BPDU configuration, for everything. So, when changing topology, it only sends a configuration BPDU with the TC bit set to draw the attention of other switches to the fact that a topology port has been moved to the transmission state.
If another switch were to work older PVST, in addition to this BPDU, a BPDU TCN would be generated. When a new connection is created between root and Switch 1, both ports are moved to a certain blocking state until they receive a BPDU of their equivalent. The proposal bit on BPDUs is only fixed and sent if a designated port is in a state of rejection or learning. This process occurs for port P0 of the main deck. As Switch 1 receives superior information, Switch 1 immediately knows that Port P1 is the new root port. Switch 1 then ensures that all ports are synchronized with this new information. If we start with the fact that all links are in a shutdown state. Let`s say fa0/0 was raised on Sw1 and Sw2. Both ports immediately go into the port-roll and the rejected port status (this is the very first role and the very first status for a non-Edge port). Both switches transfer a BPDU with the bit of the proposal. In this topology, this actually means that they have to agree on the switch that ultimately converts their port into the specified transmission and routing state, and the switch that makes their port a port in the redirect position.
This is decided by the information in the BPDU which put the proposal bit. Let`s see what`s typically in this BPDU. Switch A reacts to the root switch with a transmission agreement. Both Switch ports move quickly to the redirect state with the certainty that there will be no layer 2 loops.