Hello,
I think I understand the proposal/agreement sequence within RSTP, but not quiet sure about the below. Let's say bottom bridge had priority 20k and was root, and it increased priority to 36k. Root must become top bridge because it has 24k. From both sides (picture: left & right), bridges start to compete with each other, saying that they are root (because top bridge is silent and it does not send any packets yet, just receiving).
Below I explained two scenarios. I do not understand why in first scenario when top bridge did not agree/accept that 20k BPDU priority and still replied saying "i am root". And in second scenario when link failed on the bottom left side, top bridge advertised to the top_right that it is becoming root, top_right replied no twice, then top bridge agreed.
############ SCENARIO 1 - bottom root bridge increased priority from 20k to 36k. root must become top birdge (priority 24k) #
1. top bridge sending these packets every two seconds on its right cable towards 7c9a because it's port is desiganted.
2. At some time, bottom switch changed priority to 36k. propagation/competition for best root started on the left side. Change reached top bridge throught the left side first. Next packet it's sending saying that it's becoming root:
3. Bridge 7c9a (top right) did not receive propagation yet, and replies saying no, I'm becoming Designated port now and I have a path towards bottom root bridge with priority 20k (outdated info, 20k was increased to 36k on the bottom bridge).
4. top switch replies to the top right siwtch: no im still the root with x2 same packets in the same second. < why? how does it know that there was no link failure on the left side?
5. top right bridges sends agreement and aknowledges root bridge is top one.
########### SCENARIO 2 - LINK FAILED BETWEEN BOTTOM ROOT BR & BOTTOM LEFT
1. top bridge sending BPDUs towards top right bridge saying bottom one is root with 20k priority
2. link failed somewhere at the bottom left. propagation reached top bridge first from left, and it sends now message that it's becoming root towards top right (7c9a).
3. 7c9a did not agree and twice in a second sent reply to top birdge that bottom one with 20k is root.
4. top one sends agreement to the top right and makes it's port root
