RSTP

RSTP Topology Change:
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->TC when non-edge port goes forward only

-> Broadcasted on the network by initiator(not by root as in 802.1d)
                  Sent on Root and non-edge Designated ports

-> Flush most of the cam entries in the network immediately except for edge ports.

-> MAC entries  learned from the port where the Topology change is forwarded are flushed out.

-> Do Not clear entries for ports that received the TC BPDU

RSTP Convergence Process:
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Switch x(Root)------- a(Switch A)

Handshake:

Below mechanism for Point-to-Point Full duplex links.

1) Switch Root sends proposal bit set.(3 proposals before moving the port to blocking)
2) On Switch A "a" becomes root port goes into forwarding and send back a BPDU with agreement bit set
3) As soon as x receives the agreement it goes to Forwarding.

If they are connected to HUB then same process as 802.1d


Propagating the Handshake:
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Switch X ----- Switch A

1)  X is the root.

So when a new switch B with lower priority is connected to the switch A

Switch X ----- Switch A ----- NEW(Switch B)

2) Switch B sends proposal to Switch A proposing it as a new root.
3) Switch A realises the B priority is better than X and does 3 things simultaneously after receiving proposal from B.

             a) Block the port connecting to X.
             b) Forward the proposal to X
             c) Send back agreement to B

4) Once X sends back the agreement to A the port a will be moved to forwarding again.


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