On point to point links a proposal agreement.
Handshake sequence is used by rapid spending tree to achieve foster convergence.
So rapid spanning tree uses this to quickly transition ports to the forwarding state, whereas 82 to
1 D was just waiting for timers to expire before ports were set to the forwarding state.
In this topology, if the link between the route switch on Port zero and Port one on the switch came
up, they would send proposals to each other.
Both ports are put into designated blocking and they both send a rapid spanning tree BPD with a proposal
but set.
Now this relies on a new BPU format which contains a proposal, but in the BPD view.
So additional information in a rapid spanning tree BPU has been added.
We have the proposal, but we have the port role.
We have learning, forwarding and agreement bits as well as a topology change bit and topology change
acknowledgement, but that are part of the rapid spanning tree BPU.
So essentially what happens is when the port comes up, the switches send proposals to each other saying
I want to have the designated port on the segment.
However, because bridge, this switch here receives a BPU indicating a superior path cost.
In other words, a better path is found via the route switch then itself.
It immediately knows that port one is going to be its new route port.
So it know straight away that this port should be its root port because this port has the best path
back to the route switch.
In other words, a superior or better or lower path cost.
Bridge.
A In other words, the switch starts a sync to ensure that all of its ports are in sync with this new
information.
A port is in sync if it meets the following criteria.
It is in the blocking state.
In other words, it's discarding or it's an edge port.
So in this example, Port two is an alternate port.
Port three is a designated port and port four is a edge.
Port support two and port four already meet one of the criteria listed above.
It's in the blocking state or it's an edge port so to be in sync bridge a must block port three and
assigns it to the discarding state.
Now that all ports are in sync bridge a can unblock its newly selected route port and reply to the route
with an agreement message.
So basically a proposal is sent from the root to switch a switch.
A agrees that this is the best path back to the root.
It blocks all ports or make sure that they are in sync because they edge ports and then sends back an
agreement.
As soon as that happens, the ports can be unblocked.
So Bridge can unblock its port and replies with an agreement message.
Once the root switch receives that agreement message, it can transition immediately to the forwarding
state.
So proposal, agreement, unblock port.
In other words, set it to the forwarding state.
Now that happens very, very quickly.
Now, by the same token, switch a can send a proposal to the switch.
So this process continues.
Proposal to the switch.
The switch sends back an agreement.
The support can go to the forwarding state.
This happens very quickly because it doesn't rely on timers.
This wave of handshakes propagates quickly towards the edge of the network and quickly restores connectivity
after a change in the topology.
If a designated discarding port does not receive an agreement to its proposal, it slowly transitions
to the forwarding state using the traditional 8 to 1 D listening learning sequence.
This could happen if the remote bridge doesn't understand rapid spending tree beeps or if the remote
bridges port is blocking.
So in this topology at the moment shows spanning tree.
Gigabit zero one is a designated port in the forwarding state, but it's a shared port.
In this topology, gigabit zero zero has been shut down.
So the route put on switch three is gigabit zero one and the alternate port is gigabit zero two.
But if I no shut gigabit zero zero.
So I've enabled this port again.
Shows spanning tree notice.
Gigabit zero zero is the root port of switch three.
This port is now the designated port but is in a blocking state.
Zero two is in the blocking state.
Back up port on switch to the status has changed to blocking because on this segment this is the best
port to use to get back to the root bridge.
However, it takes longer for this process to complete than it would be if it was a point to point link.
Shade ports take longer to converge.
So let's do that again.
I'll shut the port.
So gigabit to zero zero is now going to be shut down.
Previously, Gigabit zero one in the switch was the alternate port.
Now it's the designated port, but it's blocking.
Still blocking.
Show spending tree on switch three notice.
Alternate port is blocking route port is forwarding.
Now the port is in the learning state, so traffic is still being dropped on this port, even though
this is the port to use to get to the route bridge.
Now it's forwarding can take 30 seconds on a shared port because we're using traditional ETA 2 to 1
DX listening learning sequences on that port.
To use proposals and agreements.
The ports need to be configured as point to point ports or need to negotiate to use full duplex.