On point to point links a proposal agreement handshake sequences used by rapid spending tree to achieve
fast convergence so rapid spending he uses this to quickly transition ports to the fording state where
as a two to one D was just waiting for timer's to expire before ports were set to the forwarding state
in this apology the link between the router switch on port 0 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 spending tree BPT with a proposal
but set.
Now this relies on a new BPU format which contains a proposal.
But in the BPT.
So additional information in a rapid spinning tree BPT you be noted.
We have the proposal but we have the port roll we have learning forwarding and agreement butts as well
as a topology change but and topology change acknowledgement.
But that are part of the rapid spending 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 bridgeway this switch here receives a BPT you indicating a superior Poth cost.
In other words a better path is found via the route switch then itself it immediately knows that port
1 is going to be its new route port.
So know straight away that this port should be its route 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 isn't the blocking state.
In other words it's discarding What's an H port.
So in this example port 2 is an alternate port Port 3 is a designated port and port 4 is a large port
support to and port for a ready 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 route to switch a switch a agrees that this is the best path
back to the route.
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 bridgeway can block its port and replies with an agreement message.
Once the route 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.
This port can go to the forwarding state.
This happens very quickly because it doesn't rely on timers.
This wave of hand-shakes 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 Ada to the one d.
Listening learning sequence.
This could happen if the remote bridge doesn't understand Reppert spending three BPT use with them remote
bridges port is blocking.
So an apology at the moment shows spanning tree gigabit 0 1 is a designated port in the fording state
but it's a shared port.
In this topology gigabit 00 has been shut down.
So the root port on switch 3 gigabit is 0 1 and the alternate port is gigabit 0 2.
But if I know shut gigabit 00 so I'd be unable to support again show spending tree noticed gigabit 00
is the root port of switch 3.
This port is now the designated port but is in a blocking state.
0 2 is in the blocking state backup port on switch to the status has changed to blocking because on
this segment this is the base pool 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
shit porch take longer to converge.
So let's do that again.
Shut the port so gigabit to 0 0 is not going to be shut down.
Previously gigabit zero or one on the switch was the old tenant port.
Now it's the designated port but it's blocking still blocking
show spinning tree on switch three is alternate port is blocking route Portas 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 root
bridge.
Now it's fording can take 30 seconds on a shared port because we using traditional Ada to the one d
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.