- [Narrator] In this video we want
to review Rapid Spanning Tree Protocol
which we covered back in your CCNA studies.
And remember Cisco's specific implementation
of Rapid Spanning Tree Protocol is called rapid PVST+
because Cisco's PVST+, which allows us to have
a different instance of spanning tree for every VLAN.
It's going to be used as the basis for rapid PVST+,
but the convergence can happen quicker
because of the the rapidness of rapid PVST+.
Now let's review some of the basics
of Rapid Spanning Tree Protocol.
We still have a root bridge,
just like we do in traditional Spanning Tree Protocol,
we still have designated ports, which is a port
on a segment that's closest to the root in terms of cost,
we still have a root port, which is a port
on a non root bridge, that's the closest to the route
in terms of cost, we still have a disabled
or a port that is shut down.
However we have a couple of different types
of blocking ports, we have an alternate port
and we have a backup port.
Let's make sure we understand the difference between those.
Take a look at that link between SW2 and SW3.
Notice the end on SW2 is a blocking port,
but it's an alternate port, whereas the link
on the right hand side of SW3 going up to the hub,
that's also a blocking port, but it's a backup port.
The difference is we're connecting
to a hub with the backup port, and we're connecting
to a switch with the alternate port.
You see if we just connected the two ports
from a switch into a hub, we're going to have a layer 2 loop,
that traffic is just going to come through the hub
and right back into the other port.
We don't want that.
So we need to block one of those ports.
But if we're going to a hub, a shared media device,
that's called a backup port,
but if we're going into a switch,
which is probably about 100% of the time these days,
that's going to be called an alternate port.
Let's also define some of the different port states.
Remember with traditional Spanning Tree Protocol,
we had the blocking, listening learning forwarding.
Well here, instead of blocking,
we've talked about discarding, which is the same
as blocking, we're going to be dropping data packets.
And we're going to be doing that if we're the alternate state,
connecting into a blocking port on a switch.
In the backup state, connecting into a hub,
or a port that's administratively shut down or disabled.
And instead of having a listening and learning state,
those have been combined into a single learning state
that we're going to transition through.
And finally, we go to the forwarding state,
we're actively forwarding traffic.
And one way that convergence happens quicker
is that we don't rely on the route
to just stop sending BPDUs
and after a while we get suspicious thinking that, you know,
it's been a while since I heard
from the route I wonder if something is up.
Well, what we're going to do instead is allow everybody
in the topology to play a role in updating everybody else.
when there is a change in the topology,
when a switch notices that a link goes down,
any of our switches be it a root bridge or not,
can send out a topology change notification or a TCN
that can help speed up the convergence process.
A couple of other terms I want you to be aware
of before we get into the configuration in the next video,
we have a different link types.
And if we're just connecting from a switch
into another switch, and we're running it full duplex,
that's a point to point type of connection.
That's a point to point link type specifically.
If we're connecting into a hub,
which is a shared media device well,
that's going to be a shared link type.
And remember, if we're connecting into a hub
that's going to be running at half duplex,
because we have to run CSMA/CD on a hub.
And if we're connecting into an end device like a PC
or a laptop here, that's going to be an edge port.
Now, this brings up a point of confusion.
It seems like that's a full duplex port
from the perspective of switching SW2.
Yes, it's connecting to a laptop, which is an edge device
but how does it know it's not connecting to a switch?
Shouldn't that be a point to point link type?
Well, actually, it is a point to point link type
but it's a special type of port on that point
to point link type, it's an edge port.
How do we make that port an edge port?
We can figure something we talked about
in our CCNA studies called portfast.
Portfast is when we go into a switch port and we say,
I know that you want to run through the normal delays
of Spanning Tree Protocol to make sure
that we're not forming a loop before you go active
on this port, but I promise I'm not going to connect
another switch to the sport, it's going to be some sort
of an end device, you will never see
a bridge protocol data unit coming on this port.
So please don't make me wait through the delay
of Spanning Tree Protocol.
And once we give that solemn vow and we say,
trust me go active almost immediately,
and we enable portfast then we don't have to wait
for the 30 seconds that we would normally wait
with traditional Spanning Tree Protocol when we plug
in an end device.
Well if we enable port fast here, that makes
this an edge port.
Now let's take a closer look at how all
the different switches participate together
to inform one another about
a new topology once we lose our current root bridge.
Let's say that before the transition happened,
before there was a topology change,
let's say that the root bridge used to live somewhere off
the bottom of the screen.
However, something happened to that root bridge,
it went down and now we'll say that the root bridge
is somewhere off of the top of the screen.
And as a result switch SW1 has a new root port.
That's step one in this process.
We've got a new root port which is a port
on a non-root bridge that's closest
to the root in terms of cost.
You see, the bottom port on switch SW1 that used
to be the root port, because it would go out of the bottom
to get to the root bridge, somewhere off the bottom
of the screen, but now it's going to go out that top port.
Was a second step once it elects a new root port,
What's it do with the other root port?
Well, that root port just to prevent any sort
of temporary loop is going to go into a blocking state.
So let's going to block,
that's going to be step two,
and we want to proactively inform switch two
about this change.
So what we're going to do step three,
is we send out a proposal.
And this proposal says,
"I propose that I know a better way
to get to the root bridge, our other root bridge
is no longer available via the path you used to use now I'm
the best way to get to the root."
And if switch two believes that, if it accepts it,
it will send out an agreement that's going to
be step number four, will say,
"okay I agree to that."
And when it agrees it takes the port
which was previously a designated port,
because that was the end on the segment that was closer
to the root in terms of cost
when the root was off the bottom of the screen.
So step number five is we're going to transition
from a designated port to a root port
on the top of switch SW2.
That is step number five.
But switch SW1 still has a blocking state
on its bottom port.
But as soon as it receives that agreement message,
about the same time that step number five is happening.
Step number six happens, we transition from a blocking state
and we become a designated port.
Now on that segment between SW1 and SW2,
the bottom port on SW1 is the closest to the root.
That is step number six.
Now I'll not go through all those steps again,
but you can see the same kind of thing is going to happen
between switch SW2 and SW3, will go blocking
on this bottom port, will send the proposal,
will get the agreement and it'll daisy chain like
that throughout the rest of the tree.
And that's a review
of how Rapid Spanning Tree Protocol works.
Now join me in our next video where we're going to take a look
at the configuration of Rapid Spanning Tree.