A rapid spanning tree is going to give us some new port roles for us to learn.
As we see here, I've already got some of them written out.
A couple of them look familiar, root and designated.
A couple of them might not look too familiar.
We have alternate and backup.
When we go into the details of these port roles, it's important for us to
remember that
when a switch becomes part of this spanning tree topology, it's going to
identify its
upstream ports, meaning it's towards the root.
It's going to identify its downstream ports. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It's going to identify its downstream ports.
Downstream ports go away from the root, and it's as simple as that.
And so when we break down these different port roles and what ends up happening
, we want
to pay attention to this concept and the fact that in the vast majority of
cases, we're
going to identify our best upstream path as the root port.
We're going to block all the other upstream ports, and we're going to leave all
of the
downstream ports online, and that creates a loop-free topology.
And there's just a couple of things for us to know about that as we explore WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And there's just a couple of things for us to know about that as we explore
these different
root port roles.
Now one thing we noticed is not on here is the concept of the non-designated
port.
The non-designated port used to be this concept where I have an upstream port,
but I'm going
to have to block it because it's not my best upstream port.
When we get rid of non-designated ports, it doesn't show up in our list here.
So as we go through these, we'll notice, first of all, the root is still the
upstream,
and it's going to be the best, meaning that it's going to be forwarding towards WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and it's going to be the best, meaning that it's going to be forwarding towards
the root.
Our alternate port is also going to be upstream.
However, it's going to not be the best, and therefore it's going to have to be
blocking.
And yes, we should start to use the official rapid spanning tree term, so it's
going to
be discarding.
Meanwhile, the designated port is a term we should be familiar with.
That used to be these down here, and it still is.
The designated port is my downstream port, and it's also in a forwarding state. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
The designated port is my downstream port, and it's also in a forwarding state.
But then we have this backup port concept.
We're kind of out of situations, aren't we?
I mean, we've described the three situations in that diagram there, but there
is one other
one that we need to be at least remotely aware of.
This doesn't happen too often in modern networks.
I'll say that.
It is a downstream port that gets blocked.
We've basically said this doesn't happen, and so we'll have to explore the
scenario
where it does happen.
But the primary takeaway at this point is that when we look at rapid spanning WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But the primary takeaway at this point is that when we look at rapid spanning
tree, they
have defined four clear port roles, depending on whether it's upstream or
downstream, and
whether we're forwarding, or hydro blocking again, or discarding.
How about that?
So let's go ahead and go into these.
So first of all, let's talk about the root port.
And it's going to be a very quick conversation because not only is it the same
as our traditional
spanning tree, but I've already described it here within this video.
What simply put is going to be the best path to the root. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
What simply put is going to be the best path to the root.
And as always, this is going to start with the best cost.
And from here, we can look at tiebreakers if required.
Now keep in mind that the root itself will not actually have a root port
because it is
the root, and therefore it won't have any upstream ports.
It won't have a root port or an alternate port or anything along those lines.
But that would be the root port.
So next, let's go ahead and talk about the alternate port.
So the alternate port is a replacement for the non-designated port. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So the alternate port is a replacement for the non-designated port.
But it comes with a feature.
It has the ability to perform a fast failover.
The alternate port is basically identifying another path to the root.
And if it's another path to the root, well, then that means it's another
upstream port.
And we understand that we have to block those otherwise loops will end up
forming.
However, what happens if the root fails?
The root port specifically, sorry, if the root port fails, then I can very WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
The root port specifically, sorry, if the root port fails, then I can very
quickly,
immediately activate my best alternate port.
So that's the idea of the alternate.
Is that I can actually activate an alternate.
My best alternate port at that point.
So I might have many different upstream paths. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So I might have many different upstream paths.
I might have this selected as my root port, and these will be selected as my
alternate
ports.
Well, then the root port goes down.
And so I'm going to evaluate, depending on which one of these alternate ports
is better,
I'm going to immediately make that my new root port.
And because of that, it immediately transitions to forwarding.
The other side also immediately transitions into forwarding.
And so we come online very, very quickly.
Now, one thing we didn't cover in this course because it's out of scope is the
concept of
Cisco uplink fast. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Cisco uplink fast.
Uplink fast was one of those enhancements that Cisco created for traditional
spanning
tree.
And we could basically get this exact functionality out of uplink fast, even on
traditional spanning
tree.
And we don't need to worry about uplink fast or even having to configure it.
Like we still have to configure port fast or type edge, but we don't have to
configure
uplink fast because this is just baked into rapid spanning tree now.
We don't have to tell it which ports are alternate ports because it's able to
figure
that out thanks to BP DU propagation. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that out thanks to BP DU propagation.
And so this all happens automatically and it's a beautiful thing.
So next, let's look at our downstream options here.
So one of which is simply our designated port and the designated port.
There's not much to say here.
It's a port that is forwarding in the downstream direction, forwarding
downstream.
The bigger question is when does this backup port come into play?
Because at that point, we're talking about a downstream port that is blocking WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Because at that point, we're talking about a downstream port that is blocking
or discarding.
So how does this even happen?
Well as I mentioned earlier, this doesn't happen too often in modern networks
because
it's going to require a hub.
And at this point, we might even start to shut this conversation down in our
heads because
we're not going to see this a whole lot in the modern network.
However, be aware of what exactly this is for because this terminology might
still show
up somewhere. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
up somewhere.
So the idea is this.
I have a switch and maybe I have another switch on the other side.
It doesn't really matter in this situation, but I also have a hub in here.
And usually we draw this as saying, oh yeah, we both got a connection into the
hub.
But what if I have two connections into the hub?
What if I want some redundancy?
Well, that creates a problem because it creates a little loop.
It's almost like I connected this switch port to itself.
And that's going to create a problem.
So what happens is we use normal spanning tree BPDU propagation to identify WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So what happens is we use normal spanning tree BPDU propagation to identify
which port
is lower, like let's say it's ports 13 and 14.
Well, 13 is lower and therefore 13 is going to win.
14 is going to go into the blocking state.
Well, that, well, again, discarding, but either way, this becomes a backup port
from
a spanning tree terminology perspective.
So that's all it is.
It's a downstream facing port.
And yet it's going to be in a discarding state.
And that makes it a backup. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And that makes it a backup.
So now we see that we've got all of these different options.
We have alternate ports.
We have designated ports and root ports still.
And then we also have this backup port.
I would say the primary takeaway from this conversation should be a little bit
more focused
on the alternate port and how it works and allows a very fast failover because
root and
designated ports pretty well identical to what we've already learned and backup
ports
shouldn't be showing up a whole lot again in our modern networks thanks to the
requirement
of having a hub.