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A rapid spanning tree is going to give us some new port roles for us to learn.

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As we see here, I've already got some of them written out.

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A couple of them look familiar, root and designated.

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A couple of them might not look too familiar.

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We have alternate and backup.

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When we go into the details of these port roles, it's important for us to

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remember that

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when a switch becomes part of this spanning tree topology, it's going to

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identify its

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upstream ports, meaning it's towards the root.

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It's going to identify its downstream ports.
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It's going to identify its downstream ports.

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Downstream ports go away from the root, and it's as simple as that.

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And so when we break down these different port roles and what ends up happening

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, we want

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to pay attention to this concept and the fact that in the vast majority of

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cases, we're

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going to identify our best upstream path as the root port.

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We're going to block all the other upstream ports, and we're going to leave all

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of the

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downstream ports online, and that creates a loop-free topology.

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And there's just a couple of things for us to know about that as we explore
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And there's just a couple of things for us to know about that as we explore

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these different

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root port roles.

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Now one thing we noticed is not on here is the concept of the non-designated

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port.

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The non-designated port used to be this concept where I have an upstream port,

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but I'm going

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to have to block it because it's not my best upstream port.

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When we get rid of non-designated ports, it doesn't show up in our list here.

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So as we go through these, we'll notice, first of all, the root is still the

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upstream,

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and it's going to be the best, meaning that it's going to be forwarding towards
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and it's going to be the best, meaning that it's going to be forwarding towards

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the root.

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Our alternate port is also going to be upstream.

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However, it's going to not be the best, and therefore it's going to have to be

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blocking.

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And yes, we should start to use the official rapid spanning tree term, so it's

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going to

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be discarding.

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Meanwhile, the designated port is a term we should be familiar with.

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That used to be these down here, and it still is.

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The designated port is my downstream port, and it's also in a forwarding state.
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The designated port is my downstream port, and it's also in a forwarding state.

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But then we have this backup port concept.

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We're kind of out of situations, aren't we?

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I mean, we've described the three situations in that diagram there, but there

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is one other

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one that we need to be at least remotely aware of.

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This doesn't happen too often in modern networks.

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I'll say that.

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It is a downstream port that gets blocked.

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We've basically said this doesn't happen, and so we'll have to explore the

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scenario

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where it does happen.

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But the primary takeaway at this point is that when we look at rapid spanning
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But the primary takeaway at this point is that when we look at rapid spanning

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tree, they

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have defined four clear port roles, depending on whether it's upstream or

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downstream, and

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whether we're forwarding, or hydro blocking again, or discarding.

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How about that?

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So let's go ahead and go into these.

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So first of all, let's talk about the root port.

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And it's going to be a very quick conversation because not only is it the same

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as our traditional

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spanning tree, but I've already described it here within this video.

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What simply put is going to be the best path to the root.
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What simply put is going to be the best path to the root.

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And as always, this is going to start with the best cost.

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And from here, we can look at tiebreakers if required.

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Now keep in mind that the root itself will not actually have a root port

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because it is

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the root, and therefore it won't have any upstream ports.

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It won't have a root port or an alternate port or anything along those lines.

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But that would be the root port.

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So next, let's go ahead and talk about the alternate port.

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So the alternate port is a replacement for the non-designated port.
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So the alternate port is a replacement for the non-designated port.

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But it comes with a feature.

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It has the ability to perform a fast failover.

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The alternate port is basically identifying another path to the root.

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And if it's another path to the root, well, then that means it's another

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upstream port.

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And we understand that we have to block those otherwise loops will end up

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forming.

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However, what happens if the root fails?

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The root port specifically, sorry, if the root port fails, then I can very
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The root port specifically, sorry, if the root port fails, then I can very

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quickly,

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immediately activate my best alternate port.

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So that's the idea of the alternate.

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Is that I can actually activate an alternate.

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My best alternate port at that point.

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So I might have many different upstream paths.
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So I might have many different upstream paths.

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I might have this selected as my root port, and these will be selected as my

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alternate

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ports.

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Well, then the root port goes down.

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And so I'm going to evaluate, depending on which one of these alternate ports

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is better,

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I'm going to immediately make that my new root port.

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And because of that, it immediately transitions to forwarding.

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The other side also immediately transitions into forwarding.

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And so we come online very, very quickly.

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Now, one thing we didn't cover in this course because it's out of scope is the

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concept of

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Cisco uplink fast.
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Cisco uplink fast.

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Uplink fast was one of those enhancements that Cisco created for traditional

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spanning

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tree.

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And we could basically get this exact functionality out of uplink fast, even on

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traditional spanning

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tree.

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And we don't need to worry about uplink fast or even having to configure it.

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Like we still have to configure port fast or type edge, but we don't have to

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configure

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uplink fast because this is just baked into rapid spanning tree now.

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We don't have to tell it which ports are alternate ports because it's able to

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figure

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that out thanks to BP DU propagation.
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that out thanks to BP DU propagation.

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And so this all happens automatically and it's a beautiful thing.

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So next, let's look at our downstream options here.

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So one of which is simply our designated port and the designated port.

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There's not much to say here.

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It's a port that is forwarding in the downstream direction, forwarding

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downstream.

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The bigger question is when does this backup port come into play?

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Because at that point, we're talking about a downstream port that is blocking
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Because at that point, we're talking about a downstream port that is blocking

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or discarding.

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So how does this even happen?

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Well as I mentioned earlier, this doesn't happen too often in modern networks

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because

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it's going to require a hub.

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And at this point, we might even start to shut this conversation down in our

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heads because

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we're not going to see this a whole lot in the modern network.

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However, be aware of what exactly this is for because this terminology might

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still show

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up somewhere.
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up somewhere.

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So the idea is this.

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I have a switch and maybe I have another switch on the other side.

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It doesn't really matter in this situation, but I also have a hub in here.

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And usually we draw this as saying, oh yeah, we both got a connection into the

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hub.

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But what if I have two connections into the hub?

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What if I want some redundancy?

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Well, that creates a problem because it creates a little loop.

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It's almost like I connected this switch port to itself.

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And that's going to create a problem.

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So what happens is we use normal spanning tree BPDU propagation to identify
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So what happens is we use normal spanning tree BPDU propagation to identify

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which port

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is lower, like let's say it's ports 13 and 14.

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Well, 13 is lower and therefore 13 is going to win.

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14 is going to go into the blocking state.

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Well, that, well, again, discarding, but either way, this becomes a backup port

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from

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a spanning tree terminology perspective.

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So that's all it is.

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It's a downstream facing port.

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And yet it's going to be in a discarding state.

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And that makes it a backup.
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And that makes it a backup.

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So now we see that we've got all of these different options.

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We have alternate ports.

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We have designated ports and root ports still.

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And then we also have this backup port.

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I would say the primary takeaway from this conversation should be a little bit

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more focused

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on the alternate port and how it works and allows a very fast failover because

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root and

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designated ports pretty well identical to what we've already learned and backup

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ports

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shouldn't be showing up a whole lot again in our modern networks thanks to the

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requirement

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of having a hub.