WEBVTT

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 Hello and welcome to this video titled
 the rapid spanning tree protocol

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 topology changes.

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 In this video, that's exactly what
 I'm going to talk about is how does

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 rapid spanning tree protocol react
 when a topology change occurs?

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 So let's look into it.

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 So if you've watched any of my videos
 or studied rapid spanning tree on

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 your own, you'll know that.

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 The original spanning
 tree came out in 1998.

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 It was called 802 dot 1d.

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 Rapid spanning tree came out
 three years later in 2001.

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 It was called 802 dot 1d as an
 amendment to spanning tree.

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 And then it was incorporated into 802
 dot 1d three years later in 2004.

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 Now in the original version of 802 dot
 1d in 1998, basically, almost anything

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 that could happen to a port would cause
 that switch to generate a topology

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 change. If a port went blocking, if
 a port went forwarding, if it went

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 down, pretty much any event you could
 think of that could happen to a

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 port on a switch would cause that switch
 to say, oh, there's been a topology

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 change. I need to let the root
 bridge know about that.

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 And in 802 dot 1d, the original, it was
 really the job of the root bridge

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 once he found out that there had been
 a topology change to let the rest

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 of the switch topology know
 that something had happened.

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 And then what would happen as a result
 of that was something we're not

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 going to cover because this video
 is not talking about that.

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 So when 802 dot 1d w, 802 dot 1d w
 came out rapid spanning tree, they

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 changed the topology change
 process quite a bit.

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 So the first thing they changed was
 they said, okay, we want to restrict

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 this a little bit.

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 We don't want any willy-nilly change
 on a port to cause this topology

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 change process to happen that
 I'm about to describe for you.

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 So with 802 dot 1d w, which is now incorporated
 into spanning tree, they

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 said only when a non edge port goes
 forwarding, will that be considered

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 a topology change?

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 Now remember, what
 is a non edge port?

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 That is a port that does
 not have port fast.

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 So any interface on your switch running
 rapid spanning tree that does

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 not have port fast configured
 is considered a non edge port.

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 And only when that port goes into the
 forwarding state, will that switch

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 say, ah, a topology change
 has just happened.

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 Okay. So if the port goes blocking,
 if it goes down, nope, that doesn't

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 cause it. Only if it goes
 into the forwarding state.

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 Secondly, what does that
 switch do about it?

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 Well, in rapid spanning tree, if I
 am a switch, doesn't matter if I'm

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 the root bridge or not, if I'm any
 switch in the topology and all of a

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 sudden I have a non edge
 port that just comes up.

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 Now remember, if this is a non edge
 port, it's probably a port leading

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 to another switch.

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 Because if this is a port leading to
 a host device like a laptop or a

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 PC or even a port leading to a router,
 one would assume we would have

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 configured port fast
 on that interface.

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 So if a non edge port goes forwarding,
 that's most likely an infrastructure

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 port leading to another switch, either
 a new switch in the topology or

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 something. So in that case, I say, oh,
 I need to let the whole topology

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 know that there's been
 a topology change.

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 So how am I going to do that?

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 I'm going to generate a
 topology change BPDU.

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 Now, if you know what the structure
 of a regular BPDU looks like, and

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 actually let's just go back a little
 bit and take a look at one of those

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 real quickly. So here was our BPDU,
 our rapid spanning tree BPDU.

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 And if you'll recall, in the rapid spanning
 tree BPDU, we had a one byte

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 field called flags.

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 So there was our flags field.

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 And in the flags field, let's go ahead
 and look at that a little bit more

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 closely. So in the flags field,
 here was the flags field.

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 We had a topology change bit and a topology
 change acknowledgement bit.

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 So if I am a switch, whether I'm the
 root bridge or not, and I detected

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 a non edge port has just gone forwarding,
 I will immediately create a

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 BPDU. Now if I'm not the root bridge,
 I'm not going to all of a sudden

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 say, hey, I'm the root bridge,
 I'm not going to do that.

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 I will still say, hey, the root bridge
 is a switch out there, but I will

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 now set this bit right
 here to a one.

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 So we'll say yes, there has been a
 topology change and I will do that.

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 Now where am I going to send that?


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 So I've just created that BPDU, I've
 just set the topology bit to a one.

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 Where am I going to
 send that BPDU?

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 Well as you can see right here, I
 will transmit it on my root port.

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 It says all root ports.

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 Well I'm only going to have one root
 port and I will transmit it on all

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 of my designated ports
 that are non edge.

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 Let's just draw this
 here for a second.

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 Kind of wordy. So here's
 the root bridge out here.

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 He's somewhere in the cloud that I
 get to him and here's me, here's my

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 switch. So here's my root port.

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 Here I have a couple of designated
 ports, but these are edge.

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 I've got port fast
 turned on on those.

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 So maybe one's leading to a PC, maybe
 one is leading to a router.

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 Here I have a couple of other designated
 ports leading to downstream switches.

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 Maybe here I have a couple
 of blocking ports.

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 Okay so that's my steady state.

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 Now all of a sudden a new port comes up,
 most likely connected to a switch.

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 This goes forwarding.

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 It's a designated port
 and it's non edge.

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 That is my trigger.

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 I say aha, I need to create
 a topology change BPDU.

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 So as we can see, I will send that topology
 change BPDU out my root port

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 and out my non edge
 designated ports.

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 I'm not going to send it out my blocking
 ports and I'm not going to send

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 it out my edge ports.

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 That router, that PC, they wouldn't
 even know what to do with it anyway.

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 They're not running spanning tree.


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 So that's where it's going to go.

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 So the first critical piece of this
 is you need to recognize where the

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 topology change BPDU will go.

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 Now you might say, well why
 do I care where it will go?

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 Well because in rapid spanning tree,
 here's the critical thing.

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 If I transmit a BPDU out an interface,
 like in this case I'm transmitting

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 it out here, here, and here, any MAC
 addresses that I learned on those

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 interfaces gone, I flush
 them, I delete them.

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 So that's the critical thing.

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 If you don't know which interface the switch
 is going to forward his topology

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 BPDU on, you won't understand why did
 some MAC addresses just get erased?

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 Why did some MAC addresses
 not get erased?

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 Now also, let's say that
 we have this situation.

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 Okay, so here is a root port.

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 Here we have a designated port, a designated
 port, and another designated

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 port, but this is edge.

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 Okay, now I receive a
 topology change BPDU.

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 So this is me right here.

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 So some non-edge port went forward
 somewhere in the topology.

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 It wasn't on my switch, but because
 that happened, I received a topology

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 change BPDU. So first question,
 where am I going to send it?

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 Well we know. I'm not going to send it
 on my root port in this case because

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 I just received it
 on my root port.

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 I wouldn't reflect it right back out
 the same port I just received it

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 on, but I will send it out here on
 this interface and I'll send it out

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 here. These are my non
-edge designated ports.

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 Okay, according to Rapid Spanning Tree,
 any MAC addresses I learned on

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 these two interfaces where I transmitted
 it, those are wiped out.

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 So remember, my edge port,
 I did not send it there.

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 I did not forward the
 topology change BPDU.

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 So any MAC addresses I learned on
 that edge port, those are safe.

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 I'm not going to delete those.

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 Also, the port where I received the
 topology change BPDU, that's safe

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 as well. Any MAC addresses I learned
 on that interface are not going to

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 be touched. Only the MAC addresses on
 the interfaces where I transmitted

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 or forwarded that topology change BPDU,
 only those MAC addresses are wiped

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 out. And that's what this is
 talking about right here.

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 So just one more thing.

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 So let's take a look at this.

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 Now let me ask you, I'm going to ask
 you a question and then I want you

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 to pause the video and answer it.

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 Here is your question.

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 If I connect a link between switch 4
 and switch 3, so right here on switch

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 3, now we have a non-edge
 port going forwarding.

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 Here's my question to you.

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 How will that affect the MAC address
 tables that you see here?

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 So go ahead and take maybe a screenshot
 of this video, this image right

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 here, put the video on pause and what
 I want you to do is I want you to

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 draw a line through any MAC addresses
 that will be deleted in these three

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 tables as a result
 of this process.

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 So go ahead and pause the video now
 and then press play when you think

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 you've got the answer.

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 Okay, so here is the answer.

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 So number one, we know that with a non
-edge port going forwarding, that

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 means switch 3 is going to forward out,
 he's going to create a topology

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 change BPDU and he's going to forward
 it out all non-edge ports he has,

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 including his root port.

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 So he's going to forward
 it out here.

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 And he's also going to
 forward it out here.

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 Okay, because after all, this is a non
-edge port, it is a designated port.

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 Now according to his MAC address table,
 okay, he did not send it out one

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 four or one five, so those
 edge ports are safe.

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 He did transmit it on one slash three,
 so these MAC addresses right here

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 that he learned on one slash
 three, those are deleted.

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 Now I guess the root bridge, he's going
 to forward it out his non-edge

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 forwarding ports.

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 So any MAC addresses that he learned
 on one slash one are deleted, gone.

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 Gets to one slash two.

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 I'm sorry, gets to
 switch number two.

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 Switch two is going to forward it
 out this interface right here.

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 Well because the other side of that interface
 is blocking, he didn't even

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 learn any MAC addresses there.

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 And, oh look at this, port one slash
 nine, we forgot to configure port

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 fast on there. We forgot that.

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 So from switch two's perspective,
 that is a non-edge port.

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 He'll forward the topology
 change BPD out there.

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 And so consequently any MAC addresses
 he learned on that interface will

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 be deleted. So remember, the ports where
 you received the topology change

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 BPDU, those MAC addresses that are
 learned on that port are safe.

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 It's only the ports where you forward
 the topology change BPDU that those

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 MAC addresses learned on
 those ports are deleted.

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 So that concludes this video on the
 rapid spanning tree topology change

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 process. Thank you for watching.