WEBVTT

0:00:02.960000 --> 0:00:07.500000
 Hello and welcome to this video titled
 Designated and Non-designated Port

0:00:07.500000 --> 0:00:13.340000
 Election. In this video I'm going to
 talk about designated ports and how

0:00:13.340000 --> 0:00:16.240000
 they are elected between switches.


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 I'm going to show you how you can predict,
 given any topology diagram

0:00:19.560000 --> 0:00:22.860000
 and the minimum required information
 which port will be the designated

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 port. We'll talk about something called
 BPDUs as Keep-Alize, which is

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 a characteristic of BPD.

0:00:30.960000 --> 0:00:35.100000
 We'll talk about non-designated ports
 and specifically the names of non

0:00:35.100000 --> 0:00:38.160000
-designated ports and how their names
 are different from each other.

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 Let's start talking about
 designated ports.

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 A lot of times I say that designated
 ports deliver because designated

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 ports their job is to face away from
 the root bridge and to deliver BPDUs.

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 So if I'm the root bridge and I'm delivering
 a BPDU to you, my port is

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 a designated port.

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 If you get that BPDU and then you turn
 around and you deliver it to someone

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 even further downstream, that interface
 on you that's doing the delivery

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 is a designated port.

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 Now like a root port,
 it can be elected.

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 Now if I'm the root bridge,
 there is no election.

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 Between you and me, my interface will
 always be the designated port because

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 I'm creating BPDUs and I'm sending them
 to you, so clearly it has to be

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 designated. Now when we talk about election,
 what we're really referring

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 to here is something like this, where
 if I have the root bridge and I

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 have bridge A and bridge B, and please
 know that when I use the term bridge,

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 it's synonymous with switch.

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 We know that the root bridge, his ports
 are going to be designated ports,

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 they always are.

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 But every cable between two switches,
 one side of that cable has to be

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 a designated port.

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 Somebody has to put the BPDUs onto
 the wire and whoever's job that is,

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 that is going to be
 the designated port.

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 So in this situation, on this cable
 right here, the designated port is

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 going to be elected between
 switch A and switch B.

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 Now in this particular case, how that
 election is going to transpire is

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 that between those two switches, if
 one switch has a lower cost to the

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 root bridge than the other
 switch, we're done.

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 Whichever switch has the lowest cost
 back to the root bridge, he gets

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 to be the designated
 port on that wire.

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 If two switches facing each other have
 the same cost to the reach of the

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 root bridge, then it's going to simply
 be based on their name, based on

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 their bridge ID.

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 Whoever has the lowest bridge ID, that
 switch is the winner and that switch

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 ends up becoming our
 designated port.

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 Lowest port ID, the only time that
 would come into play is if you had

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 a situation like this.

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 If I had, for example, root bridge
 here, switch A right here and I had

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 a hub. And switch A had
 two links into the hub.

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 In this particular case, he's actually
 going to be getting BPDUs from

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 himself. As a BPDU comes here, it's
 going to circle back around, he's

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 going to get it on that interface and
 as he transmits a BPDU here, it's

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 going to circle back around, he's
 going to get it on that interface.

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 So either way you slice it, he's going
 to be receiving his own BPDUs back

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 on his own interfaces.

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 So in that case, he can't even use the
 lowest bridge ID because both those

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 BPDUs will have his own name in
 the sending bridge ID field.

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 So whichever of his own ports has the
 lowest port ID, that'll be elected

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 as the designated port.

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 All the other ports in the same collision
 domain go into either root port

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 or non-designated port roles.

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 Let's take a look.

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 So here we see in this topology right
 here, we've already got some root

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 ports. Now we know that the root bridge
 himself has designated ports.

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 So let's just put that right there, designated
 port, let's put that right

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 here, designated port, and put that
 right here, designated port.

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 Okay, so we're done with
 the root bridge.

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 We've already figured out our root ports
 here and if you don't remember

0:04:41.340000 --> 0:04:44.700000
 the logic, please refer back to another
 one of my videos or any other

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 instructor videos at INE for how to
 determine the root port on a switch.

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 I'm not going to cover that here.

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 All right, so let's take,
 for example, this link.

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 That link needs a designated port.


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 All right, first step is going to be
 what of these two switches, switch

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 X and the switch ending with 44-44,
 does one of those switches have a

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 lower cost to the root
 than the other one?

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 Well, 44-44, he's going
 to take his root port.

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 This is a fast ethernet link he's directly
 connected to the root, so his

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 cost, his total cost going
 that way is going to be 19.

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 So that's what he's going
 to advertise downstream.

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 He's going to say, hey, my
 neighbor, my cost is 19.

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 Switch X, he's going up one hop of fast
 ethernet, which is 19, and then

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 he has to go up another hop of gigabit,
 which is 4, so he says, my best

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 cost is 23. That's what
 he's going to advertise.

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 So in this case, the top bridge,
 44-44, will be the winner.

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 He will be the designated port because
 he has the lower cost to reach

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 the root bridge than switch X.

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 Now, what's going to happen down
 here to switch to port 0 slash 1?

0:06:06.440000 --> 0:06:09.040000
 Well, that's not a root port.

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 It's not the designated port on that
 wire, so that port has to go into

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 the blocking state.

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 That has to block.

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 All right, so that is that,
 so we can put that in there.

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 All right, so let's mark that.

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 So we know this is going to
 be our designated port.

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 We know this down here
 is going to block.

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 Okay, what about this link?

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 Who's going to win there?

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 Well, I think you're probably going
 to see this is a pretty clear cut.

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 So 44-E4, his root port
 has a cost of 4.

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 That's a gigabit link.

0:07:06.760000 --> 0:07:12.000000
 We already said that this guy's
 root port had a cost of 23.

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 So when he advertises 23, and he advertises
 downstream, 44-E4 will win.

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 So this will end up becoming
 our designated port.

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 And once again, this port right here,
 we'll go into the blocking state.

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 So let's draw that.

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 So here we'll have our
 designated port.

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 And here we'll have
 our blocking port.

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 And then we keep going.

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 What about this segment
 right here?

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 Well, I think clearly we can see that
 when it comes to this switch and

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 then we can see, when this switch, this
 guy is closer to the root bridge

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 than switch X. So both of his
 interfaces will be designated.

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 So to recap, all of our designated
 ports are here.

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 That's designated.

0:08:12.160000 --> 0:08:14.160000
 That is a designated port.

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 And that is a designated port because
 it's on the root bridge.

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 On this link right here, this
 guy became designated.

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 On this link right here, this
 guy became designated.

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 And on these two links, the top switch
 became the designated port.

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 All right, let's talk about another concept
 as it relates to rapid spanning

0:08:41.780000 --> 0:08:46.900000
 tree, which is this concept of
 BP to use as keep-a-lives.

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 Now, if we were dealing with the original
 classic spanning tree, the 1998

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 version, before it was
 updated in 2004.

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 In the original classic version of spanning
 tree, before rapid spanning

0:09:01.940000 --> 0:09:05.220000
 tree was even invented, this
 is the way it worked.

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 The root bridge created BP to use.


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 He was the only one who
 created BP to use.

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 Once that root bridge sent a BP to you,
 you would take it in, you would

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 modify a couple of things.

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 For example, if I'm the root bridge and
 I'm sending it to you, the sending

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 bridge ID and the root
 bridge ID are both me.

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 They're both my information.

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 Now, when you get that BP to you, you
 have the freedom to change the sending

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 bridge ID to your own ID, increment the
 cost a little bit, and then forward

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 that BP to you on
 to somebody else.

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 So you would then forward it.

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 So in classic pre-2004 spanning tree,
 a bridge that was not the root bridge

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 could only forward a BP to you that he
 had received from an upstream neighbor.

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 So if your upstream neighbor suddenly
 just stopped sending you BP to use,

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 you had nothing to forward downstream
 to your neighbors.

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 It was a cascading effect.

0:10:05.560000 --> 0:10:10.100000
 So in that particular situation, if,
 let's say the root bridge is not

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 me, let's say you and I are switches,
 we're connected to each other, and

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 the root bridge is somewhere
 back there, okay?

0:10:16.200000 --> 0:10:18.160000
 Not me, but back there.

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 You've been getting
 BP to use from me.

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 On this link, I've been the designated
 bridge because I'm closer to the

0:10:23.460000 --> 0:10:24.760000
 root bridge than you are.

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 So it's been my responsibility every
 time I got a group, a BP to you from

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 the root bridge to change a little
 bit, forward it to you.

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 Change a little bit,
 forward it to you.

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 Now all of a sudden,
 what if that stops?

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 What if you stopped getting
 BP to use from me?

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 Well, that could mean
 one of two things.

0:10:43.160000 --> 0:10:47.460000
 It could mean that there's something
 wrong with me that maybe my CPU is

0:10:47.460000 --> 0:10:51.140000
 pegged to 100% and I just can't
 forward those BP to use anymore.

0:10:51.140000 --> 0:10:55.420000
 Maybe my memory is messed up
 and they're getting garbled.

0:10:55.420000 --> 0:10:56.660000
 You don't even recognize them.

0:10:56.660000 --> 0:11:00.000000
 So the problem could be me or
 the problem could be him.

0:11:00.000000 --> 0:11:01.320000
 The problem could be
 the root bridge.

0:11:01.320000 --> 0:11:02.600000
 There's nothing wrong with me.

0:11:02.600000 --> 0:11:04.720000
 I'm just not getting
 BP to use from him.

0:11:04.720000 --> 0:11:07.240000
 So I have nothing
 I can send to you.

0:11:07.240000 --> 0:11:08.900000
 Either way, you didn't know.

0:11:08.900000 --> 0:11:13.860000
 All you knew was that you weren't
 getting BP to use from me.

0:11:13.860000 --> 0:11:17.220000
 Well, with rapid spanning tree, they
 decided to take a slightly different

0:11:17.220000 --> 0:11:25.360000
 approach. In rapid spanning tree, if
 I am your designated port, it is

0:11:25.360000 --> 0:11:30.660000
 my responsibility to send you BP to use
 regardless of whether I'm getting

0:11:30.660000 --> 0:11:33.020000
 them from him or not.

0:11:33.020000 --> 0:11:35.460000
 See, here's the difference.

0:11:35.460000 --> 0:11:39.580000
 As it says, designated ports on non
-root bridges must always transmit

0:11:39.580000 --> 0:11:47.760000
 BP to use. In classic 802.1d, the old
 style, the way it worked was that

0:11:47.760000 --> 0:11:53.980000
 when you received a BP to use from
 me or I received one from the root

0:11:53.980000 --> 0:11:58.380000
 bridge, either way, every time you
 got a BP to use, it started a timer

0:11:58.380000 --> 0:12:01.760000
 that was called the max age timer.


0:12:01.760000 --> 0:12:04.080000
 I'll write that right here.

0:12:04.080000 --> 0:12:10.960000
 Max age. And back in classic, so
 I'm going to write that here.

0:12:10.960000 --> 0:12:16.420000
 This is a classic behavior that Max
 age timer was set to 20 seconds.

0:12:16.420000 --> 0:12:20.140000
 It was very, very conservative
 timer.

0:12:20.140000 --> 0:12:24.320000
 So the way that timer worked was,
 okay, I just sent you a BP to you.

0:12:24.320000 --> 0:12:26.500000
 You start the 22nd timer.

0:12:26.500000 --> 0:12:28.260000
 Now, BP to use go out
 pretty frequently.

0:12:28.260000 --> 0:12:30.580000
 They go out every two seconds.

0:12:30.580000 --> 0:12:32.860000
 That's the default,
 every two seconds.

0:12:32.860000 --> 0:12:35.500000
 So within two seconds, you should get
 another BP to you from me again.

0:12:35.500000 --> 0:12:39.740000
 And if you do, you restart the
 max age timer all over again.

0:12:39.740000 --> 0:12:43.160000
 But if for some reason I stopped sending
 you a BP to you, you would wait

0:12:43.160000 --> 0:12:48.220000
 20 seconds before you said, hmm, there
 must be something wrong with Keith

0:12:48.220000 --> 0:12:52.460000
 or maybe with the root bridge, either
 way, I'm not getting BP to use from

0:12:52.460000 --> 0:12:54.440000
 Keith anymore and I'm
 tired of waiting.

0:12:54.440000 --> 0:12:56.040000
 Max age has expired.

0:12:56.040000 --> 0:12:58.620000
 And now you would do
 something about that.

0:12:58.620000 --> 0:13:01.920000
 Well, they decide to do
 something different.

0:13:01.920000 --> 0:13:04.540000
 In rapid spanning tree, they
 said, hey, you know what?

0:13:04.540000 --> 0:13:10.520000
 Most protocols out there that use some
 sort of hello or keep-a-life mechanism,

0:13:10.520000 --> 0:13:15.600000
 a lot of times the way they work is
 that if you miss three, if you miss

0:13:15.600000 --> 0:13:18.220000
 three hellos or three keep
-a-lives, that's it.

0:13:18.220000 --> 0:13:20.200000
 You're done. Restart.

0:13:20.200000 --> 0:13:22.580000
 Why don't we do the same
 thing with spanning tree?

0:13:22.580000 --> 0:13:23.600000
 So that's what they did.

0:13:23.600000 --> 0:13:28.860000
 In rapid spanning tree, they said, all
 right, a designated port, regardless

0:13:28.860000 --> 0:13:32.220000
 of whether that designated port is
 on the root bridge himself or some

0:13:32.220000 --> 0:13:37.300000
 other non-root bridge, has to send
 BP to use every two seconds.

0:13:37.300000 --> 0:13:41.540000
 If three BP to use are
 lost, game over.

0:13:41.540000 --> 0:13:46.560000
 So if you actually capture a rapid spanning
 tree, BP to you and like wire

0:13:46.560000 --> 0:13:49.880000
 shark or a sniffer trace or something,
 you will see there's a timer in

0:13:49.880000 --> 0:13:51.160000
 there called max age.

0:13:51.160000 --> 0:13:55.660000
 And by default, it's still set
 to 20, but it's meaningless.

0:13:55.660000 --> 0:13:59.060000
 In rapid spanning tree, the max age timer
 is no longer used for anything.

0:13:59.060000 --> 0:14:02.260000
 It's more of a carryover from
 legacy spanning tree.

0:14:02.260000 --> 0:14:06.480000
 It's used for interoperability
 with a legacy switch.

0:14:06.480000 --> 0:14:10.460000
 But if you and I are doing rapid spanning
 tree, we don't even look at

0:14:10.460000 --> 0:14:11.460000
 that timer anymore.

0:14:11.460000 --> 0:14:12.680000
 It's not used for that.

0:14:12.680000 --> 0:14:15.800000
 Now you say, Keith, you're
 my designated port.

0:14:15.800000 --> 0:14:18.380000
 Keith, you told me that you're closer
 to the root bridge than I am.

0:14:18.380000 --> 0:14:22.360000
 Okay, so Keith, you need to send
 me BP to use every two seconds.

0:14:22.360000 --> 0:14:26.120000
 If you lose three BP to
 use from me, that's it.

0:14:26.120000 --> 0:14:30.500000
 Game over, now you're going to
 take some action based on that.

0:14:30.500000 --> 0:14:39.180000
 So like in this picture right here, we
 can see that these designated ports

0:14:39.180000 --> 0:14:44.580000
 have to send BP to use every two seconds
 over and over and over again.

0:14:44.580000 --> 0:14:49.560000
 And these designated ports have to
 send BP to use every two seconds.

0:14:49.560000 --> 0:14:56.420000
 Now in classic spanning tree, if this
 BP to you here stopped, all right,

0:14:56.420000 --> 0:14:58.300000
 so I'm just going to delete that.

0:14:58.300000 --> 0:15:00.840000
 Boop, it's gone.

0:15:00.840000 --> 0:15:04.720000
 Well then bridge 43, 44 wouldn't have
 anything he could send downstream

0:15:04.720000 --> 0:15:13.460000
 to switch X. But in rapid spanning
 tree, he says, okay.

0:15:13.460000 --> 0:15:15.740000
 So BP to just came down.

0:15:15.740000 --> 0:15:19.300000
 All right, I'll create my own
 BP to you, send it down.

0:15:19.300000 --> 0:15:23.420000
 Now all of a sudden
 two seconds expires.

0:15:23.420000 --> 0:15:25.820000
 Huh, I haven't gotten a BP to you.


0:15:25.820000 --> 0:15:29.020000
 Doesn't matter. I'm still going
 to send my own downstream.

0:15:29.020000 --> 0:15:32.380000
 I'm still going to advertise the root
 bridge as being the root bridge

0:15:32.380000 --> 0:15:36.100000
 because maybe he's still there
 and just having some problems.

0:15:36.100000 --> 0:15:37.880000
 Another two seconds expires.

0:15:37.880000 --> 0:15:39.340000
 So now I've lost two BP to use.

0:15:39.340000 --> 0:15:40.060000
 Well, it doesn't matter.

0:15:40.060000 --> 0:15:43.900000
 I still have to send my own switch
 X still has to know I'm here.

0:15:43.900000 --> 0:15:47.500000
 Finally, another two
 seconds expires.

0:15:47.500000 --> 0:15:50.240000
 So three BP to use have been lost.


0:15:50.240000 --> 0:15:55.660000
 At that point, this guy's going
 to say, ah, there's a problem.

0:15:55.660000 --> 0:15:59.400000
 Maybe I should take over and
 become the new root bridge.

0:15:59.400000 --> 0:16:03.840000
 So three BP to use were lost, but even
 while they were lost, he was still

0:16:03.840000 --> 0:16:08.300000
 sending his own BP to use downstream
 on his own designated ports.

0:16:08.300000 --> 0:16:12.880000
 So he was using them as a keep alive,
 so switch X would know that 4344

0:16:12.880000 --> 0:16:16.140000
 was still there and still alive.

0:16:16.140000 --> 0:16:27.020000
 Now let's talk about blocking ports for
 a second, which we call non-designated

0:16:27.020000 --> 0:16:29.940000
 ports because in spanning
 tree, it's not root.

0:16:29.940000 --> 0:16:31.120000
 It's not designated.

0:16:31.120000 --> 0:16:33.140000
 It is non-designated.

0:16:33.140000 --> 0:16:39.060000
 So ports are neither are called non-designated
 and their role is discarding.

0:16:39.060000 --> 0:16:42.600000
 Now you might read some papers that
 say their role is blocking.

0:16:42.600000 --> 0:16:45.660000
 That is a classic spanning
 tree term.

0:16:45.660000 --> 0:16:51.700000
 So in the 1998 version of 802.1d, a port
 was in the blocking state, which

0:16:51.700000 --> 0:16:54.840000
 meant no user data can be
 transmitted out that port.

0:16:54.840000 --> 0:16:57.000000
 No user data can come
 in that port.

0:16:57.000000 --> 0:16:59.860000
 It's like that port
 doesn't even exist.

0:16:59.860000 --> 0:17:01.540000
 So it's stripped out of the tree.

0:17:01.540000 --> 0:17:03.580000
 It's removed from redundancy.

0:17:03.580000 --> 0:17:04.800000
 So they called it blocking.

0:17:04.800000 --> 0:17:08.300000
 Well, when rapid spanning tree came
 out, they said, no, we're going to

0:17:08.300000 --> 0:17:12.320000
 call that discarding instead because
 it makes a little bit more sense.

0:17:12.320000 --> 0:17:14.580000
 It's discarding user traffic.

0:17:14.580000 --> 0:17:18.140000
 Now, as far as non-designated ports
 are concerned, there's actually two

0:17:18.140000 --> 0:17:20.520000
 different flavors of
 non-designated ports.

0:17:20.520000 --> 0:17:22.620000
 They're both in the
 discarding state.

0:17:22.620000 --> 0:17:26.920000
 There are alternate ports
 and there's backup ports.

0:17:26.920000 --> 0:17:32.160000
 So an alternate port is a port that's
 in the discarding state because

0:17:32.160000 --> 0:17:36.880000
 it received a superior BPDU
 on its non-root port.

0:17:36.880000 --> 0:17:40.780000
 And the BPDU is received from
 a different sending bridge ID.

0:17:40.780000 --> 0:17:45.860000
 Versus a backup port basically means
 you received a BPDU from yourself.

0:17:45.860000 --> 0:17:46.780000
 Let's take a look at this.

0:17:46.780000 --> 0:17:48.900000
 Let's finish up this video
 and describe this.

0:17:48.900000 --> 0:17:52.740000
 So look at the situation
 right here.

0:17:52.740000 --> 0:18:04.060000
 So here, the bridge on the bottom,
 he is receiving BPDUs down from the

0:18:04.060000 --> 0:18:06.860000
 root bridge. OK?

0:18:06.860000 --> 0:18:09.840000
 So we know he's going to have
 to elect a root port.

0:18:09.840000 --> 0:18:11.360000
 First thing he's going
 to have to do.

0:18:11.360000 --> 0:18:14.540000
 He's going to say, all right, well,
 my cost on these two links is the

0:18:14.540000 --> 0:18:20.380000
 same, but the sending port ID of 01
 is lower than the sending port ID

0:18:20.380000 --> 0:18:26.000000
 of 0 slash 2. So I'm going to
 elect this as a root port.

0:18:26.000000 --> 0:18:31.020000
 0 slash 3, that will go into
 the discarding state.

0:18:31.020000 --> 0:18:34.580000
 If you say blocking, no one's
 going to laugh at you.

0:18:34.580000 --> 0:18:36.900000
 Blocking is also accurate.

0:18:36.900000 --> 0:18:41.660000
 Now in this case, this port is discarding
 and he knows that the BPDUs

0:18:41.660000 --> 0:18:44.580000
 he's getting are not from himself.


0:18:44.580000 --> 0:18:49.720000
 All right, the BPDU has a sending
 bridge ID ending with AAAAD.

0:18:49.720000 --> 0:18:51.140000
 He says, well, that's not me.

0:18:51.140000 --> 0:18:53.200000
 My bridge ID ends with FFFF.

0:18:53.200000 --> 0:18:56.440000
 So these BPDUs I'm getting
 are from somebody else.

0:18:56.440000 --> 0:19:00.520000
 So that is called
 an alternate port.

0:19:00.520000 --> 0:19:04.120000
 And that's what you're
 going to see.

0:19:04.120000 --> 0:19:06.880000
 99.99% of the time.

0:19:06.880000 --> 0:19:12.560000
 If a port is blocking or discarding,
 it's because it's an alternate port.

0:19:12.560000 --> 0:19:15.600000
 Now, let's ignore
 that for a second.

0:19:15.600000 --> 0:19:17.520000
 I'm going to delete all that.

0:19:17.520000 --> 0:19:26.100000
 We can just put down
 here discarding.

0:19:26.100000 --> 0:19:35.040000
 And we can put right
 here, root port.

0:19:35.040000 --> 0:19:41.580000
 All right. Well, look
 at what happens here.

0:19:41.580000 --> 0:19:49.680000
 So when FFFF receives the BPDU on the
 discarding port, he does process

0:19:49.680000 --> 0:19:52.020000
 it, but then he throws it away.

0:19:52.020000 --> 0:19:54.480000
 He can't forward that BPDU.

0:19:54.480000 --> 0:19:59.540000
 Now he receives the BPDU
 on the root port.

0:19:59.540000 --> 0:20:02.180000
 Okay, what are these
 ports going to be?

0:20:02.180000 --> 0:20:07.360000
 Well, they can't be root ports because
 he's not even getting any BPDUs

0:20:07.360000 --> 0:20:12.380000
 from this guy. So those are going
 to be designated ports.

0:20:12.380000 --> 0:20:14.840000
 Those are going to be
 designated ports.

0:20:14.840000 --> 0:20:20.460000
 So let's mark those
 as designated ports.

0:20:20.460000 --> 0:20:28.700000
 So DP for that one.

0:20:28.700000 --> 0:20:33.740000
 And DP for that one.

0:20:33.740000 --> 0:20:39.600000
 Now we know as designated ports,
 he has to transmit BPDUs.

0:20:39.600000 --> 0:20:41.260000
 Well, let's take a look
 at this one right here.

0:20:41.260000 --> 0:20:44.420000
 Now, he's going to transmit BPDUs on
 both, but let's just look at this

0:20:44.420000 --> 0:20:45.880000
 one to begin with.

0:20:45.880000 --> 0:20:48.980000
 The heat transmit
 on zero slash one.

0:20:48.980000 --> 0:20:56.140000
 Now, this BPDU is going into a server
 that has two net cards, two ethernet

0:20:56.140000 --> 0:21:00.520000
 net cards. Inside that server, this server
 is running some sort of virtualization

0:21:00.520000 --> 0:21:05.260000
 software like VMware ESXi
 or something else.

0:21:05.260000 --> 0:21:08.020000
 And we have some virtual machines.


0:21:08.020000 --> 0:21:10.940000
 You don't don't worry if you don't know
 anything about virtual machines.

0:21:10.940000 --> 0:21:12.580000
 That's kind of irrelevant.

0:21:12.580000 --> 0:21:16.300000
 But those virtual machines are tied
 together with something called a V

0:21:16.300000 --> 0:21:22.820000
 switch. Now, a V switch, unlike a normal
 switch, does not participate

0:21:22.820000 --> 0:21:25.180000
 in spanning tree.

0:21:25.180000 --> 0:21:30.900000
 In ESXi terminology, they call it a V
 switch, but it's actually more like

0:21:30.900000 --> 0:21:37.840000
 a hub. A hub because on a V switch,
 the logical connections, if they're

0:21:37.840000 --> 0:21:40.480000
 all in the same group,
 it's like a hub.

0:21:40.480000 --> 0:21:45.500000
 Anything that comes in on logical connection
 A will go out other logical

0:21:45.500000 --> 0:21:49.100000
 connection A's. We actually call those
 port groups, but you don't have

0:21:49.100000 --> 0:21:50.780000
 to worry about that.

0:21:50.780000 --> 0:21:56.880000
 So, let's just say for the sake of argument
 that all of these connections

0:21:56.880000 --> 0:21:59.080000
 are in the same port group.

0:21:59.080000 --> 0:22:04.420000
 We'll just call it port group
 A on this V switch.

0:22:04.420000 --> 0:22:12.500000
 This one right here.

0:22:12.500000 --> 0:22:17.600000
 And this one right here.

0:22:17.600000 --> 0:22:22.660000
 Okay, so because all those logical
 internal ports on the V switch are

0:22:22.660000 --> 0:22:26.460000
 in the same port group, port group A,
 that means that anything that enters

0:22:26.460000 --> 0:22:31.380000
 any one of those interfaces will be
 reflected just like a hub out all

0:22:31.380000 --> 0:22:33.000000
 the other interfaces.

0:22:33.000000 --> 0:22:42.280000
 Which means that when our BPDU, for example,
 comes in this way, it's going

0:22:42.280000 --> 0:22:45.660000
 to end up coming out that way.

0:22:45.660000 --> 0:22:50.540000
 And when this BPDU comes in here,
 it's going to come out here.

0:22:50.540000 --> 0:22:55.160000
 So, in this situation, switch FFFF is
 actually going to be receiving on

0:22:55.160000 --> 0:23:00.580000
 port 01 and 02 BPDUs from himself.


0:23:00.580000 --> 0:23:04.060000
 He can't have both of those
 ports as designated ports.

0:23:04.060000 --> 0:23:08.100000
 If both of those ports were designated
 ports, now we'd have a loop, where

0:23:08.100000 --> 0:23:10.240000
 stuff could go around and around
 and around like this.

0:23:10.240000 --> 0:23:11.900000
 That can't be the case.

0:23:11.900000 --> 0:23:19.080000
 So, he needs to take one of those designated
 ports out of that state and

0:23:19.080000 --> 0:23:23.960000
 convert it into a blocking port.

0:23:23.960000 --> 0:23:26.720000
 In this case, it'll be 0 slash 2.

0:23:26.720000 --> 0:23:28.580000
 The lower number port
 will be allowed.

0:23:28.580000 --> 0:23:30.720000
 That's the winner because
 it's lowered.

0:23:30.720000 --> 0:23:35.780000
 And this one here will be placed
 into the discarding state.

0:23:35.780000 --> 0:23:40.120000
 Now, if I was actually on switch F and
 I did a show spanning tree command,

0:23:40.120000 --> 0:23:46.540000
 0 slash 3 was an alternate port because
 it was discarding, because it

0:23:46.540000 --> 0:23:49.980000
 received a BPDU from
 a different switch.

0:23:49.980000 --> 0:23:54.980000
 0 slash 2 is discarding because he received
 his own BPDU reflected right

0:23:54.980000 --> 0:24:00.680000
 back to him. So, that is
 called a backup port.

0:24:00.680000 --> 0:24:09.500000
 And that is the difference between
 backup and alternate ports.

0:24:09.500000 --> 0:24:12.440000
 So, that concludes this video.

0:24:12.440000 --> 0:24:13.120000
 Thank you for watching.