WEBVTT

0:00:03.340000 --> 0:00:08.900000
 Welcome to this video titled Designated
 and Non-designated Port Election.

0:00:08.900000 --> 0:00:12.560000
 So we're continuing our discussion
 here about rapid spanning tree.

0:00:12.560000 --> 0:00:16.000000
 All this is also backwards compatible
 with the earlier version of 802

0:00:16.000000 --> 0:00:17.760000
.1d spanning tree.

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 And now we're going to focus on designated
 and non-designated ports.

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 So what is a designated port?

0:00:24.460000 --> 0:00:28.120000
 It is a port that is leading
 away from the root bridge.

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 So if you think about the path that
 BPDUs are flowing, remember the root

0:00:32.700000 --> 0:00:38.320000
 bridge initially creates a BPDU, then
 it just follows and floods along,

0:00:38.320000 --> 0:00:43.940000
 down away from the BPDU, any interface
 that is transmitting a BPDU is

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 a designated port.

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 Now, like your root port, there is
 an election process that happens to

0:00:50.360000 --> 0:00:52.460000
 determine which port
 will be designated.

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 Now, number one, the port with
 the lowest root path cost.

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 And let me just pause right here.

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 This is why all the interfaces on the root
 bridge itself are always designated

0:01:04.420000 --> 0:01:09.240000
 ports. Because if I am an interface that
 lives on the root bridge, nothing

0:01:09.240000 --> 0:01:13.240000
 can beat me. Nothing has a lower cost
 to the root bridge than me because

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 I'm right there.

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 I am the root bridge.

0:01:16.020000 --> 0:01:22.240000
 That's why all of his ports
 are always designated ports.

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 Other than that, it's all supports with
 the lowest port cost, lowest bridge

0:01:25.780000 --> 0:01:28.020000
 ID, or lowest port ID.

0:01:28.020000 --> 0:01:29.020000
 Now, what's that referring to?

0:01:29.020000 --> 0:01:30.680000
 What's that really talking about?

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 So what we're talking about here is
 if this is my root bridge, and here

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 we have bridge A and we have bridge
 B, we know bridge A and bridge B are

0:01:42.240000 --> 0:01:44.440000
 going to need root ports.

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 Let's just for the purposes of argument,
 let's just say that these are

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 the root ports for
 these two bridges.

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 And we also know that the root bridge,
 he's got designated ports.

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 Now, every single link needs
 to have a designated port.

0:02:00.780000 --> 0:02:02.460000
 Every link needs to have one.

0:02:02.460000 --> 0:02:05.880000
 Now we connect a link
 between A and B.

0:02:05.880000 --> 0:02:10.180000
 One of these interfaces is going to have
 to take on the role of the designated

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 port. And that's what we're
 talking right here.

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 How has that election done?

0:02:15.720000 --> 0:02:20.140000
 A and B are first going to exchange
 with each other the cost of the root

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 ports. Who's got the lowest
 cost to the root bridge?

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 If one of them is clearly the winner,
 if one of them is lower than the

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 other, we're done.

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 Whoever has the lowest cost wins.

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 They become the designated port.

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 If their costs are the same, then
 they compare each other's names.

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 They compare each other's
 bridge IDs.

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 And typically, that's
 where we're done.

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 Whoever has the lowest
 bridge ID wins.

0:02:45.160000 --> 0:02:47.380000
 Definitely, in this case,
 that would be the winner.

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 The only time we would have lowest
 port ID be a factor is if you have

0:02:51.500000 --> 0:02:55.560000
 a switch with two interfaces
 are connecting into a hub.

0:02:55.560000 --> 0:03:01.820000
 So if I'm actually receiving BPDUs on
 two different interfaces from myself,

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 both of them had the exact same sending
 bridge ID, which is me, then I

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 will go down to whichever port on
 myself is the lowest port ID.

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 But chances are you probably
 are not going to see that.

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 So we've elected our root ports.

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 We've elected our
 designated ports.

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 Everything that's left over
 is a non-designate port.

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 And it will go into
 the blocking role.

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 Now, in the terminology of rapid spanning
 tree, we don't call it that.

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 Logically, functionally,
 it's the same.

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 But in rapid spanning tree, if a port
 is not a root port and it's not

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 a designated port, then we actually
 call it one of two things as either

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 an alternate or a backup port.

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 We're going to talk about
 that in just a moment.

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 But both alternate and backup ports,
 their functional role is the same.

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 They are discarding traffic.

0:03:55.280000 --> 0:03:56.020000
 We actually say that.

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 We don't say they're blocking.

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 We say they are discarding.

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 So the concept of blocking was replaced
 by the concept of discarding in

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 rapid spanning tree.

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 So let's take a look here
 at how this works.

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 All right. So, Disney
 port election.

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 We'll start with the root bridge.

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 We already know by default, his
 ports are designated ports.

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 We've got our root ports already
 assigned right here.

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 Okay. So we know right here that the port
 leading away from the root bridge

0:04:31.440000 --> 0:04:33.720000
 is going to be a designated port.

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 And here's another thing.

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 Whenever you see a root port, you know
 that by default, the other side

0:04:40.320000 --> 0:04:43.280000
 of a root port is always
 a designated port.

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 So on a point-to-point connection like
 this, once I identify the root

0:04:46.480000 --> 0:04:50.220000
 port, I know the other side
 has to be a designated port.

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 Now, I still need to figure
 out this link right here.

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 So let's figure that out.

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 Okay. So between these two switches,
 43, 44 and switch X, who is going

0:05:03.720000 --> 0:05:06.620000
 to be the designated
 port on that wire?

0:05:06.620000 --> 0:05:11.840000
 Well, 43, 44 is going
 to send down a BPDU.

0:05:11.840000 --> 0:05:21.820000
 And his BPDU is going to say, hey, my
 cost to the root is four, because

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 that's his root port.

0:05:23.660000 --> 0:05:26.780000
 It's a gigabit ethernet link directly
 to the root, which is a cost of

0:05:26.780000 --> 0:05:29.040000
 four. That's why he's going
 to say his cost is.

0:05:29.040000 --> 0:05:34.720000
 Now switch X, the best cost he can
 come up with based on his root port

0:05:34.720000 --> 0:05:41.200000
 is a cost of 19 to reach his neighbor,
 because this black line is fast

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 ethernet, and his
 neighbor is four.

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 So his root port is actually
 four plus 19, which is 23.

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 So that's the best
 he can advertise.

0:05:52.240000 --> 0:05:58.840000
 So in this case, 43, 44 is clearly the
 winner, because four is lower than

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 23. So 43, 44, his port will be the
 designated port in charge of sending

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 down BPDUs. Now, what about
 on this link right here?

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 Well, so bridge 44, 44, his cost
 of the root bridge is 19.

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

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 And once again, switch X, the
 best he could do is 23.

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 That was his cost.

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 So we end up seeing that port zero slash
 two right here is our designated

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 port. So the same thing is going
 to happen over here on the right.

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 Switch 44, E4, his cost
 of the root is four.

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 That's what he's going
 to advertise.

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 The best switch X can do is 23.

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 So this will be our
 designated port.

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 So now we have figured out
 our designated ports.

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 Just erase some of the stuff here
 so it's a little bit clearer.

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 Now we know who our designated
 ports are, as you can see.

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 But that still leaves us with the question
 of what's the final state of

0:07:36.560000 --> 0:07:38.720000
 these interfaces right here?

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 Well, we can surmise one thing.

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 Those interfaces are not root ports
 and they're not designated ports.

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 So they have to be
 one of two things.

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 They are either alternate
 ports or backup ports.

0:07:54.340000 --> 0:08:00.720000
 Regardless of which one they are, we
 know they're in the discarding state.

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 Discarding, which is the
 same thing as blocking.

0:08:09.360000 --> 0:08:12.400000
 For those of you familiar
 with that terminology.

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 Now before we go into the difference
 of what is the difference between

0:08:16.440000 --> 0:08:18.240000
 alternate and backup.

0:08:18.240000 --> 0:08:22.420000
 One other thing I want to point out about
 designated ports that's important

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 to know. Before rapid spanning tree
 came along, BPDUs were only created

0:08:32.960000 --> 0:08:35.480000
 by the root bridge.

0:08:35.480000 --> 0:08:40.520000
 Once the root bridge created BPDU and
 let it go, that BPD was forwarded

0:08:40.520000 --> 0:08:43.480000
 by non-root bridges.

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 Now there could be situations temporary,
 hopefully, situations where the

0:08:48.720000 --> 0:08:52.520000
 root bridge would not be
 able to send a BPDU.

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 Like maybe the root bridge, his CPU
 was really, really high for a few

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 seconds. Like maybe for six seconds
 or 12 seconds, his CPU was at like

0:09:00.740000 --> 0:09:06.420000
 99%, something bad was going on
 and he couldn't create a BPDU.

0:09:06.420000 --> 0:09:11.600000
 So in classic spanning tree, if the
 root bridge was not sending BPDUs,

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 nobody had anything to forward.

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 There was nothing going on.

0:09:15.660000 --> 0:09:22.060000
 Well, if that happened, so let's say
 I am a switch and the root bridge

0:09:22.060000 --> 0:09:24.280000
 is like five switches
 away from me.

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 Let's say I go through you
 to get to the root bridge.

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 So my connection to you,
 this is my root port.

0:09:31.800000 --> 0:09:33.440000
 You are the designated ports.

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 Your job to forward
 those BPDUs to me.

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 All of a sudden I say, hey, haven't
 gotten any BPDUs in a while.

0:09:40.720000 --> 0:09:42.400000
 What's going on?

0:09:42.400000 --> 0:09:44.660000
 Now I have a question.

0:09:44.660000 --> 0:09:46.640000
 Did the root bridge fail?

0:09:46.640000 --> 0:09:51.340000
 Did you fail? You know, maybe I have
 electrical connectivity to you, but

0:09:51.340000 --> 0:09:52.800000
 maybe something's wrong with you.

0:09:52.800000 --> 0:09:59.300000
 I don't know. So in rapid spanning tree,
 they said, hey, let's do this.

0:09:59.300000 --> 0:10:04.540000
 Once a port is elected as a designated
 port, even if it's not on the root

0:10:04.540000 --> 0:10:13.860000
 bridge itself, that port must always
 send BPDUs as keep-a-lives.

0:10:13.860000 --> 0:10:15.460000
 Say, what are you talking
 about, Keith?

0:10:15.460000 --> 0:10:16.900000
 Well, here's what we're
 talking about.

0:10:16.900000 --> 0:10:22.260000
 If the root bridge was behind you, if
 that root bridge stops sending BPDUs

0:10:22.260000 --> 0:10:27.080000
 for a couple of seconds, it's still
 your job to basically pretend as if

0:10:27.080000 --> 0:10:31.920000
 he was alive and send BPDUs to me.


0:10:31.920000 --> 0:10:34.740000
 Now here's how this worked.

0:10:34.740000 --> 0:10:39.020000
 In classic spanning tree, where the
 only one who was allowed to create

0:10:39.020000 --> 0:10:44.900000
 BPDUs was the root bridge, in classic
 spanning tree, the hello time for

0:10:44.900000 --> 0:10:49.420000
 BPDUs was every two seconds.

0:10:49.420000 --> 0:10:56.100000
 But there was a huge grace period if
 some seconds went by and the root

0:10:56.100000 --> 0:10:59.600000
 bridge did not transmit BPDUs.

0:10:59.600000 --> 0:11:04.860000
 That was called the max age timer.


0:11:04.860000 --> 0:11:07.920000
 And by default, that
 was 20 seconds.

0:11:07.920000 --> 0:11:14.320000
 So in classic spanning tree, I was expecting
 you, my neighbor, to forward

0:11:14.320000 --> 0:11:16.600000
 me those BPDUs every two seconds.

0:11:16.600000 --> 0:11:21.320000
 If all the second, three seconds elapsed,
 then five seconds elapsed and

0:11:21.320000 --> 0:11:25.460000
 nine seconds elapsed, and I'm not getting
 those BPDUs from you, I would

0:11:25.460000 --> 0:11:27.160000
 wait up to 20 seconds.

0:11:27.160000 --> 0:11:30.940000
 If 20 seconds expired and I said, oh,
 I haven't gotten a BPDUs from you

0:11:30.940000 --> 0:11:32.580000
 in a while, 20 seconds.

0:11:32.580000 --> 0:11:34.800000
 Then I'd say, time
 to make a change.

0:11:34.800000 --> 0:11:36.220000
 Clearly, there's something
 wrong with you.

0:11:36.220000 --> 0:11:38.920000
 I need to reconfigure
 my spanning tree here.

0:11:38.920000 --> 0:11:43.900000
 Well, in rapid spanning
 tree, they said this.

0:11:43.900000 --> 0:11:48.580000
 Now that I know that regardless of what's
 happening behind you, it's your

0:11:48.580000 --> 0:11:52.000000
 job to send BPDUs to me, here's
 what we're going to do.

0:11:52.000000 --> 0:11:55.620000
 We're going to do it the way pretty
 much every other protocol does it.

0:11:55.620000 --> 0:12:00.660000
 Most protocols, not all of them, but
 sort of a general rule of thumb is

0:12:00.660000 --> 0:12:06.860000
 that if a protocol relies on keep-alives
 or hello messages, generally

0:12:06.860000 --> 0:12:12.780000
 speaking, after three hellos or three
 keep-alives are lost, usually that

0:12:12.780000 --> 0:12:14.660000
 protocol says that's it.

0:12:14.660000 --> 0:12:15.680000
 My neighbors did.

0:12:15.680000 --> 0:12:17.360000
 Can't rely on him anymore.

0:12:17.360000 --> 0:12:19.800000
 That's not a hard and fast rule,
 but that's a general rule.

0:12:19.800000 --> 0:12:22.180000
 You'll find that in routing
 protocols a lot.

0:12:22.180000 --> 0:12:25.160000
 Three missed keep-alives, three
 missed hellos, you're done.

0:12:25.160000 --> 0:12:28.740000
 We're over. So rapid spanning
 tree took the same approach.

0:12:28.740000 --> 0:12:33.220000
 It said, look, as long as you keep sending
 me BPDUs, I'm happy with you.

0:12:33.220000 --> 0:12:36.920000
 But if I missed three of them,
 then you and I are done.

0:12:36.920000 --> 0:12:39.440000
 I need to reconfigure
 my spanning tree.

0:12:39.440000 --> 0:12:44.520000
 So now rapid spanning tree doesn't
 really rely on max age anymore.

0:12:44.520000 --> 0:12:47.860000
 Now, hellos just have every
 three hellos, right?

0:12:47.860000 --> 0:12:51.800000
 If I lose three hellos, which in this
 case would be six seconds of no

0:12:51.800000 --> 0:12:55.880000
 hellos, then it's time for
 me to make a change.

0:12:55.880000 --> 0:12:59.240000
 So that's a big difference between
 rapid spanning tree and the classic

0:12:59.240000 --> 0:13:05.680000
 version of it. Loss of three BPDUs
 triggers an action on a downstream

0:13:05.680000 --> 0:13:12.320000
 switch. So if we go back here,
 we can fill this out.

0:13:12.320000 --> 0:13:15.880000
 And now we see our downstream ports
 are blocking technically discarding.

0:13:15.880000 --> 0:13:17.740000
 So let's talk about those.

0:13:17.740000 --> 0:13:25.000000
 So there are two different types of
 non-designate ports in the world of

0:13:25.000000 --> 0:13:26.300000
 rapid spanning tree.

0:13:26.300000 --> 0:13:30.920000
 Alternate ports, which means you've
 received a superior BPDU on a non

0:13:30.920000 --> 0:13:37.660000
-root port with a different sending
 bridge ID and backup ports.

0:13:37.660000 --> 0:13:43.320000
 So let's just look at the in a sort of
 a visual sense what the difference

0:13:43.320000 --> 0:13:46.420000
 is between these two.

0:13:46.420000 --> 0:13:54.220000
 All right. Let's see here.

0:13:54.220000 --> 0:13:56.660000
 So how is this going to work?

0:13:56.660000 --> 0:14:01.880000
 Okay, so here we have
 our root bridge.

0:14:01.880000 --> 0:14:07.980000
 Support 0 slash 4 is going
 to be my root port.

0:14:07.980000 --> 0:14:16.260000
 And port 0 slash 3 now, it
 says, all right, 0 slash 3.

0:14:16.260000 --> 0:14:17.880000
 I'm not a root port.

0:14:17.880000 --> 0:14:19.580000
 I'm not a designated port.

0:14:19.580000 --> 0:14:24.440000
 So I need to pick between
 alternate and backup.

0:14:24.440000 --> 0:14:26.140000
 So how do I pick that?

0:14:26.140000 --> 0:14:31.100000
 Well, when a BPDU comes down
 to me, here comes my BPDU.

0:14:31.100000 --> 0:14:33.560000
 Or I should say A BPDU.

0:14:33.560000 --> 0:14:36.540000
 I look at the sending
 bridge ID field.

0:14:36.540000 --> 0:14:47.440000
 Okay, in this case, the sending
 bridge ID is this.

0:14:47.440000 --> 0:14:51.840000
 I compare it with my own bridge ID and
 I simply ask myself a very simple

0:14:51.840000 --> 0:14:55.720000
 question. Are they the same
 or are they different?

0:14:55.720000 --> 0:14:57.720000
 In this case, the answer is clear.


0:14:57.720000 --> 0:14:58.680000
 They are not the same.

0:14:58.680000 --> 0:14:59.800000
 They are different.

0:14:59.800000 --> 0:15:04.660000
 So because I received a BPDU from a
 switch that's not myself, I will put

0:15:04.660000 --> 0:15:08.220000
 this port as an alternate port.

0:15:08.220000 --> 0:15:15.940000
 Alternate. And then I'll put it into
 the discarding state, which is the

0:15:15.940000 --> 0:15:18.000000
 same as blocking.

0:15:18.000000 --> 0:15:23.640000
 All right. Now, things get
 a little bit tricky here.

0:15:23.640000 --> 0:15:29.040000
 Because here, let's
 say I take a BPDU.

0:15:29.040000 --> 0:15:34.540000
 And I forward it out, zero slash one,
 into this server, which is running

0:15:34.540000 --> 0:15:38.440000
 VMware or something, which
 has a V switch in it.

0:15:38.440000 --> 0:15:43.200000
 Well, a V switch operates
 kind of like a hub.

0:15:43.200000 --> 0:15:45.680000
 It's not really, it's not
 doing spanning trees.

0:15:45.680000 --> 0:15:50.340000
 So if I actually send a BPDU into a V
 switch, guess what's going to happen?

0:15:50.340000 --> 0:15:54.500000
 I'm going to see that same BPDU
 pop out the other side.

0:15:54.500000 --> 0:15:59.220000
 And same thing here, when zero slash
 two, when he sends in a BPDU, it's

0:15:59.220000 --> 0:16:01.100000
 going to pop out the other side.

0:16:01.100000 --> 0:16:08.780000
 So now my switch on the bottom there,
 he's receiving two BPDUs.

0:16:08.780000 --> 0:16:12.920000
 And he says, well, I can't have both zero
 one and zero two in the forwarding

0:16:12.920000 --> 0:16:15.840000
 state, because if they were,
 I'd have a bridging loop.

0:16:15.840000 --> 0:16:17.920000
 I have to block one of these.

0:16:17.920000 --> 0:16:21.680000
 So the next thing he says is, oh, okay,
 well, let me look at the cost.

0:16:21.680000 --> 0:16:24.300000
 Well, both of these
 BPDUs came from him.

0:16:24.300000 --> 0:16:26.560000
 So the cost is identical.

0:16:26.560000 --> 0:16:29.340000
 It's the cost of his root
 port, whatever that is.

0:16:29.340000 --> 0:16:31.300000
 Cost is not going to help him.

0:16:31.300000 --> 0:16:35.440000
 So now he looks at the sending bridge
 ID and says, oh, wait a second.

0:16:35.440000 --> 0:16:37.860000
 Both these BPDUs have
 the exact same name.

0:16:37.860000 --> 0:16:41.860000
 They're me. They're
 my own bridge IDs.

0:16:41.860000 --> 0:16:42.800000
 So this is okay.

0:16:42.800000 --> 0:16:46.420000
 Well, I'm going to have to end
 up blocking one of these ports.

0:16:46.420000 --> 0:16:47.940000
 Lower is better.

0:16:47.940000 --> 0:16:50.540000
 So I'm not going to block
 the port that's lower.

0:16:50.540000 --> 0:16:55.620000
 So I will keep the lower port
 as a designated port.

0:16:55.620000 --> 0:17:00.940000
 That's the one they'll
 be forwarding.

0:17:00.940000 --> 0:17:04.080000
 But I'm going to block or I'm going
 to say, I'm going to put this port

0:17:04.080000 --> 0:17:06.880000
 here into the discarding state.

0:17:06.880000 --> 0:17:12.600000
 And I'm going to mark
 that as a backup port.

0:17:12.600000 --> 0:17:19.340000
 So notice both backup and alternate
 go into the discarding state.

0:17:19.340000 --> 0:17:23.280000
 The only difference between them is one
 is discarding because you received

0:17:23.280000 --> 0:17:25.620000
 a BPU from an upstream switch.

0:17:25.620000 --> 0:17:30.220000
 The other is discarding because you actually
 received your own BPDU reflected

0:17:30.220000 --> 0:17:32.160000
 right back to you.

0:17:32.160000 --> 0:17:35.180000
 Which could happen in an environment
 like this, where you're connected

0:17:35.180000 --> 0:17:39.640000
 via a couple of different nicks
 into a single V switch.

0:17:39.640000 --> 0:17:48.480000
 So that concludes this discussion about
 designated ports and discarding

0:17:48.480000 --> 0:17:50.380000
 ports and the differences
 between them.