WEBVTT

0:00:02.820000 --> 0:00:07.220000
 Hello and welcome to this video
 on root port election.

0:00:07.220000 --> 0:00:11.320000
 In this video I'm going to cover the
 actual election process, the logic

0:00:11.320000 --> 0:00:14.860000
 behind how rapid spanning
 tree elects a root port.

0:00:14.860000 --> 0:00:17.880000
 We're going to talk about something
 called cost values which are very

0:00:17.880000 --> 0:00:21.700000
 critical for spanning tree
 in order to build its tree.

0:00:21.700000 --> 0:00:26.420000
 And we're going to look at given a particular
 topology, how would we predict

0:00:26.420000 --> 0:00:32.140000
 what port or become root ports.

0:00:32.140000 --> 0:00:34.600000
 So let's start with the root port.


0:00:34.600000 --> 0:00:36.380000
 What is a root port?

0:00:36.380000 --> 0:00:40.000000
 So if you've watched any of my other videos
 or you've studied rapid spanning

0:00:40.000000 --> 0:00:44.440000
 tree in a book or through some other
 author, you know that the very first

0:00:44.440000 --> 0:00:48.200000
 thing that happens is that a root
 bridge is elected in the topology.

0:00:48.200000 --> 0:00:51.160000
 Everything has to know
 who the root bridge is.

0:00:51.160000 --> 0:00:54.840000
 Once the root bridge is elected, he continues
 to send out bridge protocol

0:00:54.840000 --> 0:00:58.940000
 data units, BP DUs, so everybody
 knows that he's still alive.

0:00:58.940000 --> 0:01:03.540000
 Now at that point, all of his interfaces
 become designated ports.

0:01:03.540000 --> 0:01:05.560000
 Designated ports deliver.

0:01:05.560000 --> 0:01:06.480000
 What do they deliver?

0:01:06.480000 --> 0:01:08.940000
 They deliver BP DUs.

0:01:08.940000 --> 0:01:12.520000
 So the root bridge, his job is
 pretty much done at that point.

0:01:12.520000 --> 0:01:15.940000
 All the other switches, however, who
 lost, who are not the root bridge,

0:01:15.940000 --> 0:01:18.500000
 they have a little bit more work to
 do to figure out what the tree is

0:01:18.500000 --> 0:01:20.580000
 ultimately going to look like.

0:01:20.580000 --> 0:01:25.600000
 So all non-root switches, the next thing
 they have to do is elect an interface

0:01:25.600000 --> 0:01:30.120000
 on themselves, a local interface
 called a root port.

0:01:30.120000 --> 0:01:31.540000
 Now what is a root port?

0:01:31.540000 --> 0:01:35.520000
 Well if I'm a switch and I've got lots
 of interfaces which are connecting

0:01:35.520000 --> 0:01:40.060000
 into the network infrastructure and the
 root bridge is out there somewhere,

0:01:40.060000 --> 0:01:43.720000
 there's a very good chance that I'm receiving
 BPUs from him on this interface

0:01:43.720000 --> 0:01:46.400000
 and this interface and this
 other interface right here.

0:01:46.400000 --> 0:01:50.900000
 So I might say hey, clearly there's
 a lot of ways, a lot of paths for

0:01:50.900000 --> 0:01:53.240000
 me to take to get to
 that root bridge.

0:01:53.240000 --> 0:01:57.140000
 So the first thing I have to figure
 out is which is the best path?

0:01:57.140000 --> 0:02:01.640000
 Which path will get me back to that root
 bridge the fastest, the quickest?

0:02:01.640000 --> 0:02:06.020000
 Whatever interface I have that leads
 me to that path, that is my root

0:02:06.020000 --> 0:02:09.860000
 port. So it's an upstream interface,
 if we think of the root bridge as

0:02:09.860000 --> 0:02:12.760000
 being up there and everybody being down
 from him, so think of a tree being

0:02:12.760000 --> 0:02:15.900000
 flipped upside down with the roots here
 and everything hanging down from

0:02:15.900000 --> 0:02:21.140000
 it, then it's the upstream port
 leading towards the root bridge.

0:02:21.140000 --> 0:02:25.760000
 And it's selected based on
 lowest root path cost.

0:02:25.760000 --> 0:02:27.900000
 So we'll talk about that
 in just one second.

0:02:27.900000 --> 0:02:31.380000
 So it's not based on hop count, how
 many hops of bridges do I have to

0:02:31.380000 --> 0:02:33.360000
 get through? Nope, we
 don't care about that.

0:02:33.360000 --> 0:02:38.220000
 It's not based on like time, like okay it's
 0.5 microseconds and 0.2 microseconds

0:02:38.220000 --> 0:02:41.200000
 that way, it's not based on that at
 all, it's based on a factor called

0:02:41.200000 --> 0:02:47.100000
 cost. And each switch, not the root bridge,
 the root bridge does not have

0:02:47.100000 --> 0:02:50.940000
 any root ports. He doesn't need any ports
 leading back to himself, unless

0:02:50.940000 --> 0:02:53.000000
 he's psychotic or something
 like that.

0:02:53.000000 --> 0:02:55.680000
 All of his ports are going
 to be designated ports.

0:02:55.680000 --> 0:02:59.080000
 So we're talking about every switch
 except for the root bridge is going

0:02:59.080000 --> 0:03:03.520000
 to have a single root port pointing
 back to that root bridge.

0:03:03.520000 --> 0:03:08.480000
 All right, so the cost is based on the
 cumulative cost of all the links

0:03:08.480000 --> 0:03:10.340000
 to get to the root.

0:03:10.340000 --> 0:03:13.000000
 So let me just draw something here
 real quick just to demonstrate this

0:03:13.000000 --> 0:03:19.840000
 idea. If this is my root bridge right
 here and let's say we have something

0:03:19.840000 --> 0:03:27.080000
 like this, okay, and here we're on bridge
 X or switch X, I just use the

0:03:27.080000 --> 0:03:32.720000
 words bridge and switch interchangeably
 here, okay, and we have A, B and

0:03:32.720000 --> 0:03:37.320000
 C. All right, so we know that root bridge,
 he's transmitting bridge protocol

0:03:37.320000 --> 0:03:45.760000
 data units. And the cost is actually
 a field inside the BPdu, it's one

0:03:45.760000 --> 0:03:50.540000
 of the fields. And the cost is when somebody
 transmits a BPU they're saying,

0:03:50.540000 --> 0:03:55.120000
 hey, let me tell you what it costs
 me to get to the root bridge.

0:03:55.120000 --> 0:03:59.260000
 Well, the root bridge doesn't cost
 him anything to get to himself.

0:03:59.260000 --> 0:04:03.380000
 So his cost is always going to be zero
 whenever the BPU pops out of him.

0:04:03.380000 --> 0:04:08.120000
 But that cost will increment, and we'll
 see just in a second how it increments.

0:04:08.120000 --> 0:04:13.940000
 But at this point, switch X is getting
 BPUs on port one and port two.

0:04:13.940000 --> 0:04:18.780000
 And maybe the one he's getting on port
 one by the time, so when A advertises

0:04:18.780000 --> 0:04:21.840000
 it to him, A says,
 hey, my cost is 30.

0:04:21.840000 --> 0:04:26.520000
 And when C advertises it, he
 says, hey, my cost is 20.

0:04:26.520000 --> 0:04:30.560000
 Well, part of factoring this in is
 that bridge X will have actually a

0:04:30.560000 --> 0:04:34.620000
 local cost on his own interfaces.

0:04:34.620000 --> 0:04:38.380000
 Okay, so whatever that cost is, maybe
 the cost is 10 on both of them.

0:04:38.380000 --> 0:04:41.120000
 His own interfaces have
 a local cost of 10.

0:04:41.120000 --> 0:04:45.320000
 So when he gets in those BPUs, he's
 going to say, okay, well, if I go

0:04:45.320000 --> 0:04:50.620000
 out interface number one, my total
 cumulative cost will be 40.

0:04:50.620000 --> 0:04:54.140000
 My neighbor said it's 30 to get through
 him, but it costs me an additional

0:04:54.140000 --> 0:04:56.420000
 10 to get up to that neighbor.

0:04:56.420000 --> 0:05:02.900000
 So that's 40. Whereas if I go out interface
 number two, my total cost

0:05:02.900000 --> 0:05:08.660000
 going that way is 30, my
 total cumulative cost.

0:05:08.660000 --> 0:05:12.220000
 So in this case, bridge X will say,
 all right, clearly interface number

0:05:12.220000 --> 0:05:17.720000
 two is better. So it says, I'm going
 to select that as my root port.

0:05:17.720000 --> 0:05:26.240000
 That will be his root port because
 the total cost is the lowest.

0:05:26.240000 --> 0:05:29.040000
 And cost is based on
 inverse bandwidth.

0:05:29.040000 --> 0:05:33.300000
 In other words, the faster the bandwidth,
 the faster the link, the lower

0:05:33.300000 --> 0:05:37.700000
 the cost. One key thing to remember about
 spanning tree is lower is always

0:05:37.700000 --> 0:05:41.120000
 better. Whenever you're looking at
 two values and trying to determine

0:05:41.120000 --> 0:05:45.020000
 what's going to win for whatever it
 is I'm doing, the lower value will

0:05:45.020000 --> 0:05:49.020000
 always win. And it's not linear.

0:05:49.020000 --> 0:05:57.320000
 What that means is that you might think,
 oh, okay, well, if I have a fast,

0:05:57.320000 --> 0:06:01.820000
 ethernet interface, which is
 100 million bits per second.

0:06:01.820000 --> 0:06:07.480000
 And I have a gigabit interface,
 which is 10 times faster, right?

0:06:07.480000 --> 0:06:10.080000
 It's 1 billion bits per second.

0:06:10.080000 --> 0:06:14.500000
 Well, and that would mean that the
 cost of my fast, ethernet interface

0:06:14.500000 --> 0:06:17.440000
 is different by 10 times.

0:06:17.440000 --> 0:06:18.780000
 You might think this, right?

0:06:18.780000 --> 0:06:23.340000
 You might think, all right, here's fast,
 ethernet, which is 100 million

0:06:23.340000 --> 0:06:25.140000
 bits per second.

0:06:25.140000 --> 0:06:29.140000
 You might think, oh, okay, if the cost
 of that, I'm just going to make

0:06:29.140000 --> 0:06:30.100000
 up a number here.

0:06:30.100000 --> 0:06:38.940000
 If the cost of that is 10, then a gigabit
 interface, which is 1 billion,

0:06:38.940000 --> 0:06:46.020000
 1 billion bits per second, you think,
 all right, well, that's got to be

0:06:46.020000 --> 0:06:49.160000
 1 10th, right? Because the faster
 the speed, the lower the cost.

0:06:49.160000 --> 0:06:51.560000
 So you assume, okay, it
 must be linear then.

0:06:51.560000 --> 0:06:54.260000
 This guy is 10 times faster.

0:06:54.260000 --> 0:06:57.280000
 So the cost should be 1 10th,
 which is, you know, just one.

0:06:57.280000 --> 0:07:01.360000
 It's not. In rapid-spain tree,
 the cost is not linear.

0:07:01.360000 --> 0:07:04.520000
 And I'm going to show you here in just
 a second, a chart that shows you

0:07:04.520000 --> 0:07:11.340000
 this. What if there's
 a tie in the cost?

0:07:11.340000 --> 0:07:15.800000
 So for example, what if we have
 the root bridge right here?

0:07:15.800000 --> 0:07:19.760000
 Here's switch A, here's switch B.

0:07:19.760000 --> 0:07:25.440000
 They're both getting Bp'd use from the
 root bridge with a cost of zero.

0:07:25.440000 --> 0:07:30.560000
 Okay, now those switches take a look
 at their receiving interface that

0:07:30.560000 --> 0:07:35.320000
 receive the Bp'd U, which might have
 a local cost of 10, let's say.

0:07:35.320000 --> 0:07:42.040000
 So when they go on and advertise those
 Bp'd use down to switch X, they're

0:07:42.040000 --> 0:07:47.120000
 going to advertise a
 cost of 10, right?

0:07:47.120000 --> 0:07:49.440000
 Because it's their cost to
 get to the root bridge.

0:07:49.440000 --> 0:07:52.660000
 Switch A says, well, my neighbor told
 me it's a cost of zero to get to

0:07:52.660000 --> 0:07:56.580000
 him, but to get to him,
 I got to get 10.

0:07:56.580000 --> 0:07:58.380000
 So 10 plus zero is 10.

0:07:58.380000 --> 0:07:59.360000
 So that's what they're
 advertising.

0:07:59.360000 --> 0:08:01.760000
 Well, what switch X
 going to do here?

0:08:01.760000 --> 0:08:05.040000
 Well, switch X is going to look at his
 own local interface where he received

0:08:05.040000 --> 0:08:09.720000
 those Bp'd use, but what the cost
 on both of those is the same.

0:08:09.720000 --> 0:08:11.600000
 Let's say it's five.

0:08:11.600000 --> 0:08:17.080000
 Then he'll say, okay, well, if I
 go this way, my total cost is 15.

0:08:17.080000 --> 0:08:21.520000
 And if I go this way,
 my total cost is 15.

0:08:21.520000 --> 0:08:22.820000
 That's what they're
 saying right here.

0:08:22.820000 --> 0:08:28.340000
 If there's a tie in the cost, then bridge
 X is going to go back to these

0:08:28.340000 --> 0:08:30.560000
 two Bp'd use he just received.

0:08:30.560000 --> 0:08:34.420000
 He's going to say, okay, is there something
 else I can use in these Bp'd

0:08:34.420000 --> 0:08:38.280000
 use to compare them against each other
 that will help me determine which

0:08:38.280000 --> 0:08:40.280000
 Bp'd U is better?

0:08:40.280000 --> 0:08:44.220000
 And if there is, well, then
 that's my tiebreaker.

0:08:44.220000 --> 0:08:51.500000
 In this case, the tiebreaker would
 be the lowest upstream bridge ID.

0:08:51.500000 --> 0:08:56.220000
 So in this situation, bridge X would
 say, all right, if these letters

0:08:56.220000 --> 0:09:00.780000
 here represent my bridge IDs, if A
 is a bridge ID and B, then switch X

0:09:00.780000 --> 0:09:05.780000
 will say, okay, well, A is lower
 than B, so I'll choose this.

0:09:05.780000 --> 0:09:07.820000
 This will be my root port.

0:09:07.820000 --> 0:09:12.840000
 Cost is the same, but the bridge ID
 of my neighbor A is lower than the

0:09:12.840000 --> 0:09:15.280000
 bridge ID of my neighbor B.

0:09:15.280000 --> 0:09:20.520000
 But sometimes even that isn't
 going to work for you.

0:09:20.520000 --> 0:09:25.080000
 For example, what if I have
 something like this?

0:09:25.080000 --> 0:09:27.080000
 What if here's my root?

0:09:27.080000 --> 0:09:35.160000
 Here's bridge X and he has
 two links to the root.

0:09:35.160000 --> 0:09:39.300000
 Now we know that Bp'd use are going
 to be flying down those two links.

0:09:39.300000 --> 0:09:45.300000
 As they come down, the initial
 cost is going to be zero.

0:09:45.300000 --> 0:09:49.680000
 So, bridge X is going to take a look
 at his own local interfaces.

0:09:49.680000 --> 0:09:53.000000
 You can say, okay, well, what's
 my cost to get back to that guy?

0:09:53.000000 --> 0:09:55.500000
 All right, well, the cost of his own
 local interface here, let's just

0:09:55.500000 --> 0:09:57.340000
 make up a number, 10.

0:09:57.340000 --> 0:09:59.500000
 Maybe the cost here is also 10.

0:09:59.500000 --> 0:10:02.600000
 He's going to say, all right,
 I have a tie here.

0:10:02.600000 --> 0:10:08.360000
 Because if I go up to the
 left, my total cost is 10.

0:10:08.360000 --> 0:10:11.100000
 If I go up to the right,
 my total cost is 10.

0:10:11.100000 --> 0:10:12.880000
 All right, well, that's
 not going to help me.

0:10:12.880000 --> 0:10:15.840000
 So he says, all right, well, the next
 thing I'm going to do is I'm going

0:10:15.840000 --> 0:10:19.340000
 to look at the Bp'd use as they came
 down to me and I'm going to look

0:10:19.340000 --> 0:10:21.200000
 at the sending bridge IDs.

0:10:21.200000 --> 0:10:22.520000
 Well, guess what?

0:10:22.520000 --> 0:10:24.380000
 They both have the same
 sending bridge ID.

0:10:24.380000 --> 0:10:28.360000
 This one has a bridge ID of root because
 the root's the one who sent it

0:10:28.360000 --> 0:10:33.900000
 to him. And this one has a sending
 bridge ID of root, whatever his Mac

0:10:33.900000 --> 0:10:35.420000
 and priority is.

0:10:35.420000 --> 0:10:38.020000
 That's not going to help bridge X.


0:10:38.020000 --> 0:10:40.240000
 So what's bridge X going to do?

0:10:40.240000 --> 0:10:45.500000
 Well, there is one more component to
 these Bp'd use he could look into

0:10:45.500000 --> 0:10:47.420000
 to find a tiebreaker.

0:10:47.420000 --> 0:10:51.500000
 And that is what's
 called the port ID.

0:10:51.500000 --> 0:11:01.880000
 So on a bridge, let's say, for example,
 this is port 0 slash 3 and this

0:11:01.880000 --> 0:11:05.200000
 is port 0 slash 8.

0:11:05.200000 --> 0:11:14.760000
 These Bp's that come down just like
 we had a bridge ID was composed of

0:11:14.760000 --> 0:11:20.700000
 two pieces. A priority and a Mac.

0:11:20.700000 --> 0:11:27.180000
 Well, also in that Bp'd use, there
 is a port ID and I like to call it

0:11:27.180000 --> 0:11:32.800000
 a sending port ID because whatever
 bridge is sending you the Bp'd use,

0:11:32.800000 --> 0:11:34.620000
 he puts a sending
 port ID in there.

0:11:34.620000 --> 0:11:37.660000
 And that also has a priority.

0:11:37.660000 --> 0:11:44.320000
 But not a Mac. We just call this
 second half an ID field.

0:11:44.320000 --> 0:11:51.240000
 Well, do you remember when it comes
 to bridge IDs, do you remember what

0:11:51.240000 --> 0:11:55.780000
 the default priority value was?

0:11:55.780000 --> 0:11:57.420000
 Hopefully you said yes, Keith.

0:11:57.420000 --> 0:12:02.080000
 I do remember that was 32,768.

0:12:02.080000 --> 0:12:03.300000
 Well, guess what?

0:12:03.300000 --> 0:12:08.880000
 There's also a default port
 priority, which is 128.

0:12:08.880000 --> 0:12:13.500000
 Yeah, I know in a lot of numbers you
 got to memorize here, but 128.

0:12:13.500000 --> 0:12:19.660000
 So both these Bp'd use are coming
 down looking like this.

0:12:19.660000 --> 0:12:21.120000
 This one's coming down.

0:12:21.120000 --> 0:12:23.760000
 I'll just say here a port ID.

0:12:23.760000 --> 0:12:28.980000
 It'll say 128. This one
 will also say port ID.

0:12:28.980000 --> 0:12:31.020000
 128. That'll be the priority.

0:12:31.020000 --> 0:12:33.560000
 And if I don't change it, they
 both have the same priority.

0:12:33.560000 --> 0:12:37.940000
 But the one on the left will
 say something like 0 slash 3.

0:12:37.940000 --> 0:12:40.940000
 The one on the right will say
 something like 0 slash 8.

0:12:40.940000 --> 0:12:44.920000
 Now it won't exactly be
 0 slash 3 and 0 slash 8.

0:12:44.920000 --> 0:12:50.300000
 But whatever number goes in this section
 right here will be derived in

0:12:50.300000 --> 0:12:53.520000
 some way from 0,3, and 0,8.

0:12:53.520000 --> 0:12:55.300000
 You don't have to know
 how to predict it.

0:12:55.300000 --> 0:13:00.460000
 All you have to know is if their priorities
 are the same, bridge X will

0:13:00.460000 --> 0:13:06.020000
 prefer this as the root
 port right here.

0:13:06.020000 --> 0:13:10.800000
 Because the Bp'd he received on that
 interface had a lower sending port

0:13:10.800000 --> 0:13:14.620000
 ID. 0,3 was lower than 0,8.

0:13:14.620000 --> 0:13:17.020000
 That's what this is talking about.


0:13:17.020000 --> 0:13:21.060000
 So let's just run through this a couple
 of times to make sure you've got

0:13:21.060000 --> 0:13:25.020000
 it. Oh, first of all, we got
 to talk about cost values.

0:13:25.020000 --> 0:13:30.080000
 So I've already mentioned this that
 when I forward a Bp'd you to you,

0:13:30.080000 --> 0:13:36.920000
 I tell you what my cost is
 to reach the root bridge.

0:13:36.920000 --> 0:13:41.160000
 And 802.1d if you actually check out
 the specification does have some

0:13:41.160000 --> 0:13:43.600000
 cost values in it.

0:13:43.600000 --> 0:13:48.160000
 And I've highlighted here in this graphic
 the ones are most relevant.

0:13:48.160000 --> 0:13:51.020000
 Certainly if you're studying for any
 kind of an exam, a networking exam

0:13:51.020000 --> 0:13:55.740000
 like Network Plus or Cisco CCNA or
 something of that nature, you will

0:13:55.740000 --> 0:13:58.300000
 want to have these
 numbers memorized.

0:13:58.300000 --> 0:14:06.720000
 So as you can see, a regular 10 megabit
 ethernet language, you probably

0:14:06.720000 --> 0:14:07.960000
 won't see those anymore.

0:14:07.960000 --> 0:14:11.960000
 Those are really old have
 a default cost of 100.

0:14:11.960000 --> 0:14:17.740000
 Fast ethernet, which is still fairly
 prevalent, has a cost of 19.

0:14:17.740000 --> 0:14:25.620000
 One gig has a cost of four and
 10 gig has a cost of two.

0:14:25.620000 --> 0:14:30.360000
 So for example, if I go into
 one of my switches right now.

0:14:30.360000 --> 0:14:40.260000
 Okay, so if I go into the switch and
 I say show spanning dash tree and

0:14:40.260000 --> 0:14:43.260000
 this selective VLAN that I
 know that the switch has.

0:14:43.260000 --> 0:14:48.120000
 Here we can see all the interfaces
 that spanning tree is including as

0:14:48.120000 --> 0:14:49.380000
 part of its tree.

0:14:49.380000 --> 0:14:54.580000
 They're all fast ethernet, so
 they all have a cost of 19.

0:14:54.580000 --> 0:15:02.200000
 So notice the output in this value is
 only your own local interface cost.

0:15:02.200000 --> 0:15:04.720000
 Now I happen to be on the root bridge,
 but let's go to a switch that's

0:15:04.720000 --> 0:15:05.980000
 not the root bridge.

0:15:05.980000 --> 0:15:16.780000
 Show spanning dash tree VLAN 1.

0:15:16.780000 --> 0:15:23.620000
 Okay, so notice his cost to reach the
 root is 19, but that's not what

0:15:23.620000 --> 0:15:27.560000
 this is. This is his
 local interface cost.

0:15:27.560000 --> 0:15:30.740000
 Let's see here. Show CDP neighbor.


0:15:30.740000 --> 0:15:33.920000
 He's going out fast ethernet 0
.13 and we can see right here.

0:15:33.920000 --> 0:15:35.840000
 This is his root port.

0:15:35.840000 --> 0:15:37.100000
 So this is root information.

0:15:37.100000 --> 0:15:44.920000
 He says to get to the root, my root
 port is fast ethernet 0.13.

0:15:44.920000 --> 0:15:51.400000
 All right, so let's do this interface
 range fast ethernet 0 slash 13 through

0:15:51.400000 --> 0:15:55.020000
 15. Let's shut down those ports.

0:15:55.020000 --> 0:16:05.920000
 So we're on switch 3 right now.

0:16:05.920000 --> 0:16:12.720000
 We're right here and he was, no,
 we're on switch number one.

0:16:12.720000 --> 0:16:14.920000
 We're not on switch number
 three, we're right here.

0:16:14.920000 --> 0:16:16.300000
 Switch number one.

0:16:16.300000 --> 0:16:23.980000
 And he was saying that his root port
 was this fast ethernet 0.13 and we

0:16:23.980000 --> 0:16:26.520000
 know that this guy here
 is the root bridge.

0:16:26.520000 --> 0:16:30.460000
 Now what I just did is I shut
 down all these links.

0:16:30.460000 --> 0:16:33.800000
 So now in order for switch one to get
 to the root bridge, he's going to

0:16:33.800000 --> 0:16:35.340000
 have to go this way.

0:16:35.340000 --> 0:16:37.840000
 Now let's think about it.

0:16:37.840000 --> 0:16:44.880000
 So switch 3 is advertising BPUs on
 each one of these three links here.

0:16:44.880000 --> 0:16:50.000000
 Each one of those BPUs has a cost of
 0 because he is the root bridge.

0:16:50.000000 --> 0:16:55.140000
 When switch 2 gets it, he says, okay,
 what's my local interface cost on

0:16:55.140000 --> 0:16:57.340000
 those three interfaces that
 just receive those BPUs?

0:16:57.340000 --> 0:17:02.220000
 Well, they're all fast ethernet,
 so they're all 19.

0:17:02.220000 --> 0:17:06.520000
 So switch 2 has to figure out what
 his root port's going to be.

0:17:06.520000 --> 0:17:10.440000
 He says, well, it's 19 no matter
 which path I take here.

0:17:10.440000 --> 0:17:12.420000
 So the next thing he's going to look
 at, he's going to look at each one

0:17:12.420000 --> 0:17:15.420000
 of those BPUs and try
 to find a tiebreaker.

0:17:15.420000 --> 0:17:17.220000
 The tiebreaker is going
 to be the bridge ID.

0:17:17.220000 --> 0:17:19.920000
 Well, he's going to look at the bridge
 ID and say, okay, did these all

0:17:19.920000 --> 0:17:22.320000
 come from different switches?

0:17:22.320000 --> 0:17:24.100000
 Oh man, they didn't.

0:17:24.100000 --> 0:17:28.360000
 The sending bridge ID of all these
 BPUs is exactly the same thing.

0:17:28.360000 --> 0:17:32.000000
 Switch 3. So that's
 not going to help.

0:17:32.000000 --> 0:17:36.800000
 Then he's going to go into those BPUs
 and look at the sending port IDs.

0:17:36.800000 --> 0:17:42.940000
 And he'll end up saying, oh, this BPU
 here is the better one because that

0:17:42.940000 --> 0:17:46.300000
 has a, it was sent on port 016.

0:17:46.300000 --> 0:17:51.820000
 These other ones here 017
 and 018 are not as good.

0:17:51.820000 --> 0:17:55.100000
 So if we go over to switch 2 and I'll
 go over there right now, we'll see

0:17:55.100000 --> 0:17:59.640000
 that fast Ethan at 016, I'm assuming
 that all three of these links are

0:17:59.640000 --> 0:18:05.540000
 up. If they are, we'll see that fast
 Ethan at 016 is the root port.

0:18:05.540000 --> 0:18:10.540000
 Keep in mind, it's not based
 on his own port IDs.

0:18:10.540000 --> 0:18:17.900000
 If I had something like this, if I had
 016 here, 018 here, but over here

0:18:17.900000 --> 0:18:23.960000
 I had something like 0
 slash 4 and 0 slash 9.

0:18:23.960000 --> 0:18:27.500000
 Sometimes people mistakenly think,
 oh, okay, well, if the cost is the

0:18:27.500000 --> 0:18:31.700000
 same, and if both BPUs had the exact
 same sending bridge ID, it must be

0:18:31.700000 --> 0:18:33.600000
 this interface that's
 going to win.

0:18:33.600000 --> 0:18:38.220000
 No, it's not based on the interface where
 you receive the BPUs, it's based

0:18:38.220000 --> 0:18:43.000000
 on your neighbor's upstream interfaces
 that transmitted those BPUs to

0:18:43.000000 --> 0:18:46.660000
 you. So let's just go over to switch
 2 real quick and we'll confirm that

0:18:46.660000 --> 0:18:50.160000
 016 is his root port.

0:18:50.160000 --> 0:19:05.180000
 Show spanning dash tree, BLAN 1, and
 there we see his root port is indeed

0:19:05.180000 --> 0:19:09.620000
 fast Ethan at 016.

0:19:09.620000 --> 0:19:14.940000
 So this is his root
 port right here.

0:19:14.940000 --> 0:19:16.060000
 So what did he do?

0:19:16.060000 --> 0:19:22.400000
 Switch 2 turned around and sent 3 BPUs
 downstream this way, because remember,

0:19:22.400000 --> 0:19:25.200000
 we shut off these interfaces
 right here.

0:19:25.200000 --> 0:19:29.040000
 Switch 1 said, okay, clearly I can reach
 the root bridge through switch

0:19:29.040000 --> 0:19:32.380000
 2. What should my root port be?

0:19:32.380000 --> 0:19:38.520000
 Well, all three of these BPUs
 said 19 as far as the cost.

0:19:38.520000 --> 0:19:41.040000
 Why did they all say
 the same thing?

0:19:41.040000 --> 0:19:46.380000
 Because switch 2 told him, let me tell
 you about the cost of my root port.

0:19:46.380000 --> 0:19:50.380000
 That root port is fast Ethan at 0,
 0, because remember, the BPU had a

0:19:50.380000 --> 0:19:55.240000
 cost of 0 initially when it came down,
 and then switch 2 said, well, for

0:19:55.240000 --> 0:19:59.640000
 me to get back that way, for me to go
 this way, I have to look at my own

0:19:59.640000 --> 0:20:02.980000
 local cost, which is 19, because
 that's fast Ethan at.

0:20:02.980000 --> 0:20:09.420000
 So that 19 is what he advertises downstream
 to all of his neighbors.

0:20:09.420000 --> 0:20:12.240000
 Switch 1 is going to say, well,
 the cost is the same.

0:20:12.240000 --> 0:20:14.360000
 He said, I'm going to look
 at my own local cost.

0:20:14.360000 --> 0:20:16.960000
 Well, these are also
 fast Ethan at ports.

0:20:16.960000 --> 0:20:19.740000
 So he says, well,
 19 plus 19 is 38.

0:20:19.740000 --> 0:20:25.280000
 If I go that way, 38, if I go that
 way, and 38, if I go that way.

0:20:25.280000 --> 0:20:30.440000
 So my total cumulative cost is the
 same, no matter which path I take.

0:20:30.440000 --> 0:20:34.460000
 So he's going to do the same
 logic that switch 2 did.

0:20:34.460000 --> 0:20:36.960000
 He's going to take a look at
 all three of these BPUs.

0:20:36.960000 --> 0:20:39.400000
 He's going to say, all right, what
 about the sending bridge ID?

0:20:39.400000 --> 0:20:40.580000
 Are they different?

0:20:40.580000 --> 0:20:42.760000
 Can I use that as a tiebreaker?

0:20:42.760000 --> 0:20:47.960000
 No, I can't. Sending bridge ID of all
 three of those is switch 2, because

0:20:47.960000 --> 0:20:49.800000
 he's the one who's sending it.

0:20:49.800000 --> 0:20:52.440000
 All right, let me look at
 the sending port IDs.

0:20:52.440000 --> 0:20:54.820000
 And here's where he's going
 to find his tiebreaker.

0:20:54.820000 --> 0:20:58.520000
 Because the one that was received on
 this interface right here has a port

0:20:58.520000 --> 0:21:02.720000
 ID of 0 slash 10, which is
 lower than the other ones.

0:21:02.720000 --> 0:21:06.240000
 So now we're going to see that switch
 1 is using fast Ethan at 0 slash

0:21:06.240000 --> 0:21:09.180000
 10 as his root port.

0:21:09.180000 --> 0:21:14.400000
 And we should see his
 total cost is 38.

0:21:14.400000 --> 0:21:17.580000
 It's kind of chicken scratch
 right there, 38.

0:21:17.580000 --> 0:21:27.920000
 So let's take a look on switch 1.

0:21:27.920000 --> 0:21:32.620000
 All right, so there 0 slash 10 and
 actually says right down here, fast

0:21:32.620000 --> 0:21:35.060000
 Ethan, 0 10 root.

0:21:35.060000 --> 0:21:36.820000
 See, now look at the difference.

0:21:36.820000 --> 0:21:41.920000
 This is his total cumulative cost to
 get all the way back up to the root

0:21:41.920000 --> 0:21:48.300000
 bridge. This is just his
 local interface cost.

0:21:48.300000 --> 0:21:49.780000
 So there's a difference.

0:21:49.780000 --> 0:21:58.540000
 Your local interface cost,
 total cumulative cost.

0:21:58.540000 --> 0:22:02.560000
 Now you might be wondering about Keith,
 what if I have some interface

0:22:02.560000 --> 0:22:06.480000
 that doesn't fall into
 this range here?

0:22:06.480000 --> 0:22:09.960000
 You know, for example, I've studied
 a little bit about switching.

0:22:09.960000 --> 0:22:14.800000
 I know that with switches, you can have
 multiple interfaces all bundled

0:22:14.800000 --> 0:22:17.460000
 together into something
 called an ether channel.

0:22:17.460000 --> 0:22:21.980000
 Or maybe you don't know that.

0:22:21.980000 --> 0:22:24.000000
 Well, now you do.

0:22:24.000000 --> 0:22:31.280000
 So if all four of these links as an example
 are fast Ethan at and I bundle

0:22:31.280000 --> 0:22:36.740000
 them into an ether channel, spanning
 tree is going to see that as 400

0:22:36.740000 --> 0:22:43.920000
 megabits. 100 megabits per second
 bundled together times four.

0:22:43.920000 --> 0:22:46.940000
 I don't see 400 megabits
 anywhere on this chart.

0:22:46.940000 --> 0:22:48.560000
 So what's that going to be?

0:22:48.560000 --> 0:22:52.620000
 Well, the IEEE specification
 doesn't give you that.

0:22:52.620000 --> 0:22:56.880000
 The IEEE specification does not give
 you an actual mathematical formula

0:22:56.880000 --> 0:23:01.320000
 to say, this is my bandwidth, plug into
 a formula, here's the resulting

0:23:01.320000 --> 0:23:05.000000
 cost. All it does is give
 you a chart like this.

0:23:05.000000 --> 0:23:07.160000
 So here's the only thing
 that we would know.

0:23:07.160000 --> 0:23:13.780000
 Looking at our chart, we can infer
 that a 400 megabit per second link,

0:23:13.780000 --> 0:23:18.280000
 well, 100 megabits is 19.

0:23:18.280000 --> 0:23:21.800000
 10 times that a gigabit is four.

0:23:21.800000 --> 0:23:25.860000
 So all we know is that a 400 megabit
 is going to be some number between

0:23:25.860000 --> 0:23:30.580000
 four and 19. It might be 17, it might
 be 12, it might be 11, it'll be

0:23:30.580000 --> 0:23:33.420000
 some number between those two.

0:23:33.420000 --> 0:23:41.180000
 Now, one other thing I want to mention
 here, before I leave this slide,

0:23:41.180000 --> 0:23:47.160000
 is that the chart that I'm showing
 you is actually an old chart.

0:23:47.160000 --> 0:23:55.060000
 That comes from the original
 1998 802.1D specification.

0:23:55.060000 --> 0:23:57.740000
 And if you're watching in my other videos
 or read some books, you'll know

0:23:57.740000 --> 0:24:03.560000
 that Spanning Tree 802.1D was actually
 updated back in 2004, 15 years

0:24:03.560000 --> 0:24:05.720000
 ago as of this recording.

0:24:05.720000 --> 0:24:10.500000
 And in that specification, the cost
 values are different than this.

0:24:10.500000 --> 0:24:12.800000
 So why am I showing you this?

0:24:12.800000 --> 0:24:16.040000
 Well, because we just saw that even
 though the cost values were updated

0:24:16.040000 --> 0:24:22.380000
 back in 2004, as far as the IEEE is concerned,
 at least on Cisco switches,

0:24:22.380000 --> 0:24:27.280000
 they're still using the older
 cost values, four, 19 and 100.

0:24:27.280000 --> 0:24:30.180000
 So certainly if you're working with
 a Cisco switch and you're running

0:24:30.180000 --> 0:24:35.300000
 rapid Spanning Tree, you have to remember
 those old values because that's

0:24:35.300000 --> 0:24:37.740000
 what the switches are
 still working with.

0:24:37.740000 --> 0:24:41.900000
 So let's finish it off here by going
 to just another quick demonstration

0:24:41.900000 --> 0:24:44.040000
 here on the graphic.

0:24:44.040000 --> 0:24:47.420000
 So let me ask you this question.

0:24:47.420000 --> 0:24:49.680000
 Let me draw in some stuff.

0:24:49.680000 --> 0:24:58.360000
 Let's say that the switch X on the bottom
 was connected like this to these

0:24:58.360000 --> 0:25:01.340000
 three upstream switches.

0:25:01.340000 --> 0:25:05.920000
 Let's just put in some port
 numbers here, 0 slash 2.

0:25:05.920000 --> 0:25:12.880000
 Let's say 0 slash 1,
 0 slash 4, 0 slash 7.

0:25:12.880000 --> 0:25:17.860000
 Let's say that these black links
 represent fast ethernet.

0:25:17.860000 --> 0:25:26.700000
 And let's put some port
 numbers down below here.

0:25:26.700000 --> 0:25:41.940000
 Let's say 0 slash 1, 0 slash
 3, 0 slash 2, and 0 slash 4.

0:25:41.940000 --> 0:25:45.900000
 Pause this video for a moment and see
 if you can determine for switch

0:25:45.900000 --> 0:25:50.820000
 X and for the other three switches,
 the ones in the middle, we already

0:25:50.820000 --> 0:25:53.460000
 see our root bridge that's
 been defined for us.

0:25:53.460000 --> 0:25:58.820000
 So if all four downstream switches,
 what would your root ports be?

0:25:58.820000 --> 0:26:08.780000
 And then press play when you think
 you've identified the root ports.

0:26:08.780000 --> 0:26:11.240000
 All right, so let's work on this.

0:26:11.240000 --> 0:26:17.240000
 So we know that on the root bridge
 himself, he's not going to have any

0:26:17.240000 --> 0:26:23.680000
 root ports. We know that on the root
 bridge, every port that he has is

0:26:23.680000 --> 0:26:25.300000
 going to be a designated port.

0:26:25.300000 --> 0:26:27.380000
 Designated ports deliver.

0:26:27.380000 --> 0:26:34.360000
 He's going to be delivering BPDUs out
 this way, this way, and this way.

0:26:34.360000 --> 0:26:38.040000
 All right, let's start
 with switch 44, 44.

0:26:38.040000 --> 0:26:42.520000
 Well, we could start adding up all
 sorts of cost values, but this one

0:26:42.520000 --> 0:26:44.040000
 should be pretty easy.

0:26:44.040000 --> 0:26:47.980000
 We know that when the BPDU comes down,
 it's going to have a cost of 0.

0:26:47.980000 --> 0:26:52.140000
 He's connected via fast ethernet,
 which has a local cost of 19.

0:26:52.140000 --> 0:26:54.100000
 That's his local cost.

0:26:54.100000 --> 0:26:59.480000
 So his total cost to get to the root
 bridge that way is going to be 19.

0:26:59.480000 --> 0:27:06.340000
 19 plus 0. Any other path he takes,
 you don't even have to do the math.

0:27:06.340000 --> 0:27:08.960000
 You could if you wanted to, but you
 know that any other path he takes

0:27:08.960000 --> 0:27:13.440000
 going this way is going to be
 something higher than 19.

0:27:13.440000 --> 0:27:18.580000
 So he's not going to use those
 paths as his root port.

0:27:18.580000 --> 0:27:27.300000
 So we can safely say that his root port
 will be port 0 slash 1, and he'll

0:27:27.300000 --> 0:27:29.900000
 place that into the
 forwarding state.

0:27:29.900000 --> 0:27:34.260000
 The same thing holds true for the other
 two switches in line with him.

0:27:34.260000 --> 0:27:42.640000
 43, 44. He's going to say, well, my
 local cost is 4, because he received

0:27:42.640000 --> 0:27:45.540000
 that BPDU on a gigabit
 ethernet interface.

0:27:45.540000 --> 0:27:51.780000
 It came down with a cost of 0, so
 he'll say, well, 4 plus 0 is 4.

0:27:51.780000 --> 0:27:56.620000
 Any other path he tries to take would
 be higher than 4, so he'll say,

0:27:56.620000 --> 0:27:59.500000
 this will be my root
 port right here.

0:27:59.500000 --> 0:28:02.740000
 And I'll place that in
 the forwarding state.

0:28:02.740000 --> 0:28:06.340000
 The same thing with bridge 44E4.

0:28:06.340000 --> 0:28:07.520000
 He'll do the exact same thing.

0:28:07.520000 --> 0:28:14.160000
 He'll say, my total cost is 4, because
 what came down to me had a cost

0:28:14.160000 --> 0:28:16.120000
 of 0 inside the BPDU.

0:28:16.120000 --> 0:28:20.100000
 I added my own local interface, which
 was a cost of 4 locally, which gave

0:28:20.100000 --> 0:28:21.440000
 me a total cost of 4.

0:28:21.440000 --> 0:28:23.380000
 That's the lowest I've got.

0:28:23.380000 --> 0:28:25.360000
 That'll be my root port.

0:28:25.360000 --> 0:28:27.260000
 I'll put that in the
 forwarding state.

0:28:27.260000 --> 0:28:32.360000
 Now let's look at the BPDU as they
 continue to flood down here.

0:28:32.360000 --> 0:28:42.340000
 Hopefully, you determined that the best
 path that SwitchX had was 0 slash

0:28:42.340000 --> 0:28:46.640000
 3. That is his root port.

0:28:46.640000 --> 0:28:49.420000
 That's what goes into
 the forwarding state.

0:28:49.420000 --> 0:28:53.260000
 Now in case you weren't able to
 figure that out, here's why.

0:28:53.260000 --> 0:29:03.580000
 If he goes off to the left, his
 total cost that way is 38.

0:29:03.580000 --> 0:29:05.460000
 Fast ethernet is 19.

0:29:05.460000 --> 0:29:07.580000
 That's his local cost.

0:29:07.580000 --> 0:29:11.400000
 His neighbor is advertising
 19 to him.

0:29:11.400000 --> 0:29:15.940000
 19 plus 19 this way is 38.

0:29:15.940000 --> 0:29:22.120000
 Whereas if he goes any other direction,
 it's going to be one hop of fast

0:29:22.120000 --> 0:29:28.780000
 ethernet, which is 19 plus one
 hop of gigabit, which is 4.

0:29:28.780000 --> 0:29:33.820000
 So all these other paths
 will be a cost of 23.

0:29:33.820000 --> 0:29:35.920000
 Total cost of 23.

0:29:35.920000 --> 0:29:39.320000
 Let's put those numbers in here.

0:29:39.320000 --> 0:29:43.420000
 23. 23. So now he has a tie.

0:29:43.420000 --> 0:29:45.200000
 We can rule out the left side.

0:29:45.200000 --> 0:29:47.600000
 That's definitely not going
 to be used as his root port.

0:29:47.600000 --> 0:29:49.420000
 He's got a tie of 23.

0:29:49.420000 --> 0:29:51.780000
 So what do we do as a tie breaker?


0:29:51.780000 --> 0:29:54.940000
 Well, the first thing is we go back
 to the BPDUs and we take a look at

0:29:54.940000 --> 0:29:56.520000
 the sending bridge ID.

0:29:56.520000 --> 0:30:00.660000
 And we try to say, okay, do any of these
 BPDUs have a sending bridge ID

0:30:00.660000 --> 0:30:04.640000
 of a neighbor that's lower
 than anybody else?

0:30:04.640000 --> 0:30:11.260000
 Well, the two BPDUs that came down this
 way have a sending bridge ID of

0:30:11.260000 --> 0:30:15.940000
 this guy. Let me just delete
 some of this stuff here.

0:30:15.940000 --> 0:30:27.680000
 All right. So the BPDUs that came down
 this way had this as a sending

0:30:27.680000 --> 0:30:32.100000
 bridge ID. But the BPD that came down
 this way had this as a sending bridge

0:30:32.100000 --> 0:30:39.740000
 ID. If you actually take a look at
 those numbers, 43 is lower than 44.

0:30:39.740000 --> 0:30:42.220000
 So that rules out this link.

0:30:42.220000 --> 0:30:45.200000
 So we know that's not going
 to be the root port.

0:30:45.200000 --> 0:30:50.200000
 So now we've narrowed down the root
 port to being one of these two links

0:30:50.200000 --> 0:30:55.500000
 right here. It's going to be
 either that one or that one.

0:30:55.500000 --> 0:31:01.160000
 And if you recall the port numbers
 I put in, it was 0, 1 here.

0:31:01.160000 --> 0:31:03.460000
 And I believe 0, 2 here.

0:31:03.460000 --> 0:31:07.160000
 Now I intentionally, on the bottom,
 I don't remember what I put in.

0:31:07.160000 --> 0:31:14.680000
 I think I put in something like 0,
 3, and 0, 2, something like that.

0:31:14.680000 --> 0:31:18.140000
 Because I want you to think, is it
 based on, so we take a look at the

0:31:18.140000 --> 0:31:22.300000
 BPDUs, we take a look at
 the sending bridge ID.

0:31:22.300000 --> 0:31:24.760000
 Well, they both have the exact
 same sending bridge ID.

0:31:24.760000 --> 0:31:26.620000
 The cost is the same.

0:31:26.620000 --> 0:31:29.480000
 Whether I go up the left or up the
 right, the cost is the same.

0:31:29.480000 --> 0:31:31.180000
 Sending bridge ID is the same.

0:31:31.180000 --> 0:31:33.020000
 So now we look at the
 sending port ID.

0:31:33.020000 --> 0:31:35.360000
 It's not based on
 your own port IDs.

0:31:35.360000 --> 0:31:38.160000
 Your own port ID is irrelevant.

0:31:38.160000 --> 0:31:42.720000
 And my neighbor's port ID of 0, 1 is
 better than my neighbor port ID of

0:31:42.720000 --> 0:31:49.200000
 0, 2. So for that reason, that's why
 we ended up with this port here as

0:31:49.200000 --> 0:31:58.820000
 my root port. So that
 concludes this video.

0:31:58.820000 --> 0:32:00.120000
 Thank you for watching.