STP Election Process

12 minutes
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All right, welcome back everyone. Now, when dealing with whatever spanning tree you're using, there is an election process and that's what we're going to get into here. Okay. Got as I said earlier, STP comes alive only when there's redundancy. It's working always, but it really doesn't start doing its job of blocking ports and all that good stuff and root bridges and all that until there is redundancy. Okay, once this redundancy, alright spanning tree is going to say okay, I need to select the root bridge.

The root bridge is the center of everything. That's where all the information is coming from. Okay, the root bridge, so let's call that all right, all the other switches are not root bridges for right now right root bridge or non root bridge. Now, how do you become Route bridge? Well, there's the first part of the election. It looks at something called a bid or a bridge ID.

Okay, now that bridge ID consists of two things, the bridge party number, which is 32 769, and a MAC address, whatever it is, okay? I just use this as an example. All right, so it'll look at the bridge ID, but we'll look at the priority number of the bridge ID. Now 32 769 is the default for all switches, default for all switches for Cisco, okay? That's if they're all the same, well, then I can make a decision because it's got to pick the lowest setting in the lowest bridge priority number. So if you don't change this, then it has to base it on what the lowest MAC address Last How do you know which is the law as well?

And this is us use your way and the other one is 0001 or 000. Then that's lor. Okay? So when you're given a scenario to say, hey, which of the following is the root bridge? And they'll they'll have the bridge IDs, right? Whether which is consists of these two numbers right here.

All right, and then you choose Okay, well, this bridge parties are all the same, and the MAC address, okay, boom, and then you pick the lowest number, the lowest bridge ID, okay, which is a combination of these two. Now, like I said, we'll first look at the party number. And then if the party number is the same, it will look at the lowest MAC address now, once it does that, once it says okay, you are their lowest Mac or you're the lowest bridge, party number, you are the root bridge. Now those parts, those parts on that root bridge We'll be designated forwarding period, all those ports will be designated 40. All right, but now I gotta do my job as SDP. So I got to find a switch that I need to block a port on.

So information stops being forwarded. So we don't have that broadcast storm, because that's what's going to happen. All right, so Okay, so what do I do? Well, it'll look at the same thing. It'll look at the bridge either the bridge priority if they're all the same, then I'll look for the highest bridge priority or again either the same. Now look at that highest MAC address.

Once I look at the highest MAC address, there says okay, you switch number 22. You I will choose to block a port on Well then, how does it know how much you support well is based on two things. Bandwidth And port number. Okay. And which doesn't say in the book. And if it's facing any non root bridge, if it's ports are facing the root bridge, they cannot be blocked.

Again, non root bridges whose ports are facing the root bridge, those ports cannot be blocked. Those parts will be placed into root ports. That's what it'll be called root ports. All right. So it says, All right, you got the highest MAC address. Cool.

We're gonna pick a port. All right. Let me see. We're running Fast Ethernet. We all have the same cost of 19. So can you see that we're running the same bandwidth So what we'll go for, let's go for the highest port number.

All right, let's pick the highest port number cheaper, the highest port number is facing the root bridge. I can do that. So I'll just pick the one that's not facing the root bridge and choose that as my port to block. Okay, see how that comes in? We have a visual, visual. Now everyone else say no, you gotta do it on costs.

If you have a triangle, picture a triangle, which we're going to see in a second as your triangle. If this is the switch down here, on the right to left triangle, okay, 19 Okay, 1919 that's 38 Oh, there's a highest cost path. So as who isn't gonna block this guy right down here. See, I'm not even adding let me say, well, you're facing the root bridge, you're not going to get blocked. So it must be one of these guys down here facing some other place. Okay, that's going to get blocked.

But if you do the math, because I saw Cisco Does it they base it on the highest cost path, because each bandwidth has its own cost, which we'll look at in a second. If you do the math to get to the root bridge, the one that has the highest cost path to the root bridge, that's the one that will get blocked. I do it easy, especially if it's in a triangle is very easy to do. Well, you're going to be blocked because if this way you can because this port is facing the root bridge. And Either way, it's just 19. But if you go 1919, that's 38.

Well, you got the highest cost path, buddy. All right, and you're not facing the root bridge. So you're it but if you look at our MAC address, sometimes it will be a lower port number with a lower MAC address. Sometimes, okay, but all the times No, but when I mean costs, and here's a little table. All right, it goes all the way from four megabits to 10 gigabit. Wow, put two G's in there.

Okay. No big deal. right but you're normally going to be seeing 100 megabits per second or one gigabit right 100 megabits that's what I see I say 19 right Fast Ethernet, and then gigabit is for gigabit is for definitely if you have Fast Ethernet and gigabit fast season is going to get blocked. All the rest is facing the rubric. Period. That's just the way it is.

All right, well, you have this information there's a nice table to have. Okay, so you know, because you know you're in college or wherever you're at they may say well, what about 10 gigabits okay well 10 gigabits I mean, what we're running on data centers that coming in through no see like what you know, I mean, know why but you know, people like to play around is 00 for the old, two for the new. I came up with these numbers. I have no idea. Okay, just memorize them. Okay.

But that's what they come on these these guys. They say, Hey, we're going to give this a cost. Okay. So that's what the cost is. Cool. So now let's take a look at a visualization of it.

Alright, let's escape out of here. And let's open up the packet tracer. There we go. Now these are three switches, three layer three switches that I just pop on there. Now using this, it's pretty simple to see that that amber light right there, that's our block port. Okay.

So, our blog part, you know, this is not the root bridge. I just said that root bridge, right? So he's blocked he's not the root bridge. He's facing this guy. He's now the real bridge. He's the bridge.

Why do you hear this? Yeah, let's see. That's true. Let's see if that's true. Okay, I'm just gonna open it up and then I'll get out your way because I know you guys keep telling me lies Get out the way sure shows spanning tree okay. 00 a 41 all of this I want to use there was a way for you on a dz this bridge is a route this information right here that information deals with the root bridge, you will see this and every switch you go into this information is just for that this information not here is for the MAC address for the MAC address.

Okay, now you can see that since this is the root bridge, you can take a look at both ports designated forwarding and there's your cost right there 19 Okay, because it's Fast Ethernet. So and this is the actual the root bridge and this is the MAC address of the switch. There you go is the same thing so we're going to see this and all the other switches so let's take a look at the other switches. I said only the one that got blocked let's see what my goodness Yes. Okay, Alright, enable shows. Oops.

All right. Now, you see that that was the root bridge right there. Now the root bridge, but look at the MAC address here. 00 e, I'm pretty high, right? This root, oh, it's facing the root bridge, alternating block, meaning it could change, okay, but the cost is 19. But you can see that the MAC address is higher than here.

This is triple zero, this is double zero he, so he was chosen to be blocked. Okay, and this guy, let's see what his MAC addresses. Okay, here's his 006 less than 00 he, so that's why he didn't get blocked, which is ports, all the other ports or non root bridges that don't get bought or set a designated forwarding except the ports that are Facing the root bridge, which gets set to root. Okay, when you're facing it, look at the cost again 19. Now we're just talking about spanning tree. Okay?

No big deal is easy to see in the packet tracer, which one is longer, which is not because you got the little lights, obviously, but in your certification, not gonna have little lights. All right, you're gonna have the bridge IDs, right, the priority number and the MAC address. So you got to take a look at those numbers and decide who's who always remember that the root bridge will have the lowest priority or the lowest Mac, the one that's blocked will have either the highest priority or the highest Mac, right, and everybody else will be non root bridges. And there'll be sent to designate a 40 designate a 40. And the root bridge will always have all his ports on designated 40 period. Why there's no discussion on that.

Okay, but it is fairly simple. There's really simple now we're just talking about it. Okay spanning tree. So you can get, you know, guess more like a review or start getting all I remember spanning tree. Yeah, blogging reports and all that. But in the next section, we're actually going to go ahead and configure spanning tree.

We'll talk about the configuration, and then why can you do a lab with spanning tree. We'll see you then.

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