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Well, if you studied spanning tree before, you're probably aware that there are

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some

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famous or maybe infamous tiebreakers that are out there.

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Because it's great when we say, "Hey, I've got a fast path to the root and I've

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got

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a maybe a less fast path to the root," then I would just take the faster of the

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two.

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But as we can imagine, especially in a network with lots of equal cost links,

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there's going

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to be a lot of equal cost opportunities, meaning that I'm going to have to

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decide between

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this path being a total cost of, let's say, 8.
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this path being a total cost of, let's say, 8.

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And this cost also being a total cost of 8.

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So if it's 8 this way and 8 this way, what am I going to do?

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And yes, that is going to lead us into a conversation around tiebreakers.

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So let's take a look at one specific type of example, and then we're going to

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look at

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a different type of example that uses the same principles.

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So in this network, let's say that these are 1 gig links.

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1 gig links have a path cost of 4 by default, so that would be our short path

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cost.
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cost.

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Let's say we're using that here, and we've got a root up here at the top, and

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then we've

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got another 1 gig link down here.

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So what ends up happening is we send our BPDU down to these two switches,

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advertising our

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root ID, which might be necessary if this network just came online.

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And I need to prove that I am the root, maybe I've got a priority value of 4000

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here, 4096.

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And so I advertise the root ID, and then I advertise my cost of zero.
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And so I advertise the root ID, and then I advertise my cost of zero.

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So both of these switches are going to receive this cost of zero, and they're

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going to add

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the four and say, hey, I've got a cost of four.

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Both of them are thinking the same thing here.

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And so they determine that this is going to be the root port here for at least

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right

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now, unless I hear of a better path.

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And either way, then I am very quick to advertise out that I have a path cost

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of four.

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So I say, hey, I've got a cost of four, and this switch also says, hey, I've
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So I say, hey, I've got a cost of four, and this switch also says, hey, I've

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got a cost

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of four, and now we have a little bit of a problem.

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Which of these switches is going to block their link?

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Is it going to be a block on the right here?

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Is it going to be a block on the left?

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We're not going to both block because, again, think about spanning tree back in

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the day.

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There could be a hub in here.

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We could have hosts.

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Those hosts would have no way off the network if they both block.

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So somebody has to block this even in modern networks where we just have a

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point to point

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link.

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So the question here is going to come down to a question we ask a lot with
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So the question here is going to come down to a question we ask a lot with

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spanning tree

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protocol, which is which one of these is the superior BPDU.

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We know what a superior BPDU looks like when we're talking about root IDs.

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We recall that we're going to compare this is a concatenated set of values,

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which is

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a fancy way of saying we smash two values together.

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And so we've got a priority value and we've got a MAC address and we know that

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the lower

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of these values wins.

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Spanning tree protocols all about the lower value winning.
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Spanning tree protocols all about the lower value winning.

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And so this tells me that I've got the superior BPDU compared to you two

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switches because

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my priority MAC combo is better than yours.

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And so remember BPDU is flow one way.

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And so I stopped trying to send BPDU upstream because I know that your BPDU is

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superior

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to mine.

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But down here, which of these is going to be the superior and which one's going

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to be

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the inferior.

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So this is where we need to consider a tiebreaker because we have a tie.
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So this is where we need to consider a tiebreaker because we have a tie.

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And frankly, at this point, the only thing we care about the speed by which we

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can get

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to the root, that's equal.

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And so this tiebreaker can be completely arbitrary.

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We are really just trying to break the tie so that somebody blocks the port.

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That's what we care about at this point.

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I just need one of you to block it because the user experience is going to be

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the exact

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same if I go this way as if I go this way.

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Doesn't matter to users who might be here in the middle on a hub.
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Doesn't matter to users who might be here in the middle on a hub.

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And that is the primary thing we care about user experience.

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We're going to take the shortest path.

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If the paths are the same, now it can be arbitrary.

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So what are we going to base it on?

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Well, we're going to base it on the bridge ID.

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Now we just said that the root is sending out its bridge ID and that bridge ID

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actually

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becomes the root ID for the whole network because I take my priority value and
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becomes the root ID for the whole network because I take my priority value and

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I add

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my MAC address to it and I have become the root because my bridge ID is the

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best in the

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network.

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Well, now we're going to use the bridge ID in a slightly different way here.

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So remember, when we send a BPDU, both of these switches sending a BPDU to one

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another,

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they're going to include things like the root ID and the path cost to the root.

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But I'm also going to include my own bridge ID.
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But I'm also going to include my own bridge ID.

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Now I'm not trying to become the root at this point.

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I'm just telling you this is who it's coming from.

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My bridge ID, just like the root ID, is my local priority value concatenated

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with my

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system MAC address.

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So I might find that I set this switch to B4K, this switch to B8K, 8192, and

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this switch

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to B16K.

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So in this case, when I exchange the two, even though the path cost is the same

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, the

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root ID is the same, the path cost is the same, now I'm going to have to get
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root ID is the same, the path cost is the same, now I'm going to have to get

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down and

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compare the bridge ID.

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So who has the better bridge ID?

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Well, it's the person with the lower priority.

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So this is going to be considered the superior BPDU and this would be

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considered the inferior

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BPDU.

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Because I received a superior BPDU on my interface, I'm going to stop sending

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my inferior BPDU

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and I'm going to block that port.

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So that is how we resolve this issue.

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It's how we decide which port on the link is going to block in the case that by
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It's how we decide which port on the link is going to block in the case that by

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the

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both of these switches have the exact same path cost to the root.

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Now keep in mind that this conversation isn't quite done because what happens

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if we both

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have the same priority value.

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Maybe I just left this as the default, 32K here and 32K here.

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At that point, who's going to win the battle of superior BPDUs?

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Well, when I look at the bridge ID, if the priority is the same, I have to
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Well, when I look at the bridge ID, if the priority is the same, I have to

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compare the

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MAC.

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And the system MAC is always going to be unique per the rules of Ethernet.

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And so one of these will have the lower MAC address.

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And so the bridge ID is first and foremost going to take a look at the priority

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.

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But if the priority is the same, then it's going to look at the system MAC

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address.

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So in our case, let's say that we have system MAC address, we'll just keep this

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easy.

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We'll say this is 0.0.5 and over here on the right, this system MAC is 0.0.B.
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We'll say this is 0.0.5 and over here on the right, this system MAC is 0.0.B.

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So remember their hexadecimal values, B stands for 11, A would be 10, B would

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be 11, C would

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be 12.

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And so 12 is a higher number than 5.

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Therefore this switch still wins and we still block the same port.

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But if this side had been 0.0.4, well then it would have won and the other side

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would

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have been blocked.

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Again, this is all very arbitrary because it doesn't matter which side blocks.
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Again, this is all very arbitrary because it doesn't matter which side blocks.

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Again, we just need somebody to block.

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Now this triangle example is a good way of seeing how we might have two

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switches that

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are sending BP to use to one another.

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And we're kind of fighting over which one actually has the superior BP to use.

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But we might have another situation where we've got four switches here.

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And all of these are one gig links.

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So path cost of four, path cost of four, four and four.
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So path cost of four, path cost of four, four and four.

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And so I advertise down to this switch that hey I've got a cost of four to the

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root.

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And this switch on the right says the same thing.

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I've got a path cost of four to the root.

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So now we have to decide hey, now this isn't a matter of me saying I have a

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superior BP

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to you.

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I don't even have a good way to the root at this point.

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I'm trying to decide my path to the root.

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So I see that you're telling me it's a four.

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I add my four.

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And so I know this is eight to go this way.
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And so I know this is eight to go this way.

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Well, I do the same thing over here.

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And I see that I've got a path cost of eight going this way.

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So how do I decide which way to go?

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Well, it's going to be the exact same concept except now I'm not comparing the

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bridge ID

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to my bridge ID.

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I'm comparing this bridge ID to this bridge ID.

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So the BP to you that's coming in from both sides that includes the path cost

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to the root

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out and I will have two different bridge IDs.

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So now let's say I have zero dot zero dot D here and I've got zero dot zero dot
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So now let's say I have zero dot zero dot D here and I've got zero dot zero dot

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five.

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Well D which would be thirteen here from a decimal perspective is going to be

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greater

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than five.

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So I'm going to view this BP to you as superior because it has a lower bridge

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ID.

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Now label this one as inferior meaning that I'm going to block this port.

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Now when I say it's inferior I don't mean to my own BP to you because what's
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Now when I say it's inferior I don't mean to my own BP to you because what's

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going

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to happen here is I'm going to assign this as my root port and technically I'm

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going to

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try I being the switch we'll call the switch four.

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Switch four is going to try to claim that it has a path to the root because it

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does.

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It has a path cost of eight.

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So now if we zoom in on these two switches let's make it a little more clear

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down here.

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So now I have a root path root port here root port here and those both go to

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the root so

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we understand that.
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we understand that.

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So now we have to decide between this link here and that's exactly the same

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scenario

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we just had.

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The difference is that I'm advertising that I have a path cost of four and I'm

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advertising

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down here our switch four here is advertising a cost of eight because I know

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that my path

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cost as we said is going to be eight going that direction.

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Well I don't even have to compare bridge IDs because the better path is going

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to win.

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Again imagine there's a hub here imagine there's hosts and imagine which way
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Again imagine there's a hub here imagine there's hosts and imagine which way

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they'd want to

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go with everything being equal we know that this would be more hops and

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therefore we'd

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really rather just go this way.

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But it's not about how account it's about the path cost.

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And so I look at the fact that you're telling me it's a cost of four I know I

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have a cost

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of eight so my BPD is inferior and so I am going to block here otherwise the

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other side

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would have to block but in my case I know I'm going to have to block this side.
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would have to block but in my case I know I'm going to have to block this side.

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So these are the two different sort of topologies where we can start to see

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this shake out.

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One is when we have two switches directly competing on the same link another

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one is

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when we have a switch comparing multiple uplinks it might be two it might be

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three or four

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but either way if the path costs being advertised are the same I'm going to

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look at the bridge

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ID to break the tie.