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All right, we still have one more situation where everything we've talked about

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up till

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now is not quite good enough.

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And we are still going to need one more mechanism to help us out.

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So we have said that if we're getting two different BPDUs into the switch, let

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's call

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this switch before, why not?

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And we're advertising a cost of four or eight or 132 doesn't matter.

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So long as these numbers are the same.

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These are equal costs.
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These are equal costs.

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And so we're going to compare the bridge ID.

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And we're going to look at the bridge ID.

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But what happens if both of the bridge IDs have a priority field of 16 K?

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Well we know at that point I look at the MAC address.

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Well what if the MAC address on the left is zero dot zero dot A and the MAC

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address on

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the right is zero dot zero dot A.

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Well wait a second here.

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We said that MAC addresses are always unique.

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So how did this even happen?

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And we've got the tiebreaker as a result in a tie.
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And we've got the tiebreaker as a result in a tie.

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That's no good.

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So how can this happen?

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Well it's pretty simple.

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It happens when the upstream device is the same on both links.

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This we call switch three.

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So now we have two connections between these two switches.

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And we maybe connected them for redundancy.

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Now not to skip ahead, spoiler alert, we'd really like to just ether channel

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this and

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not worry about it.

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That's great.

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And that is exactly what we do in a real world scenario.
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And that is exactly what we do in a real world scenario.

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But spanning tree was invented in an era where ether channels maybe weren't as

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prolific.

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And the reality is that ether channels aren't required.

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I can't point to this and say you have to configure an ether channel.

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Spanning tree has to figure this out.

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And the information that we've had until now, the path cost doesn't help.

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It's a tie.

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The bridge ID also doesn't help.

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It's a tie.

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We need a new mechanism.

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And that new mechanism is known as the port ID.
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And that new mechanism is known as the port ID.

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This is going to be another arbitrary tiebreaker.

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If truly the cost is the same coming in.

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And yeah, that assumes that these are both the same bandwidth.

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If one was a gig link, the other is a 10 gig link, we would want to choose the

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10 gig link.

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But if everything else is the same, we just need to pick one of these to block.

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Either block this one or block this one.

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If that doesn't matter, just pick one and block.

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And so we're going to need another arbitrary way of breaking that tie.
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And so we're going to need another arbitrary way of breaking that tie.

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And that would be the port ID.

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The port ID, just like the bridge ID is going to consist of two different

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values, it's going

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to have a priority value, which is concatenated with a port number.

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This port number is an internal value.

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And unfortunately, we don't always know what exactly the port number is going

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to be.

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It's internally assigned by the switch.

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So usually, fortunately, I can look at like gig zero slash one and compare that
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So usually, fortunately, I can look at like gig zero slash one and compare that

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to gig

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zero slash two.

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And I know that gig zero slash one is less than zero slash two that just numer

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ically speaking.

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And so I can compare those two ports and I understand which one is lower.

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But what if there's a gig zero in our face?

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How do I compare that to a gig zero one?

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Or how do I compare a gig one slash zero with gig zero slash two?
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Or how do I compare a gig one slash zero with gig zero slash two?

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So we see that there can be problems in here.

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And if the operating system and the hardware manufacturer in our case Cisco is

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using logic,

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then hopefully we'll be able to figure out that maybe one zero is still higher

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than zero

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tube, treat these numbers as if they matter more, so on and so forth.

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But the port number that gets mapped, it still can't be G zero slash one.

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It has to be a numerical value.

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It has to be something like 11.

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And so as long as they have been internally assigned appropriately, we should
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And so as long as they have been internally assigned appropriately, we should

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have a port

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number here.

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The priority value is going to be just like some of the other priority values

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we've talked

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about where we can assign this, but it's going to have a default value.

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The default value is 128, which is a little bit off compared to the other

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priority values,

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which all have a default value of 32,768.

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Well, it's 128 because the full range for our priority value is zero to 255
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Well, it's 128 because the full range for our priority value is zero to 255

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because it's

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an eight bit ID.

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So we're not using the full range of IDs like we have with our priority values

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over

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here.

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Instead, we are looking at an eight bit value rather than the 16 bit value.

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So when we have eight bits, zero to 255, however, Cisco has once again said,

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you know, we require

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this to go on increments over here of 40, 96 for our switch priorities, we're
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this to go on increments over here of 40, 96 for our switch priorities, we're

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going to

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require increments of 16 here.

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So I could assign a value of, let's say zero, I could do 16, I could do 128.

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I could do 240, but 240 is going to be the highest number I can go to because I

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can't

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go to 256.

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That's 255 itself is not a multiple of 16, but these are the priority values

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that I can

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map.

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And just like with everything, the lower value is better.
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And just like with everything, the lower value is better.

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Now here is the very important part of port IDs.

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The port ID that I'm going to look at is the egress or the advertised port ID.

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What this means is the upstream switch is pushing down the port ID.

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In other words, the port ID here and the port ID here don't matter whatsoever.
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In other words, the port ID here and the port ID here don't matter whatsoever.

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It's going to be the port ID up here and the port ID ID here that matters

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because this

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is where I'm pushing the BPD use from.

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I'm going to advertise the port IDs.

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And this is really easy for us to forget when we're doing our configurations.

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If we have a lab task, spoiler alert, we probably will, where I have to control

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which

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one of these I want to block, well, I'm going to have to come up here to switch
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one of these I want to block, well, I'm going to have to come up here to switch

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three and

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do the configuration up here.

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If I want to control which of these ports get blocked between the two switches,

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I can't

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just get down here and change the port ID.

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So on egress, the port ID gets advertised.

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And as a result, it's the upstream port ID that ends up mattering.

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So here's the question.

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Let's kind of get into a almost a CCIE mindset.

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If we had a lab task that said get on to switch four and control which port
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If we had a lab task that said get on to switch four and control which port

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gets blocked on

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ingress.

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Is this possible?

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It's not going to be possible with port IDs.

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Port IDs are an egress concept.

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I can't change the port ID here or here to change which port gets blocked.

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But it is possible to change the inbound cost.

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So always keep that in mind.

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Remember, I said this was a one gig link and a one gig link.

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And so by default, it's going to have a cost of four and a cost of four.
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And so by default, it's going to have a cost of four and a cost of four.

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Well, let's say switch three has some kind of cost of eight upstream.

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And so it's advertising the port IDs.

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It also advertises a cost of eight.

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So this switch sees two options.

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Its two options are to go this way and it costs 12 or I can go this way and it

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also costs

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12.

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Why 12?

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Well, because switch three has a cost of eight and I'm adding four on this side

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to get 12.
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to get 12.

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I'm adding four on this side to get 12.

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So if I want to control this, let's say I really want to block the port on the

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left.

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But with the configuration on switch three, the current currently I'm blocking

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the port

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on the right.

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How do I force this link on the left to get blocked?

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Well, all I have to do is make it so this has a much higher cost.

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So I can say that the cost is increased or decreased depending on what I want

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to do.

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I can use those manual cost adjustments.
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I can use those manual cost adjustments.

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So let's say that I override this now and I say, hey, this no longer has a cost

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of four.

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Instead it has a cost of 400, something like that.

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Well now my cost to get out this link, we would say it's advertising eight.

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So I add 400 to it has 408 compared to 12.

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Now the port priority doesn't matter anymore and I can take the faster

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perceived path, which

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is the path on the right because that still has a cost of 12.
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is the path on the right because that still has a cost of 12.

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So keep that in mind.

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I have the ability to control this on egress via port ID.

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I can change the priority value.

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I can't change the port ID just like I can't change a port number here.

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I can't change that in the same way I can't change a system MAC address.

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But I do have configuration control over the priority.

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That's how I would do it with egress.

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If I had to configure switch forward to do this, I can configure on egress.

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It won't involve port IDs.

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It will involve cost.

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If I could break the tie at the cost level, I don't even need to worry about
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If I could break the tie at the cost level, I don't even need to worry about

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the tiebreaker

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at the port ID level.