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All right, let's jump back into the lab here.

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We're going to focus on this part of the lab topology.

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It really is the full topology still.

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We've got the root bridge in switch one and such.

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But which ports are blocking here?

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We left switch for blocking on this interface.

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But let's go ahead and see what else we're blocking on

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between those two ports that go to switch to.

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And I did reset the configuration.

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I got rid of all the cost configurations

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on those links to switch to.

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So if I do a show, spanning tree,

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let's just do blocked ports,
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let's just do blocked ports,

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then we see that we're blocking on gig 02.

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So is this the port that we were expecting?

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Because it's really easy to look at this and say,

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well, this is the lower numbered port.

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I thought we were supposed to favor lowered number ports.

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But notice how this is configured.

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We actually have the higher number port

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connected to the lower number port this way.

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And so the fact is that we're going to pay attention

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to the switch to configuration because the BPDUs

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are flowing this direction.

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And again, port priorities, that's an outbound concept.
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And again, port priorities, that's an outbound concept.

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So I look at the fact that gig 03 is the higher interface.

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And importantly, gig 02 is the lower interface.

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Therefore, I keep my local port on the right there,

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gig 03 in an online forwarding state.

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And I place gig 02 into the blocking state.

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So that's exactly what we'd expect.

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But how could I change this without changing

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the configuration on switch 4?

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I could use costs.

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And we've demonstrated that in the last lab video here.
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And we've demonstrated that in the last lab video here.

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But I want to go to switch 2 now.

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And I want to know more information

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about what's going on on gig 02 and gig 03.

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What we're going to do is we're going to do a show spanning

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tree interface.

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And I can compare gig 02 and gig 03.

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So when I look at gig 02, I see that it's

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using a priority of 128.

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We said that's the default.

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And then this internal number that we talk about is number 3.

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If I look at interface gig 03, we'll
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If I look at interface gig 03, we'll

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see that it's using an internal number of 4.

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And also, apparently, the other VLANs

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aren't actually running on that link.

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But that's fine.

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We'll not worry too much about that here.

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Focus is on VLAN 1.

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So we have the priority value.

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We have the number.

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And we do see why gig 02 is favored

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because gig 02 has a lower number, number 3, internally,

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compared to gig 03, which is number 4.

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And fortunately, we can look at gig 03 and gig 02
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And fortunately, we can look at gig 03 and gig 02

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and tell which one is higher and which one is lower.

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But if there's ever a question, this command

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will tell us exactly what it's using.

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We could also just do a normal show spanning tree command.

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And that information shows up right here.

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In fact, we can directly compare multiple interfaces

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using this command to see the relative port numbers.

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But now, let's say we want to flip it,

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and we want to flip it without changing switch

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4's configuration.

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I have to change switch 2's configuration.

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What do I do here?
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What do I do here?

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Well, in order to switch which port is being blocked,

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I'm going to have to get on here.

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And again, we have port priorities of 128 and 128.

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And I want to make it so that this side gets blocked,

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which means this has to have a higher number relative

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to the other side.

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So I could lower the priority on gig 03.

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I could raise the priority on gig 02.

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Either way is an option.

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Let's just go ahead and we'll just raise it on gig 02.
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Let's just go ahead and we'll just raise it on gig 02.

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So I'm going to say config T here, interface gig 02,

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and spanning tree.

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Again, I can do a VLAN 1 here if I want to.

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I could just do it for all the VLANs.

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And then when I run a question mark,

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we just ran cost in the previous lab video.

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Now I'm going to change the port priority.

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And I see that it's between 0 and 240.

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And yes, technically we say it's between 0 and 255

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because it's an 8-bit number.

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But Cisco mandates increments of 16.

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And so I can't call it all the way up to 256.

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So 240 is the highest number I can go with.
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So 240 is the highest number I can go with.

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Let's just do something in the middle.

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I'll say 160.

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That's a multiple of 16.

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And so I have now raised the number, which makes it worse

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on gig 02.

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So gig 02 should no longer be favored.

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Now, switch 2 is not going to block either port.

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If I do a show spanning tree for VLAN 1 here,

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it's still forwarding.

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But we do see now that gig 02 has a priority of 160.

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And so if I go to switch 4 and I run the exact same command,
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And so if I go to switch 4 and I run the exact same command,

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show spanning tree blocked ports,

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we do see that we have blocked gig 03, which

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is exactly what we set out to do.

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So as we keep saying, keep in mind that port priority

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is for outbound changes to the blocked ports.

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Cost is for inbound changes.

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We can leverage either one in this scenario.

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But if we're ever told, hey, this configuration

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has to be done on one of these switches or the other,

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we're going to have to pick the right method

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and apply it correctly in order to change the blocked interface.
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and apply it correctly in order to change the blocked interface.