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All right, we want to configure timers on our network.

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Now in the real world, we don't do a whole lot of timer tuning.

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In fact, we want to be very careful about tuning timers,

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and we'll talk about that here coming up.

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But ultimately, if we do want to change our timers,

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keep in mind that it's only the root that really matters.

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From the perspective that it pushes those timer values down.

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So we're on switch one here.

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We've made switch one the root.

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It's got this priority value of 20k still.

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We see that it has no root port, and so we

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know that it is indeed the root.
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know that it is indeed the root.

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If it doesn't have an upstream port,

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it must be the most upstream switch.

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And so what we're going to do is configure timers on this.

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We'll validate that those timers get pushed down.

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Just keep in mind that if we do change timers

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in the real world situation, we probably

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have a backup root as well.

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So do I need to configure these timers only on the root?

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Well, if the root is online, then that would be correct.

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But what happens if that root goes down

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and the secondary takes over at that point?

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It will determine what the timers are.

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So we usually at least want our timers configured
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So we usually at least want our timers configured

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on primary and secondary routes.

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If we have a catastrophic failure

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and we've lost both of those routes,

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we probably have more issues to worry about than what

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the SDP timers are.

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So either way, let's go ahead and do a show spanning tree

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for our VLAN 1 here.

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Separate the 1 from the VLAN.

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And we will see the timers as configured.

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So our hello time is two seconds.

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Our max age is 20.

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Our forwarding delay is 15 seconds.
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Our forwarding delay is 15 seconds.

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Here is my local configuration as well.

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Now this will match because I am the root.

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But just keep in mind that this is what is going to matter.

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Whereas this is just a local configuration

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and all the other switches.

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Incidentally, if we're messing around with studying

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topology change notifications, we'll

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notice the aging time is listed here.

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That would be our MAC address aging timer.

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And if we're in a topology change status,

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then we'll see that aging timer change.

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So let's go ahead and run some debug.

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In order to confirm that everything's working like we think.
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In order to confirm that everything's working like we think.

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We've got a hello time of two seconds.

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So I'm going to say debug spanning tree.

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And I can do a BPdu transmit.

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Now we'll see transmit happening.

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In fact, I'm just going to go ahead and very quickly

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undebug this.

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So I'll say undebug all and hit Enter.

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And let's go ahead and evaluate what just happened.

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So if I scroll up a little bit, we'll

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see that this all started at 31 seconds.

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Now notice how many BPUs are getting transmitted.

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We've got one for VLAN 10, VLAN 20, VLAN 30, and VLAN 1.
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We've got one for VLAN 10, VLAN 20, VLAN 30, and VLAN 1.

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And then we also have another one on VLAN 1.

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Why do we have another one on VLAN 1?

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Well, this has to do with the fact

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that I need to send one for my STP,

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pure VLAN spanning tree instance.

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But I'm also sending one for the sake of aligning

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with any non-Cisco devices out there.

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So that would be my backward compatibility version.

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Either way, we do see that we're sending

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BPdu on a per VLAN basis, which is why if we have 300 VLANs,

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we're sending 300 BPUs within a hello timer.
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we're sending 300 BPUs within a hello timer.

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Now again, we sent this all at 31 seconds after the minute.

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So if I go down to the next batch of BPdu's,

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we see it happened at 33 seconds.

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So 31 to 33 is two seconds.

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33 to 35 is another two seconds.

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35 to 37.

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OK, so we are sending our BPdu's every two seconds.

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So that's good to know.

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And that's the way we can validate that.

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So if I wanted to, I could say config T, spanning tree VLAN 1.
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So if I wanted to, I could say config T, spanning tree VLAN 1.

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And I had a question mark.

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I see I can configure my timers here.

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So I can change the hello time to, let's say, four seconds.

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So let's come down here.

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We'll do our debug command again.

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Now this is going to be interesting,

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because I only changed the hello time on VLAN 1.

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So let's see what ends up happening here.

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I'm just going to go ahead and cancel the debug's.

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And then we'll go and evaluate it.

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So let's scroll up and see what happened here.

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So we happen to catch.
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So we happen to catch.

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It looks like right here.

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OK, so we've got our VLAN 1 BPdu sent out.

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It looks like 1 got sent out at 01, 1 got sent out at 02.

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So that's kind of interesting.

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03 here is when we sent VLANs 10, 20, and 30.

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And then 05, we're also sending 10, 20, and 30.

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But notice here, we've got VLAN 1 now

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sending on the 06.

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We did not send one here in 03.

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So 02 looks like this is about where we actually are talking

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about this 01 was probably off by a few milliseconds.
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about this 01 was probably off by a few milliseconds.

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It's basically 02.

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So 02 to 06 is where we actually--

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we send it every four seconds.

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We shouldn't see another one now at 7, see it VLAN 10, 20,

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30, 10, 20, 30, and then 10 down here.

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So notice that it did change it to every four seconds.

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So that's pretty cool.

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It's good to know that changing the Hello timer

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actually does work.

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So now what I can do is test the forwarding delay.

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So I can say debug spanning tree events.
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So I can say debug spanning tree events.

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And let's get onto an interface here.

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I'm just going to get onto interface gig 00.

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This goes to switch 2.

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I'm going to do a shutdown.

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Now notice what just happened here.

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At 26 seconds afterwards, we generated a TC trap

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that topology change trap like we said.

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And we-- yeah, just other information about this.

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So we are the spanning tree root,

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sending topology change notifications

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on the different VLANs.
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on the different VLANs.

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And let's go ahead and say no shut.

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So when I do the no shut, what's interesting here

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is we're going to see that we progress through the listening

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learning states.

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And notice this is happening on every VLAN, VLAN 1, VLAN 10,

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VLAN 20, VLAN 30.

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It also started at 50 seconds after the minute.

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So we'd expect to transition to the learning 15 seconds later,

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which would be five seconds after the next minute.

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So now we're in learning.

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And then at about 20 seconds here after the minute,
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And then at about 20 seconds here after the minute,

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that is when we should be back into a forwarding state.

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So this is a great way that we can use this debug command

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to verify that the timer is already working.

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And so there we go.

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We're at the 20 second mark.

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And remember, when we transition to forwarding,

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that's going to trigger another TC trap.

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So if I do a show spanning tree VLAN 1, VLAN 1, there we go.

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We do notice that the aging timer at 15 seconds, which again,

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reflects the fact that we're in a topology change,

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like we talked about.
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like we talked about.

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Notice our hello time is at four seconds.

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Let's go ahead and go over to switch to show spanning tree

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VLAN 1, see what it thinks is happening.

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So it sees that our root is suggesting a hello time of four

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seconds, but locally I'm configured at two seconds.

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So the question here is how often am I actually

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sending my VPDUs?

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I've got my local configuration at two.

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Hello time is at four.

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So once again, I can use that same debug command.

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So debug spanning tree, VPDU transmit.
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So debug spanning tree, VPDU transmit.

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How often are we sending it for VLAN 1?

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Well, here's one.

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Oh, you know what?

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Always make sure your service time stamps are on.

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If you're going to be tracking with time stamps,

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that's going to be important.

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All right, so it looks like at 10 seconds,

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we sent a BPDU for VLAN 1, 14 seconds.

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All right.

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And we're not sending BPDUs for the other VLANs.
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And we're not sending BPDUs for the other VLANs.

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Might not be actually configured on this switch.

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So every four seconds, we see that we are sending our BPDUs.

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So it's not the local configuration,

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but it is indeed the root configuration

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that drives our timers.

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So the hello timer especially is going to be relevant

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because I don't actually send BPDUs on my own.

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I wait for a BPDU to be received from the upstream root.
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I wait for a BPDU to be received from the upstream root.

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So lastly, let's test max age.

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And in order to do that, let's take a look

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at our diagram again.

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Things have changed a little bit here.

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We no longer see switch three as the root,

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but instead switch one is the root

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because switch one is the root,

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switch four is probably blocking a port.

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And I don't remember what values

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all of our priorities are at.

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It's going to be one of these interfaces.

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Let's go ahead and take a look at switch four.

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Show spanning tree blocked ports.
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Show spanning tree blocked ports.

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VLAN 1, it is blocking a couple of ports.

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So we do have actually another switch in this topology.

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We'll see this later on in the,

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in when we get to the lab here towards the end of the skill,

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but this would be our gig three slash one.

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So that's apparently blocking as well.

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We don't need to worry about that for right now.

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What we do need to know is that we're blocking gig zero two.

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Gig zero two is this connection here.

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So this is our blocked port.
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So this is our blocked port.

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So if that's our blocked port, how do we test max age?

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I want to see how long it takes for this port

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to come back online in the event of an indirect outage.

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If I just shut down this link,

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we'll pick this back up in 30 seconds

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'cause we'll immediately transition it over to learning

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because it's my next best upstream connection.

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However, what if this link over here goes down?

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What if switch three, this is switch three's gig zero two port?

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And so if I were to take that down,

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switch three all of a sudden has no path to the root
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switch three all of a sudden has no path to the root

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and it's going to try to come this way.

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But we can't come this way 'cause switch four

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is trying to go this way to get to the root.

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And so we're all going to be kind of stuck for a little while.

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And so we want to again see how long it takes

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switch four to realize that there's a problem here.

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And so what we're going to do is we're going to enable

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our debug's on switch four.

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I'm going to again do debug spanning tree events.

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And I'm going to get on here and make sure
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And I'm going to get on here and make sure

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that service timestamps are configured.

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(keyboard clicking)

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All right, and again, we're expecting something to happen

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here with gig zero two coming out of block.

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So let's go ahead and get on to switch three,

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interface gig zero two, and we're going to actually,

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let's just do real quick, do show spanning tree root.

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All right, so indeed that is my root port for VLAN one.

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So I'm going to go ahead and shut this down.

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And let's come over to switch four
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And let's come over to switch four

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and see what ends up happening.

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So look, right away we're actually hearing new BP to use.

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So this is really cool, actually.

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So switch three is like, hey, I lost my root port.

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So I am the root.

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I must be the root.

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And it's sending BP to use out everywhere technically,

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but out here and switch four saying, wait,

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now I'm receiving these BP to use

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that suggest that there's a new root downstream.

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So that's exactly why we're saying

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we're hearing this root down here.

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And we're actually ignoring it
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And we're actually ignoring it

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because it's an inferior BP to you.

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We have to wait for Max H to be done

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before we can pay attention to it.

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So let's take a look here.

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So we see that we started hearing these at 47.

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Max H lasts 20 seconds.

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So we would expect this to be about seven seconds after,

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and indeed about seven seconds after, hold up.

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(laughs)

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We're waiting for all of this to settle out.

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Let's go ahead and undi-bug all of this.

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So somewhere right around here, let's see here.
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So somewhere right around here, let's see here.

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All right, so right here.

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So this is when we transitioned over to listening.

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So we have four seconds and then this is 47.

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So we do the math there, we say it's about 17 seconds.

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Well, wait a second.

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We said we were gonna wait 20 seconds.

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Why only 17?

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Max H is based on the last BP to you that was received.

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So if I received a BP to you and then a second or two

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later pass and then the link goes down,

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well, the 20 seconds is relative to the last BP to you
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well, the 20 seconds is relative to the last BP to you

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I received.

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So it'll actually be a little bit less

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than the full Max H timer because of that.

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So once we really think through how all these timers work,

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you know, that makes a whole lot of sense

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how that ends up happening.

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So at this point, where were we?

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Oh, at three.

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So we transitioned to learning 20 seconds

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after we received the last BP to you,

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which was 17 seconds after the port went down,

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or after we started hearing new BP to use.

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And so 15 seconds later from 03, so about 018 here,
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And so 15 seconds later from 03, so about 018 here,

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we should see that we transitioned to learning.

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Now we're off by a second because there's milliseconds

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involved here and we're rounding the seconds.

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So no problem, we're not too worried about that.

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And then about 15 seconds later,

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we should transition to forwarding and from 19 to 34,

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indeed we do.

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So that validates for us not only how these timers work,

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but also makes us think a little bit about how,

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exactly Max Age did works with indirect failures and the like.
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exactly Max Age did works with indirect failures and the like.

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And so one of our primary takeaways here,

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besides just knowing how to test this

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and seeing the results ourselves,

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is knowing how to configure these.

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So remember, it's everything with Cisco's

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on a pervieland basis.

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Spanning tree vlan one question mark,

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I can change the forward time,

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I can change the hello time,

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I can change the Max Age all here,

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and I can do so again on a pervieland basis.