WEBVTT

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 Welcome to this video titled essential
 RSTP Cisco iOS commands.

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 We're going to look at just some basic
 commands to ensure that your switch

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 is running the rapid spanning
 tree flavor of spanning tree.

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 We're going to take a look at how to
 deterministically set a switch as

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 the root bridge, how to configure port
 fast, and some show or monitoring

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 commands to see exactly what
 spanning tree is doing.

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 So let's just jump right into it.

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 Let's start with how you enable
 rapid spanning tree.

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 Now, the moment you configure a VLAN
 on a switch and the moment that VLAN

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 has at least one active port in it,
 it could be an access port, it could

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 be a VLAN trunking port.

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 It doesn't matter.

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 The moment that VLAN says, oh, I have
 a port over which I can operate,

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 spanning tree will automatically
 start.

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 So you don't have to start
 up spanning tree.

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 The question is, will spanning tree
 by default start up as the older pre

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 2004 802.1d, or will it start
 up with rapid spanning tree?

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 Most of the time it's not
 rapid spanning tree.

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 We probably want rapid spanning tree
 because rapid spanning tree has some

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 significant benefits which
 make it more rapid.

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 It converges more quickly
 than 802.1d.

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 So we can use the command spanning
 dash tree mode rapid dash pbst.

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 So I'll go ahead and do that
 on one of my switches.

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 Spanning dash tree mode
 rapid dash pbst.

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 So notice it doesn't give you any message
 or anything to indicate that

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 anything has changed.

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 I'll show you in just a second.

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 How do you confirm?

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 Well, actually, let's just
 take a look at it now.

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 All right. So if I was on the switch,
 how would I know it was doing rapid

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 spanning tree mode?

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 The most common command is show
 spanning dash tree summary.

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 And notice it says right here in
 the output, switch is in rapid.

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 Come on, is in rapid pbst mode.

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 Now I explicitly configured that
 on this particular switch.

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 Let's take a look at switch number
 one, who I haven't configured yet,

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 and see what he displays.

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 Show spanning dash tree summary.

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 So notice in his case, in this switch's
 case, it says switch is in pbst

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 mode. So that is not doing
 rapid spanning tree.

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 Bear in mind the way that Cisco switches
 have worked since the last several

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 decades is that each VLAN is running its
 own independent instance of spanning

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 tree. That's what we call
 per VLAN spanning tree.

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 The more VLANs you create, the more
 spanning trees the CPU has to take

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 care of and monitoring track,
 which is harder for the CPU.

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 So there are some upper limits as far
 as how many spanning trees you can

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 actually have running concurrently
 in a Cisco switch.

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 Now, when it says pbst mode like this,
 that means it's doing the original

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 spanning tree, the pre-2004 version,
 the older spanning tree.

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 As we just saw, when instead it says
 rapid pbst, that means we're still

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 doing an individual instance of pbst
 on each individual VLAN, but now

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 those spanning tree instances
 are doing rapid spanning tree.

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 As it mentions, rapid spanning is backwards
 compatible with the legacy

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 pre-rapid spanning tree.

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 Now, we want rapid spanning tree to
 be able to identify our edge ports.

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 For example, if we go here on switch
 two, who's currently running rapid

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 spanning tree, show spanning dash tree,
 we have to select a particular

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 VLAN that we're interested in.

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 So notice that all of
 his ports say shared.

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 Now, a port will say shared when
 it is in half duplex mode.

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 And actually, that's
 not a good thing.

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 In order to get the best benefits of
 rapid spanning and have it converge

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 the fastest, you want your ports
 to be in point to point mode.

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 In other words, full duplex.

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 Just by the nature of this particular
 topology and how it's set up, that's

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 why they're showing as shared.

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 But you can't get around that.

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 For example, I could go to interface gigabit
 1 slash 1 and say, say, spanning

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 dash tree, let's see, what is it?

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 Link type, point to point.

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 And now, even though it's doing half
 duplex, it now sees that link as

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 a point to point link,
 which is good.

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 Because the things that make rapid spanning
 tree rapid that make it faster

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 than 802.1d, the original can only
 be done on point to point links.

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 Now, in a real network environment,
 chances are when you're connecting

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 things up to your switches, you will
 be doing it in full duplex mode.

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 It'll just default to that.

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 And so your rapid spanning tree will
 see the interfaces as point to point.

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 You won't have to do that
 command that I just did.

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 But in the event that
 you do, there it is.

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 On the interface, you say spanning
 dash tree, link dash type, point to

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 point. Now, notice though, no
 mention of edge ports here.

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 Nothing saying that anything
 is an edge port.

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 Let's go to switch
 three for a second.

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 Actually, no, we can
 stick on switch two.

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 So on switch two, if we...

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 Let's see here. Where is it?

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 Okay, so switch two, his
 gig zero two interface.

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 This one right there is actually
 connecting to an edge device.

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 It's connecting to a router.

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 So we don't have any worries about
 broadcast going to that router and

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 circling around again.

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 That's not going to happen.

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 That's no different than if we were
 connected to a laptop or a PC.

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 So what we can do is we can safely
 go to that interface and configure

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 the port fast feature.

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 What port fast does is the moment it
 detects electrical connectivity on

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 an interface, that interface instantly
 goes into the spanning tree forwarding

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 state. It bypasses all
 intermediary states.

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 That's the benefit of it.

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 And as a byproduct of that, rapid spanning
 tree sees that as an edge port.

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 And it treats that port a little bit
 differently than it would a normal

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 port. So what we need to do is we need
 to make sure the gig zero slash

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 two is viewed as an edge port.

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 There's a couple of ways
 we could do that.

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 We can go right to that interface and
 say spanning dash tree port fast.

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 If we were actually trunking to that
 router, we could say spanning tree

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 port fast trunk.

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 Or at the global configuration level,
 we can say spanning tree port fast

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 default. And any interfaces that are
 up that are not trunking ports, that

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 are access ports, will have port fast
 applied to them automatically.

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 So let's do that.

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 Interface gig size zero two,
 spanning dash tree port fast.

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 We get a little warning,
 but that's okay.

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 It's just warning us that port fast
 could invoke a bridging loop.

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 If you did it on a port that's actually
 connected to another switch and

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 not to a host, that is a thing that
 could happen with port fast if you

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 can figure down the
 wrong interface.

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 And the last thing is we probably have
 a switch that is maybe our bigger

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 switch. It has more CPU power, more
 memory, and it's like in the core

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 of our network. And that's the switch
 that we want to make our root bridge.

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 So how can we do that?

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 There's two ways.

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 One is not any better
 than the other.

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 They just operate a little
 bit differently.

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 We could say spanning dash tree
 VLAN, the VLAN ID root primary.

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 How's that work?

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 Well, let's take a look.

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 First of all, I'm going
 to go to switch one.

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 And I'm not going to explain
 this command just yet.

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 We'll do this in just a second.

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 Okay. So now let's
 go to switch two.

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 Now, how can I tell if
 he's the root or not?

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 Well, that is the command
 show spanning tree.

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 VLAN and then the VLAN you're interested
 in, the tree you're interested

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 in. I want to see the
 tree for VLAN one.

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 So we can see right now
 that he is not the root.

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 How do we know that?

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 Well, in the output of this command right
 here, this first sort of paragraph

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 of information gives you information
 about the root bridge.

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 If this switch actually was the root
 bridge, it would say, plain as day,

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 this switch is the root, or
 this bridge is the root.

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 It would tell you that.

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 Another way that you know he's not
 is that the information just below

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 that, this information is about the switch
 you're currently on right now,

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 which is switch two.

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 Well, switch two, his
 priority is 32, 769.

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 The root bridge is 12, 289.

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 That's why he was the root because
 he had a lower bridge ID.

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 So let's go ahead and make
 switch two the root bridge.

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 We actually want switch two to be the
 root bridge, not this other guy.

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 So the first command that does that
 is this, spanning-tree VLAN one root

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 primary. And what that'll do is that
 switch will say, okay, what is the

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 priority of the current
 root bridge?

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 Oh, in this case, it's 12, 289.

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 If I match that number, then we would
 see whoever's MAC address is lowest.

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 If I match that number, would I become
 the root bridge because my MAC

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 address is lower?

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 If the answer is yes, then this command
 will have him match 12, 289, and

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 he would win. But in this case, we
 can see, well, actually that would

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 work because look, if this guy right
 here was not 32, 769, but instead

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 he was 12, 289, and they ended up comparing
 each other's MAC addresses,

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 then the current root bridge who is
 00AF would be competing against my

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 local switch who is 00AA,
 and AA is lower than AF.

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 So as soon as I hit enter here,
 that's what we should see.

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 We should see this switch matches the
 priority of the current root bridge,

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 but this switch becomes the winner based
 on his MAC address being lower.

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 Let's find out. And it did.

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 See, this bridge is the root.

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 All right, what if I did that
 command on switch one?

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 Let's go ahead and configure him
 to be rapid spanning tree mode.

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 Okay. Now let's go ahead and do
 do show spanning-tree VLAN one.

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 Okay, so he says,
 look, I'm 12, 289.

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 The root bridge is 12, 289, but he beat
 me because his MAC addresses lower

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 than me. Now if I went to this guy and
 I said spanning-tree VLAN one root

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 primary, now he'd say, well, my priority
 already matches, but I lost because

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 my MAC address is too high.

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 Why don't I lower my priority?

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 So now this switch will find what the
 next increment down is from 4096.

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 You know, so if we subtracted 4096
 from 12 to 88, that would actually

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 give us, I believe 8192, I believe
 is what number that would give us.

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 And then he will win.

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 And now this switch is the root.

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 His priority was reduced to the next
 increment down that he could do,

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 which was 8192, and that allowed
 him to be the root.

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 A lot of people like that command, because
 you don't even have to worry

0:13:54.920000 --> 0:13:56.980000
 about what priority value to set.

0:13:56.980000 --> 0:14:00.660000
 The bridge will automatically do what
 he needs to do to become the root

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 bridge. All right, now
 let's go to switch two.

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 Let's do it again.

0:14:07.720000 --> 0:14:16.520000
 Root primary. All right, now this
 guy should be the winner.

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 He is. Let's do it again
 over here on switch one.

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 On and on we go switch one.

0:14:30.480000 --> 0:14:32.280000
 There's actually a reason
 why I'm doing this.

0:14:32.280000 --> 0:14:33.520000
 Switch one is now the root.

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 So now let's go back
 to switch two.

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 Do it again on switch two.

0:14:39.760000 --> 0:14:44.400000
 And notice it only
 takes effect once.

0:14:44.400000 --> 0:14:46.160000
 It's not a recurring command.

0:14:46.160000 --> 0:14:50.960000
 In other words, it doesn't make the
 root persistent on this device.

0:14:50.960000 --> 0:14:55.300000
 If somebody comes in who's better,
 they will take over the role.

0:14:55.300000 --> 0:14:58.800000
 Okay, so here's what
 I want you to see.

0:14:58.800000 --> 0:15:00.640000
 Here on switch one now.

0:15:00.640000 --> 0:15:04.920000
 Switch one says I'm not the root.

0:15:04.920000 --> 0:15:10.360000
 The root bridge has the same priority
 as me, 4096, but he's got a lower

0:15:10.360000 --> 0:15:14.420000
 MAC address. Now you might think, oh,
 okay, well, you know, Keith, you

0:15:14.420000 --> 0:15:18.400000
 did tell me that zero is a valid priority,
 which is the next increment

0:15:18.400000 --> 0:15:26.740000
 down from 4096. So if I do that command
 again, it should just knock me

0:15:26.740000 --> 0:15:29.620000
 down to zero and he should
 be the winner, right?

0:15:29.620000 --> 0:15:35.760000
 And it doesn't see that's the one downside
 of this root primary macro

0:15:35.760000 --> 0:15:42.200000
 command. The lowest it can take your
 priority is 4096 and that's it.

0:15:42.200000 --> 0:15:46.180000
 So right now we just got this message
 saying, sorry, I can't go any lower

0:15:46.180000 --> 0:15:47.300000
 than I already am.

0:15:47.300000 --> 0:15:51.700000
 So if I truly want to knock this switch
 down to zero to make him the root,

0:15:51.700000 --> 0:15:55.000000
 that's where I have to use the second
 command on the bottom here, spanning

0:15:55.000000 --> 0:16:01.200000
-tree VLAN the number priority and
 explicitly set the priority value.

0:16:01.200000 --> 0:16:09.160000
 Give him a priority of zero.

0:16:09.160000 --> 0:16:14.380000
 And now he is the root bridge.

0:16:14.380000 --> 0:16:19.860000
 Now keep in mind, if I went back to switch
 to and I also gave him a priority

0:16:19.860000 --> 0:16:21.380000
 of zero, guess what?

0:16:21.380000 --> 0:16:31.420000
 He would win because he's got the
 lower MAC address than switch one.

0:16:31.420000 --> 0:16:34.300000
 And lastly, how do we verify
 the root bridge?

0:16:34.300000 --> 0:16:37.060000
 Well, I've already shown you the second
 command, show spanning-tree VLAN,

0:16:37.060000 --> 0:16:40.280000
 VLAN ID. We've seen that.

0:16:40.280000 --> 0:16:43.220000
 This first section right here tells
 you who the root bridge is.

0:16:43.220000 --> 0:16:45.220000
 You'll either say,
 hey, I am the root.

0:16:45.220000 --> 0:16:54.300000
 I'm the winner or it will show you
 that you are not the root and what

0:16:54.300000 --> 0:16:58.040000
 interface, what root port you use to
 get to the root, which is gigabit

0:16:58.040000 --> 0:16:59.740000
 one-three in this case.

0:16:59.740000 --> 0:17:03.980000
 Another command you can do, if you say,
 hey, I want to see who the root

0:17:03.980000 --> 0:17:05.520000
 bridges are for all my VLANs.

0:17:05.520000 --> 0:17:07.200000
 Right now I'm just looking
 at VLAN one.

0:17:07.200000 --> 0:17:10.140000
 Well, if I've got like 20 VLANs, you
 tell me I have to do this command

0:17:10.140000 --> 0:17:13.660000
 20 times, show spanning-tree VLAN one,
 show spanning-tree VLAN two, VLAN

0:17:13.660000 --> 0:17:15.840000
 three. No, you don't.

0:17:15.840000 --> 0:17:20.300000
 You can type show,
 spanning-tree root.

0:17:20.300000 --> 0:17:24.040000
 And it'll show you all the VLANs you
 know about and who the root bridge

0:17:24.040000 --> 0:17:28.880000
 is. Now, how do you tell if
 you are the root bridge?

0:17:28.880000 --> 0:17:31.280000
 Well, let's go back here to switch
 one here for a second.

0:17:31.280000 --> 0:17:36.180000
 Show spanning-tree root.

0:17:36.180000 --> 0:17:38.320000
 All right, so notice this.

0:17:38.320000 --> 0:17:44.240000
 This column gives you your
 cost to reach the root.

0:17:44.240000 --> 0:17:49.640000
 Anytime your cost shows as zero, that
 means you are the root bridge.

0:17:49.640000 --> 0:17:52.120000
 It doesn't cost you anything
 to get to yourself.

0:17:52.120000 --> 0:17:55.260000
 So switch one right now is the
 root bridge for VLAN one.

0:17:55.260000 --> 0:18:00.440000
 And he's also the root bridge that looks
 like for VLANs five through or

0:18:00.440000 --> 0:18:04.620000
 95 through VLAN 99.

0:18:04.620000 --> 0:18:10.520000
 So that concludes this video on the
 basic iOS commands for configuring

0:18:10.520000 --> 0:18:16.400000
 rapid spanning-tree, verifying rapid spanning
-tree, and configuring deterministic

0:18:16.400000 --> 0:18:19.740000
 root placement within
 rapid spanning-tree.