WEBVTT

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 Okay, so let's review RSTP functionality
 and start looking into the details

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 of how rapid spanning to
 creates a loop-free tree.

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 How does it work?

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 All right, so the first step is that
 any tree that's created by Span Tree

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 has to start with the election
 of a root bridge.

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 Only one switch for the tree can be
 elected to this very special role.

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 Once the root bridge is elected, all
 of its ports become what's called

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 designated ports.

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 On all the other switches that were not
 the root bridge, each one of them

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 has to elect a root port.

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 Then they have to figure out designated
 ports between themselves and then

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 we end up blocking non
-designated ports.

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 Now there's a lot to that which we're
 going to dig into a little bit here.

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 So let's start with the
 root bridge election.

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 This is step one.

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 This is what has to happen.

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 So in Span Tree and all the flavors of
 Span Tree but focusing in on rapid

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 spanning tree, a switch will start sending
 out a special Ethernet frame

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 which we call a bridge protocol
 data unit, a BPDU.

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 This is the fundamental data structure
 that Span Tree uses to discover

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 other switches and discover who's going
 to be the root bridge and what

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 the status of all the interfaces are going
 to be by transmitting and receiving

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 BPDUs. All right.

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 So inside that BPDU, every switch is
 going to advertise itself with a

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 unique identifier.

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 We call it a bridge ID,
 a bridge identifier.

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 So every switch comes up with a unique
 bridge ID and the switch with the

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 numerically lowest bridge ID ends
 up becoming the root bridge.

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 Now what does that bridge
 ID actually composed of?

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 It's composed of two pieces of information,
 a bridge priority which is

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 a 16-bit field so that can go from
 1 to 61, 440 in increments of 4096

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 and a system ID extension.

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 That's actually all part of the priority
 field as well as a MAC address.

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 Now in the original days, just as a
 quick review here, if this was your

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 BPDU and this portion right here was
 your ID, the way it was structured

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 was that part of it
 would be your MAC.

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 So a switch or a bridge back in those
 days would dynamically determine

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 that one of its MAC addresses, usually
 the lowest one it had, would be

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 the MAC address of its bridge ID and
 then this portion right here would

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 be your priority field.

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 Now with what's called extended system
 IDs, if we take our priority field

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 and we expand it a little bit, only
 the first four bits of the priority

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 field are actually used
 for the priority.

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 So if you're modifying the bridge priority
 in a spanning tree command,

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 only these four bits are the bits
 that you can actually change.

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 The rest of the bits after that are
 what we call, well what the slide

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 called, a system dash ID extension
 in reality, that's your VLAN.

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 So for example, if this BPDU is reflecting
 the tree for VLAN 7, then all

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 these bits here are going to be zeroed
 out except 111 at the end.

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 We're going to encode the number 7 in
 there and then these bits will be

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 whatever the priority is
 that was configured.

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 Now when I give this BPDU to you, you're
 going to interpret this whole

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 thing, which is this portion right
 here, as my priority value.

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 So let's dig into that
 a little bit more.

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 So there it is, two bit, two byte
 priority, six byte MAC address.

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 You should also remember what the default
 value is for spanning tree bridge

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 priority. That default
 value being 3278.

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 So that is the default
 bridge priority.

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 So if I'm sending you a BPDU and this
 BPDU represents VLAN 1, you will

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 actually see my BPDU is VLAN 1 because
 remember it's 4 bits of priority

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 plus a whole bunch
 of bits of VLAN.

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 If I'm sending you a BPDU for VLAN
 10, you'll see my priority.

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 If I don't change it, if I'm leaving
 the default as 32778, 768 plus VLAN

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 10. Now, if you do modify this value
 via configuration, recall like the

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 last slide said, this has to be an
 increments or multiples of 4096.

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 Now you could knock this
 all the way down to zero.

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 Remember, in spanning
 tree, lower is better.

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 So you could give yourself
 a bridge priority of zero.

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 After that would be 4096.

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 After that would be 8192
 and so on and so forth.

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 You'll see that 3278 is actually an
 increment or a multiple of 4096.

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 All right, real quickly, let's just review
 the port roles and port states

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 that rapid spanning tree has.

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 So a port role is what is the job or
 purpose of this port as it relates

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 to the tree. Is it's is it's job
 to transmit BPDUs into the tree?

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 Is it's job to be listening and receiving
 BPDUs from other switches upstream

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 in the tree? What's its purpose?

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 Is it's job just a block and make
 sure there's no loops here?

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 And the port state is what is this
 what is this port's purpose as far

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 as data is concerned?

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 Is it allowed to forward data?

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 Should it block data?

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 What should it do as far as your
 actual user data is concerned?

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 First one we want to talk about
 is the designated port.

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 Now, once the root bridge is elected,
 once again, that was the bridge

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 with the lowest bridge ID.

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 So all the switches first
 look at their priorities.

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 If everybody if somebody has a lower
 priority than everybody else, we're

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 done. They win lowest
 priority wins.

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 But if you haven't changed that, everybody's
 priority will be the same.

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 So then I'll be the switch with the
 lowest MAC address that will win.

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 So there will be some winner,
 which is the root bridge.

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 Once the root bridge is elected, now
 he will place all of his ports in

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 that B land as designated ports.

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 And those the designated ports
 are in the forwarding state.

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 They are wide open for receiving
 and transmitting traffic.

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 Now other switches downstream who did
 not win, who are not the root bridge,

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 they have to figure out what interface
 they have that goes back to the

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 root bridge that's the fastest
 path back to the root bridge.

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 And those here the designated
 ports, what do they do?

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 What are they responsible for?

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 Transmitting BPDUs.

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 A designated port always
 has to transmit a BPDU.

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 Now downstream switch, he will
 elect a root port, only one.

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 So if I'm a switch, I might be, you know,
 the root bridge might be three,

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 four, five hops away from me and I might
 be receiving BPDUs from him on

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 like 25 different interfaces.

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 I'm going to elect one of those
 interfaces as my root port.

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 The root port that will get me back
 to that root bridge, the fastest.

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 And we'll talk about in just a moment,
 well, what if there's a tie?

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 We'll talk about what
 the tie breakers are.

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 But a root port, as you can
 see, is receiving BPDUs.

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 I sometimes say root
 port receives, right?

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 R-R, root receives.

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 So root port receives BPDUs.

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 Now, if he has a port that's leading
 further downstream, that will become

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 a designated port and he will be forwarding
 those BPDUs on further downstream.

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 So bridge C here, on bridge C, his connection
 leading back to bridge B,

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 that will be his root port
 that's receiving BPDUs.

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 Now, rapid spanning tree also has the
 concept of something called an edge

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 port. An edge port is a port that's
 typically leading to a host, like

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 a laptop, a PC, a server, even a router,
 something where the broadcast

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 domain ends at that host.

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 So an edge port is a port where you
 know, as a network administrator,

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 that if a broadcast goes out that port,
 it's going to stop on whatever's

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 connected to that port.

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 There's no way that broadcast is circled
 back around and hit you again

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 somewhere else in the network.

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 So once you're confident that a port on
 your switch leads to an edge device,

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 you can configure it with
 the port fast feature.

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 And a port configured with a port fast
 feature is seen by rapid spanning

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 tree as an edge port.

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 Now, that is also a designated
 port, like we see right here.

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 The remaining ports that are not designated
 ports and not root ports become

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 blocking. What we used to call blocking
 now in rapid spanning tree, the

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 terminology has changed, and we call
 it discarding means exactly the same

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 thing. We are discarding
 user data.

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 So those will be either alternate
 ports or backup ports.

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 So that completes this quick review
 and refresher of the rapid spanning

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 tree. In the next video, we're going
 to look at what are some essential

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 iOS commands just to get this running
 and maybe deterministically set

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 who the root bridge is
 actually going to be.