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Oftentimes, we're
going to want to have

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redundancy inside
of our networks,

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and in the world
of switching that

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might equate to having
more than just one path.

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So if PC-1 wants to
talk to PC-4, well,

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if this link goes out, by
having a redundant path already

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in place, that redundant
path can still be used.

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However, in the
world of switching,

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if we do have parallel
paths consider a broadcast,

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a broadcast, maybe an
ARP request that PC-1

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sends into the switch.

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That broadcast is
forwarded out the trunk,

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received by Switch-2,
forwarded out the trunk,

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received by Switch-3,
forwarded out the trunk,

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we see by Switch-1 again,
who then forwards it,

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and that could cause a
switching loop, a layer 2

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loop caused by a single
broadcast initiated

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by a PC on a VLAN.

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Well, because it's not desirable
to have frames that just loop

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forever on the network, we have
a protocol named Spanning Tree

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Protocol, whose job
it is to identify

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parallel paths on
the layer 2 network,

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and then wherever there
are redundant paths

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to go ahead and
logically block on ports,

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so there's only one path through
that network at any given time.

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And to perform
this quote unquote

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magic of identifying parallel
paths and blocking them,

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the Spanning Tree Protocol
sends and receives little BPDUs.

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That stands for a
bridge protocol data

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unit, which in the world
of the Spanning Tree

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is the language of love.

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It's how these switches
can communicate and talk

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to each other regarding
the Spanning Tree,

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and which ports
should be forwarding

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and which ports
should be blocked.

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And during that process,
one of the switches

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is going to be
identified as the root.

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Think of it as the King Kong
of the Spanning Tree topology,

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and it's referred to
as the Root Bridge.

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And the reason is
called a Root Bridge,

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is because when this was
implemented or developed

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many, many years
ago we had bridges,

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layer 2 switching
devices called bridges.

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Now we use switches instead
of bridges, however,

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the term is still
referred to as the Root

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Bridge for the King Kong of
the Spanning Tree topology.

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You can think of the Root
Bridge as the winner,

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and as a result of
winning, the Root Bridge

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is given the opportunity
of forwarding

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on all ports for that VLAN.

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In this case, we're
looking at the VLAN 100.

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So if Switch-1 is
the Root Bridge,

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the winner of the Spanning
Tree, who are the losers?

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Well, they're not quite
losers, but they're simply

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switches, others switches, that
didn't become the Root Bridge.

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And in our topology here, that
would be Switch-3 and Switch-2,

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which are the non-root bridges.

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And by using BPDUs,
Switch-2 and Switch-3

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can identify what is the
optimal or fastest path

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to reach the root.

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And in this topology, it would
be Switch-2's gig 1 slash 0,

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so that would be put
in a forwarding state,

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and Switch-3's gig 0
slash 1, and it would

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be put in a forwarding state.

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And also those ports
that are the fastest

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path to get to the root, those
are referred to as root ports.

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And then when it comes down to
this segment between Switch-3

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and Switch-2, if both
these ports are forwarding,

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we're going to have a loop.

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So Switch-2 and
Switch-3 are also

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going to have a little
face off and duke it out.

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They're going to be sending
BPUs back and forth to determine

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which of these two switch's
ports should be forwarding

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and which should be blocking.

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And as Switch-2 wins, it's going
to go ahead and be forwarding

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on its port, and Switch-3,
if it lost for this segment,

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we'd go ahead and block
from a Spanning Tree

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perspective on that port.

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So any frames that do come in
to gig 0 slash 2 from Switch-2

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would simply be dropped,
put in the bitbucket,

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and also if this
port is blocked,

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Switch-3 is not going to be
forwarding any data frames out

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that port up to Switch-2.

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And Spanning Tree is going
to keep that block in place

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until there's some kind of
other change in the network.

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An example would be this path
between Switch-1 and Switch-2

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fails, or the cables cut, which
would also cause that path

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to fail, and then Spanning Tree
could reconfigure the topology

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and then start
forwarding on this port

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and take advantage of
the additional links that

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are in the network
to forward traffic.

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So that's the basic
concept of Spanning Tree,

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to identify parallel paths
in the layer 2 network,

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and then block on
one or more ports

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so that we don't have the
possibility for a layer 2 loop

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to happen inside of
our switch network.

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In this Nugget, we've
identified that if we

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have parallel paths in
our layer 2 network,

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we could have a switching
loop, and to help identify

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parallel paths and to
block on one or more ports

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so we don't have a loop,
we have the protocol

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of Spanning Tree, the
Spanning Tree Protocol, that

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can dynamically identify
those parallel paths

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and block on one or more
ports to prevent a loop.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.