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It's often fun to
verify what we think

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is happening in a network.

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So what I'd love to do
in this Nugget with you

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is take just a moment and
take a look at A Spanning tree

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topology in our current
network for VLAN 100,

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and then cause a network failure
in one part of our network,

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and then see the results
of Spanning Tree adapting

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to that failure and choosing
an alternate path to go ahead

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and forward frames through.

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So here is our
current technology.

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We're going to focus on VLAN
100 for this discussion.

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So the switches have
all shared their BPDUs

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with each other,
and as a result have

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chosen the lucky winner, which
is Switch-1 to act as the root

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bridge.

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And the way the winner works
with BPDUs is the bridge that

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has the lowest,
like a golf score,

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the lowest bridge ID
becomes the winner

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of the election for
the root, and that's

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how that happened here
on Switch-1 as well.

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And as a result,
Switch-1 for VLAN 100

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is forwarding on all ports
associated with VLAN 100,

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including gig 0/1, 0/2,
0/0 the trunk, and gig 1

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slash 3, the other trunk.

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Switch-2 and Switch-3 both have
one root port respectively.

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That's this guy and this guy,
which are both forwarding

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frames, but over
here on this segment,

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Switch-2 is forwarding
frames on it's gig 0 slash 3.

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In this topology, we only
have a single block port,

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and that is Switch-3
on gig 0 slash 2.

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Everything else is
in a forwarding state

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with Spanning Tree.

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So what we'll do is
we'll go in, we'll

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verify that everything's
forwarding for VLAN 100

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except for that one port,
and then what we'll do

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is we'll go ahead and we'll
simulate a failure by shutting

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down one of these interfaces
between Switch-1 and Switch-2.

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Spanning Tree will
recognize that change,

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and it will move this
interface right here

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on Switch-3 out of
the blocking state,

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and move it into the
forwarding state,

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so we still have connectivity
through the network.

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Before we make any
changes, let's take a look

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at the current state.

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And to do that, we're going
to issue the command show

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space spanning dash tree
space and then add VLAN space

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100 on this Cisco switch.

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In fact, I'm going to send this
command to all three switches

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simultaneously so we can
focus on the results.

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So we'll send that command to
each of our three switches,

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and let's take a
look at the results.

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So here on Switch-1
regarding VLAN 100

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and its four interfaces that
are involved with VLAN 100,

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notice here that the
forwarding status

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on all of those ports from
a Spanning Tree perspective

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is that it is willing
to forward traffic,

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and that's partly because
this switch, Switch-1, won

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the election, and this switch is
the rot for the Spanning Tree.

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Let's take a look at Switch-2.

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Over here on Switch-2,
using the same command.

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So here on Switch-2,
we have gig 1/1,

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which is the port that's
going up to PC-4 in VLAN 100,

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and we have two trunks.

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We have gig 1/0, which is
the trunk over to Switch-1,

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and gig 0 slash 3, which is
the trunk over to Switch-3.

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And all three of those ports
are in the forwarding state,

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not being blocked.

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It's also indicating here that
the root port on Switch-2,

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that's the port that's the
fastest path to get to the root

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bridge, is gig 1 slash 0, which
based on our typology, it is.

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But somebody has
got to be blocking,

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so that's going to be Switch-3.

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Let's go take a
look at Switch-3.

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So here in Switch-3
gig 0 slash 1,

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which is the port that
connects over to Switch-1,

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it's in a forwarding state.

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It's also the root
port, the fastest path

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from Switch-3's perspective to
have access towards the root.

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We can also see
that 0 slash 2 here

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is in a current
blocking state, and it's

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because of that one port
in this topology that's

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blocking due to Spanning Tree
that's preventing a layer 2

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loop.

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So let's simulate a failure
of a link between Switch-1

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and Switch-2, and what we'll
do is we'll go to Switch-1,

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and we'll simply shut down
the gig 0/0 interface, which

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will stop this root bridge from
being able to send BPDUs over

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to Switch-2.

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It'll also remove the parallel
path, and as a result,

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the Spanning Tree topology
is going to change,

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and it's going to
move gig 0 slash 2

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on Switch-3 into a
forwarding state,

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and that's the real
focus regarding

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the change that we want to
verify in this demonstration.

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So back on the switches,
let's go over to Switch-1.

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We'll go into Configuration
mode on this Cisco switch.

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We'll go into Interface
Configuration mode

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for gig 0 slash 0, and
we'll use the command

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of shut down to go ahead
and bring that interface

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down and out of commission.

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And then based on that version
of Spanning Tree we're using

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and how it's tweaked, the actual
change from a Spanning Tree

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topology could take anywhere
from a few milliseconds

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up to many, many seconds.

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So we'll give that a moment to
settle and while it's doing so,

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we'll go over to Switch-3,
and here on Switch-3

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when we run this command again,
show Spanning Tree for VLAN

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100, we expect to
see gig 0 slash 2

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no longer to be blocking,
but rather to be forwarding.

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And with traditional
Spanning Tree, IEEE's 802.1D,

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as in David, that's the
actual Spanning Tree Protocol,

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the original flavor, and it does
take a few seconds to converge.

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And this port, gig
0 slash 2, is going

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to go out of
blocking state, it's

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going to go into something
called listening state.

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It's then going to go
into learning state,

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and then eventually
forwarding state once it knows

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the coast is clear, and there
are no loops in the layer 2

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topology.

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So what we'll do here is
we'll hit the Up arrow key,

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do a show Spanning
Tree for VLAN 100

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again, and focus on the change.

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And that is that gig 0 slash 2
used to be in blocking state,

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but it's now in forwarding state
because Spanning Tree realized

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there is no longer parallel
paths, and as a result,

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it didn't have to put any
of the ports in the topology

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into a blocking state.

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In this Nugget, we
had a chance to see

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the results of Spanning
Tree Protocol reacting

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to a topology change.

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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.

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