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[AUDIO LOGO]

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So let's go ahead and jump in
here with trouble ticket 1.

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For this trouble
ticket, we're going

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to be using MPLS topology 3.

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So let's go ahead
and take a look

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at what our ticket
actually says.

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It says that R3 is forwarding
traffic to R5 loopback

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interface without any labels.

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So what that basically
means is we're

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going to have to look at the
connection between router 3's

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loopback and router 5's
loopback, which it says

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we don't have any
labels for, which

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means we're going to be focusing
probably on this connection

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right here to see why labels
aren't being exchanged.

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So that said, let's jump
into the command line,

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and of course, we like to start
with verifying the ticket.

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So let's take a look at the MPLS
forwarding table on router 3.

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So looking at our show
mpls forwarding table,

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what do we see?

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We see that going to the
destination 10.5.5.5/32,

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it says that it's going
to be sent unlabeled.

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Well, that's interesting.

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Let's see if we have any
binding for that network.

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So we'll do a show mpls ldp
binding for 10.5.5.5/32.

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What we actually see is we are
getting labels for it, but only

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from routers 1, 2, and 4.

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Now those labels aren't going
to be used because they're not

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on the IGP forwarding table.

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In other words, our
outgoing interface

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is towards router 5, not
towards routers 1, 2, or 4.

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So even though we have
labels from our other peers,

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not having a label from
router 5 means it's not

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going to be a usable label.

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In fact, if we do a show
ip cef, since the ticket

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was originally that we're
not imposing a label.

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If we do a show ip
cef for that prefix,

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what we see is that we have a
remote adjacency to that prefix

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out our gigabit
0/0/0/0 interface.

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And here is, of course,
the important part,

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labels imposed none.

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So we are not putting a label
on going towards router 5.

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So our trouble ticket
was definitely correct.

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We would be forwarding
traffic to router 5's loopback

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as standard
unlabeled IP traffic.

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So now let's see if we
can try to figure out why.

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Well, maybe router 3 here
isn't even running MPLS or LDP

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on the appropriate interfaces.

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So let's do a show
mpls interface.

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We see here that
we are, in fact,

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running MPLS with LDAP on our
gigabit 0/0/0/0 interface.

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So let's see if we
have any neighbors.

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Let's do a show mpls
ldp neighbors brief.

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We have neighborships
with router 1, router 2,

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and router 4.

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But again, we see
router 5 is actually

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not one of our neighbors here.

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So I think we're starting to
get to where the problem may be.

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We don't have an LDAP
neighborship with router 5.

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But all of that
said, router 3 here

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doesn't seem to be the issue.

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Router 3 has MPLS enabled
on its interfaces,

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LDPs enabled on the interfaces.

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So I think it's probably
time to jump over and take

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a look at router 5.

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I have a suspicion that
that's where the problem is.

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So hopping over to router 5,
let's do some testing here.

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Let's start with our show
mpls forwarding table.

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So we have a lot
of entries here.

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However, we notice that
all of these entries

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have something in common.

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They all say no label.

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There's no outgoing label.

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So if we do a show
mpls ldp binding,

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it says TIB not enabled, which
is the tag information base.

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Just for a small
piece of history here,

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Cisco when first implementing
MPLS called this tag switching.

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So we even had a protocol
called TDP or Tag Distribution

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Protocol.

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Of course, it got replaced by
the standard MPLS with LDP.

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However, some of the
outputs, as you can see,

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this one, for example,
still has a tendency

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to refer to them sometimes
as tags rather than labels.

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And of course doesn't
affect anything as far

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as functionality.

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But sometimes you'll
see an output like this

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and be like TIB, what's the T?

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It's tag if you ever
happen to run into this.

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But at the end of the day,
what that actually means

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and the fact that we have
no labels for anything,

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really looks like
MPLS isn't running

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at all here on router 5.

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Let's try a show mpls interface.

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So that's a spectacular output.

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We have no interfaces
running MPLS.

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So clearly, this is
going to be a problem.

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So the way we need to
fix it is to enable MPLS

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on our interfaces.

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But which interfaces?

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Well, let's take a look
back at our diagram.

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And we can see
based on our diagram

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that we need to be running
MPLS on this interface, which

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is gigabit 1.3 and on
this interface, which

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is gigabit 1.4.

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So those are the two
interfaces that we're going

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to have to enable MPLS on.

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Let's jump back
over to router 5.

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And let's configure MPLS.

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So we go into configuration
mode, into interface gig 1.3,

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and all we have to do is type
MPLS IP to enable MPLS here

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on IOS XE.

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And we'll do the
same thing with 1.4.

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And look, we can see we already
have neighborships coming up.

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So it's a pretty good
indication that, that

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was at least a problem that
we have in our environment.

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Let's go ahead and verify, let's
do a show mpls interface again.

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We see that now we have
our two interfaces,

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they're running LDP, and
they are both operational.

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So now we should have
some LDP bindings.

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Obviously, there's a
lot of entries here.

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We don't have to look
at every one of them.

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But now we're getting entries
where previously this list was,

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of course, completely empty.

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And if we do our show
mpls forwarding table,

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now some of these
are still no label.

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Those would be interfaces
going to customers and such.

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However, all of the entries
for our core network

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have labels
associated with them.

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So this looks like it's
functioning properly now,

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but the ticket was for router 3.

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So we should verify this
on router 3 as well.

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So let's just verify
some of the commands

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we looked at originally.

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We'll start with our show
mpls ldp neighbor brief.

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We can see that router 5 is,
of course, now on this list.

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If we do a show
ip cef, we can see

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that we're now imposing the
implicit null label, which

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is pop or label 3.

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So that would be correct.

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If we look at our MPLS LDP
binding for that entry,

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we now see that we're receiving
the implicit null label

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or again label 3 from router 5.

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And the most important is if
we do our show mpls forwarding,

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when we look at that
entry for 10.5.5.5,

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it is now set to pop
instead of unlabeled.

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So that means that it will
be forwarding the traffic

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with any necessary VPN labels.

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So yes, it will still
pop the forwarding label,

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but any other labels
would still be intact.

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So pop is acceptable
for this situation.

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So in this video,
we saw an example

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of how our environment will
react if one of our routers

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is not properly
configured with MPLS

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on the appropriate
interfaces, or in this case,

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it didn't have it on
any of its interfaces.

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So we actually fix something
that wasn't part of the ticket,

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but was clearly a
problem because router 5

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didn't have MPLS on
either of its interfaces,

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even though technically
the ticket only specified

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that the router 3
side was an issue.

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Clearly, it was going to have
the same problem towards router

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4.

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So we fixed that along the way.

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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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