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[AUDIO LOGO]

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So moving on to
trouble ticket 2,

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we're going to see that this is
a completely different ticket.

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This time, we're
on MPLS topology 1.

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Well, no.

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That's actually the same.

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Our core protocol
this time is IS-IS.

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Well, no.

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That's the same too.

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But the ticket is
probably different.

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So this ticket says
that router 6 is not

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doing label switching
to 10.5.5.5,

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which is router 5's loopback.

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So no, this ticket
isn't different at all.

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This is the exact same
ticket as ticket 1.

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As I mentioned during
the introduction,

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these tickets might be the same.

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However, it's a
different problem.

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Different problems
are being introduced

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as we go through each ticket.

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So just keep that in mind.

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Yes, joking aside, this
is the exact same ticket.

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But I assure you, it's
a different problem.

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So let's go ahead and jump
into the command line.

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And as always, we'll start
by verifying the ticket.

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So that'll take us to router 6.

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So again, we'll start with
our trace yo 10.5.5.5, source

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it from our loopback
0 interface.

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So as we can see from the
output of the trace here,

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once again, the
ticket is correct.

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We are not doing label
switching or segment

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routing towards router 5.

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So let's start with
our troubleshooting.

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Let's jump over to router 1 and
take a look at its label table.

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So again, we'll do our show isis
segment-routing label table.

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And interestingly,
what we see here

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is we have prefix SIDs
for router 1, which

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is the router we're on,
router 2, and router 5.

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But what we don't have are any
prefix SIDs for routers 3 or 4.

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So let's go over to router 3
and see what's going on there.

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So we'll just do the show isis
segment-routing label table

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again.

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And we see that it has no labels
in its label table at all.

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So let's try a show
mpls label table detail.

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And what we see here is there's
no labels assigned to the SRGB

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or for adjacency labels.

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So we have no
labels at all here.

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Let's do a show run router isis.

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What we're looking
for specifically

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is that segment routing is
not enabled under the process

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or under the loopback interface.

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So that's definitely
going to be a problem.

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Let's go ahead and
start by fixing that.

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So we'll go into router isis 1.

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Let's start by going
into our interface

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loopback 0 into the address
family IPv4 Unicast.

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Let's configure the prefix
SID with the absolute value

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of 16,003.

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And then of course, we have
to enable it for the process

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as well.

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So let's go into address
family IPv4 Unicast,

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turn on segment-routing
for mpls.

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And we'll make sure that we
have our sr-prefer on there

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as well, just in case we're
running LDP, which we're not,

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but we'll make sure
that it's on there.

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Let's commit those changes,
exit out of configuration.

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And let's verify some
of those commands again.

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Let's look at the show
mpls label table detail.

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This, of course,
looks much better.

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We have labels allocated
for the SRGB now.

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And we can see we're also
getting adjacency labels

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from IS-IS.

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So this definitely
looks much better.

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Let's look at our
label table again.

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And we see here that now, we
have a label for routers 1, 2,

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3, and 5, but we still
don't have one for router 4.

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Now, the problem with not
having one from router 4

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is if we go back
to router 6 and we

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try to verify our ticket
by doing our trace again,

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what we see on this output is
when we're doing our equal cost

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multipathing from router 1
going to routers 3 and 4,

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when we send the
traffic to router 3,

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we are, in fact, doing
our segment routing

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and therefore our
MPLS label switching.

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We also see that we're using
our prefix SID from router 5,

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as we should be, the 16,005.

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But when we take the
path to router 4,

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there's no label being used.

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So it looks like we have a
problem on router 4 as well.

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Let's go take a
look at router 4.

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Let's just start with the
show mpls label table detail.

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And we see right away that
there's no labels allocated

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for segment routing.

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So my guess is we're having
the same problem here.

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So if we do our show
run router isis,

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we can see that once again, it's
not enabled under the process.

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And we don't have it
configured on our loopback.

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So again, we need
to fix that here.

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So again, into our
loopback interface,

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into our address
family IPv4 Unicast,

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we'll set up our prefix SID
to the absolute value 16,004.

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Then, we'll turn it
on under the process

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again, commit our
changes, and exit config.

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So now, let's look at our
label table detail again.

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Now, we have labels allocated
for segment routing.

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So again, we have
allocations for the SRGB

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as well as getting
adjacency SIDs.

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So if we go back over
to, say, router 1,

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and here, we verify
our label table again,

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now we can see that
we're getting labels

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for all of the devices.

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So we have our prefix SIDs
for routers 1, 2, 3, 4, and 5.

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This means that the problem
should be completely fixed.

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Let's go back over to router 6
again and verify one more time.

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So that trace didn't exactly
show the multipath thing.

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Let's try it one
more time, see if we

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can get a multipath response.

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That's much better.

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Now, we can see that we're using
our labels both towards router

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4 and towards router 3.

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Just remember that when you're
trying to test something

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like multipathing
with traceroute,

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it can be a little
finicky as far as

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how it shows the results.

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So if you don't see
it the first time,

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just try to do a trace again.

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And most likely, you'll
see the multipathing.

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But everything's working
now as it should.

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So that solves trouble ticket 2.

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In this video, we saw
another example of a ticket

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where devices had
segment routing

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disabled on both the routing
protocol and the advertisement

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of their prefix SID
on their interfaces.

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This time, it was on an
interim router rather than

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the destination, but we saw that
the result was very similar.

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Once we enabled it and turned
our prefix SIDs back on,

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then everything started
working as it should.

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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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