1
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2
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So let's jump in and begin
with Trouble Ticket 1.

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For this ticket, we're going
to be using the diagram MPLS

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Topology 1 and our core protocol
for this ticket is IS-IS.

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So now, let's take a look
at the ticket itself.

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So our ticket says
that R6 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 clearly, it should
be since that's

10
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going through our segment
routing core, so let's go ahead

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and jump in.

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As always, we will start with
verifying the ticket first.

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So let's start on router 6.

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So we'll go ahead and do our
trace to 10.5.5.5, sourcing it

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from our loopback 0 interface.

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And what we see from
the output of the trace

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is that the ticket
is in fact correct.

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We have IP reachability
to router 5 loopback.

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We can see, of
course here at hop 3

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we reached the destination.

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However, we can
also see that there

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are no labels listed anywhere
in the output of this trace.

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So the first thing
to do, why don't we

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jump over to router 1 which
is our next router in line.

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And let's see if it has a
label for router 5 loopback.

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So let's do a show IS-IS
segment routing label table.

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And what we see here is we have
prefix SIDs for routers 1, 2,

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

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So this is clearly an issue.

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So the router directly connected
to router 5 is router 3.

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Let's go look there.

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So once again, let's
do a show IS-IS segment

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routing label table.

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We see that it does not have a
prefix SID for router 5 either.

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Although if we do a show
IS-IS neighbor, of course,

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we know that router
5 is a neighbor here

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and we knew that
already of course,

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because we had
just routed to it.

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So we're learning the
route but what we're not

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learning is a prefix SID.

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And since we're not
learning a prefix SID,

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if we were to say show
IP cef for 10.5.5.5,

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we'll see that it's
being sent with no label.

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We don't know a label
for it, therefore

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it's not imposing a label here.

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All of this is
exactly as expected

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if we don't have a prefix SID.

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So let's go to router 5
then and see why it's not

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generating a prefix SID.

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So let's start with a show IS-IS
segment routing connected SID

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and we see that it's empty.

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So that's clearly not good.

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Let's do a show IS-IS
segment routing global block.

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We see that we're learning
global block information

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from routers 1, 2, 3, and 4.

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So again, we're
receiving information

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but this router doesn't have it.

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All of this information is
really pointing to the fact

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that maybe router 5 isn't even
configured for segment routing.

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So let's take a look.

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Let's do a show run and
include segment routing.

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As we can see here,
there's nothing

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in the running config on router
5 for segment routing at all.

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So this could definitely be
the cause of our problem.

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Let's go ahead and
fix this issue.

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So we'll go into
configuration mode.

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We'll turn on
segment routing MPLS.

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Now, segment routing
MPLS is enabled.

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Just as a note here, since it
wasn't configured here at all,

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we're going to have to enable
it for the routing protocol

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as well.

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So let's go ahead
and go into IS-IS

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and enable segment
routing there also.

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00:04:08,050 --> 00:04:10,840

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So we're going to router IS-IS
1 and do segment routing MPLS.

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So let's exit configuration.

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And if we repeat some of our
last verification commands,

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we see that segment
routing is now enabled

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on the output of our show run.

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If we look at the
global block again,

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we see that now we have all
of the routers including

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router 5 except, look
here under SID index.

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Under the SID
index, it has none.

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So this is a bit of an issue.

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00:04:43,870 --> 00:04:46,930
Since there's no SID
index, this looks

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like the advertisement of our
prefix SID for our loopback

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interface is also missing.

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So let's review our show run
command and instead of saying

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include this time
let's say section,

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notice this looks the same.

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There's nothing configured
under the global segment routing

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00:05:05,530 --> 00:05:09,470
MPLS which is where our
prefix SID should be.

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So we're going to have to
go put that back in as well.

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So we'll go back into
segment routing MPLS.

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We'll go into our connected
prefix SID map for the address

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family IPv4 and for
the prefix 10.5.5.5/32.

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We'll configure the absolute
value of 16,005 range 1.

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Let's exit configuration.

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And let's look at our
validation commands again.

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So first, let's see what the
show run should have looked

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like when we did section.

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You can see here that
the connected prefix SID

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map with our information
for the prefix SID

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00:05:46,730 --> 00:05:50,880
should have been in there when
we did this show run earlier.

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00:05:50,880 --> 00:05:54,560
And if we look at
the global block now,

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notice that our SID
index is now five.

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So this is actually
looking much better.

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Let's go back to looking
at the connected SID again.

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And we see that router
5 is listed here with

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the 10.5.5.5/32 SID
index 5 range 1.

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So this is looking much better.

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So one other thing
we could verify,

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we could do a show
IS-IS neighbor detail

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and what we're looking
for here is just the fact

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that adjacency
SIDs are now being

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advertised in the updates.

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So these are good indicators
that segment routing is now

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enabled on router 5.

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So let's just go
ahead and work our way

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backwards through the
troubleshooting we had done

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and follow it up
with verification.

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So let's go back to router 3.

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So let's do our
show IP cef again.

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And we see that
now, it's imposing

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the label implicit null,
which of course, is label 3.

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That's fine.

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It's doing PHP or
penultimate hop popping.

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The fact of the
matter is it is still

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doing MPLS label switching which
it was not doing a moment ago.

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If we look at the
label table again,

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we see we now have
an entry for router 5

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so this is looking good
back over to router 1.

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All we had really
looked at on router 1

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was the label table
to begin with.

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So let's just repeat that.

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And we see that
now, of course, we

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have that entry for router 5.

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00:07:24,530 --> 00:07:27,840
All of this looks like
we've solved the ticket.

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So let's go back and verify the
original ticket is now working.

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So that would take
us back to router 6.

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So back on router 6, we
simply do our trace again.

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We can tell from this
output that on hop 2 there,

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we are now doing
MPLS label switching

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using our prefix SID 16,005.

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So this solves trouble ticket 1.

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In this video, we saw
an example of a ticket

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where a router, in
this case router 5,

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did not have segment
routing enabled.

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And that of course, breaks
the entire label switch path

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when trying to get
to router 5 loopback.

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00:08:09,130 --> 00:08:12,680
But once we enabled it, we
were able to verify everything.

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And now we're doing
segment routing

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to router 5 loopback 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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