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[AUDIO LOGO]

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So now that we have
IS-IS configured

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to use segment
routing and we've seen

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that it has reserved
the SRGB for IS-IS use,

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our next step would be
to bring prefix sid's

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into segment routing.

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Remember, this is
a manual process,

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so we have to bring
the prefixes that we

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want to use with segment
routing in as a manual process.

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So what we're going
to do is we're

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going to bring in the loopback
interfaces on our core devices.

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So, in this case, it's going
to be routers 1 through 5.

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So let's go ahead and
bring up our topology

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just so we remember
how they're connected.

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And then, we'll jump
into the command line,

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and we'll bring those prefixes
for the loopback interfaces

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into IS-IS.

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While we're at it, we'll
actually fire up Wireshark.

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We'll take a look
at what those IS-IS

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updates look like on the wire.

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And, of course, we'll dig
into the IS-IS database

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so we can see this
information as well.

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So let's go ahead and take
a look at that topology,

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and then we'll get started.

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So once again, we're
on MPLS topology 1.

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And as we mentioned, we're going
to be advertising the loopback

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interfaces on each of these
routers into segment routing.

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So, in other words,
we're going to be giving

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each of these a prefix sid.

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Keep in mind that their IP
addresses are referenced up

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here.

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So each device is going to have
an address of, for example,

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10.1.1.1 for router 1.

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And we're going to set up our
labels to sort of follow along

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that same idea.

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We're going to do
16,000, and then we'll

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use the device ID
for the last digit.

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So again, as an example,
router one would be 16,001.

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So that's what we're
going to configure.

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Let's go ahead and jump in
and take a look at this.

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But before we start
our configuration,

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I'm going to set up a Wireshark
capture on this interface right

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here between router
1 and router 3

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so that we can catch
the IS-IS update when

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it advertises the loopback
sid over to router 3.

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So we have our Wireshark
capture running.

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Let's go ahead and
configure router 1 here

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to send a prefix sid for
its loopback interface.

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So to do this, we'll
go into router IS-IS 1,

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into our interface loopback 0,
into the address family IPv4

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unicast.

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And then we say prefix sid.

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And if we hit the
question mark here,

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we can see where we have this
choice to do absolute or index.

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Those are the two main
things we need to focus on.

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So we could do this as
an index off of the SRGB,

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or we could just set this
to an absolute value.

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Now since we sort of
specified the number

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we're going to use, let's
go ahead and do absolute.

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So we'll say absolute,
and then, of course,

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we're simply going
to put in the number.

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Notice that it's telling us that
it has to start with 16,000.

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So we'll do our 16001
commit those changes and end

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configuration.

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So now that we've
configured this

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let's do some verification.

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

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What we see here
is that we now have

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a label of 16,001 for the
local loopback interface.

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Now that's fine that
router 1 knows this,

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but is it showing up
on the other routers.

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So let's go take a
look at router 3.

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Once again, we'll
do the show ISS

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segment routing label table.

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And we see that router 3 has
also learned the label 16,001

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for router one's
loopback 10.1.1.1/32.

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If we do a show MPLS
forwarding table,

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we can actually see that we're
using the same local label

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16,001 for this
segment routing prefix.

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And notice it's even
recognizing the fact

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that that is an
index of 1 based off

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the start point for the sRGB.

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Now again, we should see
this on all of the routers

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at this point.

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We'll just verify one more.

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Let's take a look at router 2.

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So same command
here on router 2.

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And we see that it is also
aware of this label 16,001.

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Let's verify on
router 5 as well,

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just because it is a
slightly different command.

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On IOS XE, we'll do a show IS-IS
segment routing connected sid.

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But we see that it is, in fact,
learning the 10.1.1.1/32 from

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the host XR 1,
which is router 1.

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And we see that here rather
than listing the exact value,

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it's actually just listing
the sid index being one.

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So, of course, we set up
that Wireshark packet capture

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for a reason.

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So now let's jump
into Wireshark,

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do a little bit of filtering,
find that update that IS-IS

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sent out to these other
routers and take a look

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at where this is carried
in the IS-IS update.

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So here we have the whole
packet capture that we did.

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Let's filter this by IS-IS.LSP.

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So if we take a look at this
update coming from router 1.

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And if we open the IS-IS
link state PDU header

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and then we open up the
IP interface addresses,

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we see that this is
for the 10.1.1.1.

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So then, if we look at the
extended IP reachability,

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open the loopback
interface address,

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notice that here we have a
sub-TLV for the prefix sid.

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But before we open that, also
notice that under sub-TLV,

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we also have the
attribute flags.

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One of those flags
is the n indicating

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that this is also a node sid.

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But let's go ahead and
open up that prefix sid.

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And right here, we
can see where it's

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exchanging the fact that the sid
label index is, in fact, one.

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So we can see that it's
sending this information

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to its neighbors, and
this is how the labels are

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being advertised through IS-IS.

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So now, let's jump back
over to the command line

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and do some more verification.

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Since we haven't really
done it yet, let's go ahead

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and look at the adjacency
sids while we're

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taking a look at these details.

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So let's do a show
IS-IS adjacency.

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And we'll add detail.

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Notice that on each of
these neighborships,

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to router 4 to
router 3 to router 6,

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even we have an adjacency sid
listed for each one of these.

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So we can see that that's also
being exchanged through IS-IS.

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This command is a little
bit different on IOS XE,

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so let's take a look
on router 5 as well.

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Here we'll do a show
IS-IS neighbors detail.

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But notice for
each one of these.

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it's also listing the
adjacency sid values.

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So now that we've done a
little bit of investigation

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let's go ahead and configure
the rest of our routers

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to send their prefix sid for
their loopback into IS-IS.

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So let's go over to router 2.

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So we'll simply go
into configuration mode

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in the router IS-IS 1, into
our interface loopback 0,

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into our address family.

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And we'll set our prefix sid--

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this time, let's do an index.

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And let's set the index to two.

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We'll commit the changes
and exit configuration.

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This way, we can see how the
index works very similar to how

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the absolute value works.

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Let's actually go to
router 1 real quick

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and check to make sure that
it's getting router two's label

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or prefix sid correctly.

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So on router 1, we'll just
look at the show IS-IS

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segment routing label table.

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Notice how even though
we configured it

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with an index rather
than an absolute value,

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that it's still getting
the label 16,002.

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Because again, we can
put this in either way

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there's not any difference
for how it actually functions.

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So now, let's configure
routers 3 and 4.

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Again this is going
to be almost identical

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except for the
prefix sid itself.

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So I'll just copy and paste
this configuration in.

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So there's router 3.

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And again, we'll just use
absolute values on these.

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We've already seen
how index works.

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And router 4.

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So now router 5,
of course, IOS XE

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is slightly different
how we configure it.

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On IOS XE, we actually
configure this globally.

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So what we do is we go into
segment routing for MPLS

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from global configuration.

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And then, we want to modify the
connected prefix to SSID map.

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Then we go into
our address family.

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Then we Enter the subnet, so
in this case, 10.5.5.5/32.

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And then, we can choose
absolute or index.

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Again we'll just do absolute.

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The value of 16,005.

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And we can actually tell it
a range to use if we want.

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We don't need to do that, so
we'll just exit configuration.

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Then let's take a look at our
segment routing prefix map.

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Notice that now all of the
devices have prefix sid.

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If we do a show MPLS forwarding
table, here at the bottom,

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notice we have all of
those prefixes listed here

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in our table.

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Of course, not the local one
but for routers 1, 2, 3, and 4.

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Let's jump back over to router
1 and do some more verification.

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If we do our show IS-IS
segment routing label table,

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we can see that this router now
has all of the entries for all

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of the other routers as well.

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Let's do a show IS-IS database
verbose for this router XR: 1.

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And notice we can
see right here it's

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loopback interface
being advertised

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with a prefix sid index of 1.

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So this is what it looks
like in the IS-IS database.

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If we do a show MPLS
forwarding table.

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So we see here that it has all
of the Segment Routing prefix

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SSIDs.

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However, if we look at what
it's going to use for 10.5.5.5,

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we see that it's
still going to be

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using either 24,009 or 24,006.

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So it looks like
it's not actually

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using the prefix SSIDs.

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Let's verify this.

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We'll go back and do our
trace from router 6 again.

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So what do we see on this trace?

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Well, again, what we're
really focusing on

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is hop 2, where router 1 is
forwarding the traffic using

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ECMP, but it's sending the
labels 24,006 and 24,009.

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Which indicates it's
not using our sr. prefix

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sid even though we
have them configured,

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and we have verified
that they're being

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learned on all the devices.

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In this video, we saw how
to configure our prefix

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sids both on IOS XR
and iOS XE for IS-IS.

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Now interestingly,
even though we

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verified that these
are on all the devices,

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we saw here at the end
that our routers are still

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not using those labels.

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These are using
the LDP labels that

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have been here since we
started this entire section.

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Now, this is
obviously a problem.

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However, we sort of already
know what the problem is.

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The problem is
that sr. by default

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will not be preferred over LDP.

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So what's happening
here is these devices

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are preferring the LDP
label over the prefix sid.

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Now clearly, since we want to
see segment routing working,

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this is not going to
be desired behavior,

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and we're going to
want to change this.

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That's exactly what
we're going to do next.

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But until then, 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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