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[MUSIC PLAYING]

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So now that we have a better
understanding of the MPLS

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header, how it carries
the label, the EXP

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bits, the bottom
of Stack Indicator,

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the TTL, and how that's a shim
header writing between layers 2

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and 3, we can start
to think about this

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as a way of tagging traffic,
much in the same way

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that we can tag routes.

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Now, if you think about it.

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We tagged routes for
various different reasons.

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The most common, of course,
is to then filter on that tag.

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But we can also
use it to indicate

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where that route came from.

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So much like tagging
routes is useful,

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tagging traffic can
be useful as well.

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And not just for IP forwarding.

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That's why these other
applications also require MPLS.

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But here's the thing.

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You have one label
that we're going

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to be using for
actually forwarding

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the traffic through our core.

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Then we have to have additional
labels to give that information

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to these other applications.

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So that's what we're going to
be focusing on in this video.

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When one label is not enough,
we then have a label stack.

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So let's jump in and take
a look at how this works,

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and what applications
actually need to use it.

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So as we mentioned, the
primary purpose of running MPLS

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to begin with, is to support
various different applications.

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Some of those applications
we have listed here.

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One of them, of course, being
IP and IPv6 unicast routing.

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This is going to
require a single label.

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We usually call it
the forwarding label.

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And this label is
going to be exchanged

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using LDP, which we'll be
taking a much deeper look

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at in a future skill.

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Also, we have IP and IPv6
multicast forwarding.

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This will also be using
just a single label.

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So the regular forwarding,
in and of itself,

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is a single label.

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But when we start adding
on other applications,

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such as traffic
engineering, this

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is going to require
an additional label.

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Differentiated QoS.

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Again, we'll talk more about
this in the future as well.

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But this also requires a label.

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Layer 2 and Layer 3 VPN.

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Both of these require
an additional label.

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So when we add on
these services,

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we're having a minimum
of two labels--

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possibly three, if
we start combining

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these different
services together.

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Now, the reason
this is important.

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There's really no limit on the
number of additional labels

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we can have.

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However, we do
have to have a way

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to deal with these
additional labels.

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Now, right off
the bat, we simply

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call this a label
stack because there's,

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of course, more than one label.

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And this is what we
had just mentioned,

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that there's not really a limit
on how many labels you can use.

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Now, like I said, in most
of today's applications,

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we're going to be looking
at two, maybe three.

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But there's
technically no limit.

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We're hooking these together
like a train of labels.

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So really, there's no limit.

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The real limit would
be this next one,

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which is we do have to
ensure that the MTU allows

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for those additional labels.

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Keep in mind that if we have
a standard Ethernet interface,

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the MTU on that interface
is generally 1500 bytes.

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And if a device has sent
a packet at the maximum

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MTU size--

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meaning, the frame is
actually 1500 bytes--

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and then we insert
this label, which,

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again, is a shim header, that's
32 bits or 4 additional bytes,

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we have to add 4 bytes
to this for every label

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that we put on the stack.

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Next thing you know,
we're at 1508 just

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to support two labels.

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So we need to make
sure that we increase

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the MTU for however many labels
we need in our label stack.

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Just as a side note.

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On a lot of
environments today, we

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turn on what's called jumbo
frames on Ethernet, which

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is in excess of 9000 bytes.

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In which case, this really
shouldn't be an issue.

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However, if you are still
running standard Ethernet

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frames at 1500 bytes,
we do have to make sure

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that we adjust this MTU.

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Now, since there's no
set number of labels,

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that means we have to
have some way of knowing

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where the label stack stops,
and the next header begins.

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So just as an example.

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If I had my original Layer 2
header out here in the front,

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and then I start stacking
labels onto this.

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How are the devices
supposed to know

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where those labels stop, and the
original layer header begins?

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There's nothing to indicate
how many labels are actually

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in here.

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So what we do is we use this
bottom-of-stack indicator

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to let everybody know
where the last label is.

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Keep in mind that this is
also referred to as the S-bit.

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But the way it works
is simply this.

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Each of these labels
would have an S-bit of 0,

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except the last one.

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This is a simple
flag bit to indicate

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which one is the last label.

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And now the receiving devices
know that the next bits are

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the original Layer 3 header.

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This is how we know where
the label stack ends.

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So while we're getting
started with MPLS,

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we will be focusing here on
IPv4 and IPv6 forwarding.

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Because again, this
is the baseline

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to get all of our other
features up and running.

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Again, this is needed to support
all those other applications.

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The way it works is, we create a
label for every IP destination,

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specifically in the RIB,
or the routing information

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base, commonly known
as the routing table.

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But each of these
represents what's

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called an F-E-C, or a FEC, which
is a Forwarding Equivalency

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Class.

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As we mentioned before, we also
support multicast forwarding.

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This is supported with PIM, or
Protocol Independent Multicast,

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which is essentially the
routing protocol for multicast.

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We have skills on this
coming up in future domains.

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In this case, the
FEC is going to be

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the multicast destination.

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In this video, we
talked about how

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we can use multiple
labels in a stack

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to support different
applications running

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on top of MPLS.

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Keep in mind that IP forwarding,
and IPv6 forwarding itself,

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of course, uses a label as well.

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And then the other
applications have

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to have additional labels to
provide the needed information

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to those applications.

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One of the applications that
needs that is MPLS VPNs.

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And we're going to get an
introduction to those coming up

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next.

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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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