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[AUDIO LOGO]

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So in this video, let's take a
closer look at the differences

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between normal
shortest path first,

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which of course, is what
OSPF and IS-IS normally use,

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and constraint-based shortest
path first, which is what we're

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using for traffic engineering.

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Specifically, we want to look at
what some of those constraints

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are, how we can change them,
how it influences the path

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selection, and
ultimately, how we

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choose the path that our
tunnel is going to use.

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So let's go ahead and
jump in and get started

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looking at those details.

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So we already know that
traffic engineering uses CSPF.

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But what does that
actually mean?

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Well, it modifies regular
shortest path first

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so that it's not just looking at
the shortest or the best path.

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It's the shortest or best path
within defined constraints.

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Now, we have already
mentioned that the use of CSPF

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does require a link-state
routing protocol.

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So of course, that's
only OSPF and IS-IS.

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And just for terminology's
sake, another name for this

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is Constraint-Based
Routing, or CBR.

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Now, the way that CBR differs
from regular link-state routing

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is, of course, in a
few different ways.

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So first, it modifies
the use of the link cost

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to actually consider a few
things that we normally

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don't consider,
the first of which

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is bandwidth availability.

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Traditionally, we just look
at the total bandwidth.

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This is actually looking
at how much of that

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bandwidth is currently
available as opposed,

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of course, to being in use.

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It also looks at
link attributes.

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We'll be covering those
more in an upcoming video.

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And of course, it
looks at the priority

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of the tunnel, zero to
seven, as we discussed

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in our previous video.

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Now, CBR is generally going to
be used on the edge of the MPLS

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network--

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again, generally on the PEs.

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And it's going to calculate
the path to another edge.

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So again, this is normally an
edge to edge type technology.

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And what it's going
to do is give us

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a series of next-hop
addresses instead

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of just a single next hop.

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This is going to be used
to create that entire path.

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Now, there are two types
of tunnels possible.

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The first and probably the
most common is dynamic.

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This is one that's going
to be calculated, again,

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by either OSPF or IS-IS--

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in other words, your IGP.

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The other one is explicit,
where you as the administrator

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manually configure
a particular path.

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This will probably
more likely be

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done if we're talking
about setting up

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a tunnel for a specific
type of traffic.

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Now, another interesting
thing to note here,

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tunnels do not do multipathing.

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Now, understand, MPLS
itself can, of course,

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do multipathing if we have an
IGP tie, which of course, we

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mentioned earlier, is
equal-cost multipathing.

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So MPLS itself can.

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The tunnel itself will not.

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So if there is
equal-cost multipathing,

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we're going to break that tie.

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The way we're going to do
it is using this criteria.

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First, we'll look at the highest
minimum bandwidth on the path.

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Basically, we're looking
for the bottleneck.

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So whatever the
smallest link is,

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we're going to choose
the best of those.

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Next, it's going to go
to lowest hop count.

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Just between us, we're
not going to say that that

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sounds an awful lot like RIP.

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We're just not
going to say that.

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But the fact of the matter
is it's quite a bit like RIP.

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But of course, that's
still a little bit better

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than the final tiebreaker,
which is just really random.

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So it's just going
to pick a path

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to use if there's no
other tiebreakers here.

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The point is it will
only use one path.

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And that's fine because
it's a single tunnel.

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Now, two key factors
here of the TE tunnel,

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this is not considered
an interface to the IGP.

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If it was, we could
have some issues.

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And by not doing
so, we prevent them.

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First one is we prevent a
recursive lookup problem,

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where all of a
sudden, the tunnel

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itself might look
like a really good way

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to get to the
tunnel destination.

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And of course, that's a
recursive lookup problem.

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So that's not going
to be allowed.

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It also prevents triggering
an SPF calculation.

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We don't want SPF to
have to reconverge

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every time a tunnel comes up.

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So these are both
really good things

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that happen because
it's not actually

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treated as an IGP interface.

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And another big thing
that changes with CSPF

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and the changes to
our routing protocols

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is we pick up this
feature called autoroute.

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Now, we're going to talk
about traffic steering

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in a little more
detail coming up.

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But for now, let's just say
that the tunnel actually

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appears in the routing table.

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So yes, it's an interface.

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It's just not an
interface in the IGP.

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It does have its own
IGP metric, however.

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And it, of course, can be
used to forward traffic.

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All of that said,
again, it is still

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not used in link-state updates.

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Again, that's going back
to the statement up here,

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where it's not actually
an interface to the IGP,

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so that we prevent
these other problems.

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But autoroute does
give us a method

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to get traffic to
go into this tunnel.

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We'll talk about that
more in an upcoming video.

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So in this video, we
looked at the things

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that had to change between
normal shortest path first

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and constraint-based shortest
path first in order for it

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to work for traffic engineering.

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We talked about how
it will actually not

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use equal-cost multipathing.

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But it will come
down to a tiebreaker,

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even if that's just
randomly choosing a path.

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And we talked about how
it doesn't use this tunnel

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as an actual IGP interface,
therefore preventing some,

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well, rather catastrophic
problems that could occur.

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Now, we've spent a
lot of time talking

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about how this tunnel is
built, how it's set up,

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the different protocols and
decisions that have to be made.

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But at the end of
the day, the tunnel

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won't do any good
unless we get traffic

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to go through the tunnel.

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And that we call
traffic steering.

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And that's what we
get to cover 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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