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[AUDIO LOGO]

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So now it's time to jump in
and take a closer look at RSVP,

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or Resource
Reservation Protocol.

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Now, as we mentioned in
one of the previous videos,

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this is not a new protocol
for MPLS traffic engineering.

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We're actually taking this
from the IntServ model of QoS.

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Now, this is not a model
that's commonly implemented.

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And the reason for that
is simply scalability.

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We have the DiffServ
model, which we're probably

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very familiar with.

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That scales much better, where
we're prioritizing and doing

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QoS to our traffic
based on classes

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and what type of traffic
it is, where the IntServ

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model actually set up a
dedicated or reserved path

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for the traffic
through the network.

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So I'm sure you can
imagine that setting aside

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a certain amount of bandwidth
per each flow, again,

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doesn't scale real well.

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But that's exactly what we
want to do for our tunnels.

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And that's why we borrow or take
RSVP from the QoS IntServ model

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and use it here in
traffic engineering.

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And we'll see, as we go
through this discussion,

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how we use its signaling and,
really, its main function,

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which is to do
path reservations,

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to make these tunnels possible.

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So let's go ahead and
jump in and take a look

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at how MPLS traffic
engineering uses RSVP.

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So the job of RSVP in
traffic engineering

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is once the path has
been selected, again,

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that's up to the IGP.

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That's up to the
routing protocol.

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But then we use RSVP
from the head end

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to signal all of the
intermediate routers

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what the intended
path is and what

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the requirements are for the
reservation for this tunnel.

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As long as all of the
intermediate routers agree,

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RSVP then reserves
that bandwidth

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for the specific tunnel to
prevent oversubscription.

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This prevents us from
setting up more tunnels

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than we actually have bandwidth
to make sure that we're

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fulfilling the requirement.

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Now, of course, it would
be possible, particularly

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in the event of a failure, that
we end up with more tunnels

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than we have bandwidth
for, in which case,

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we do need to have a way to deal
with that because, of course,

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in most environments, some
tunnels are going to be more

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important than others.

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It could depend on
what type of traffic

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it is, that sort of thing.

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So we do have a
way to handle that.

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The way we handle that is
by using tunnel priority.

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Tunnel priorities allow one
tunnel to preempt a lower

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priority tunnel.

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Some of the guidelines here--

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the priorities are a
range of zero to seven.

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And the lowest
value wins, meaning

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a two priority would be
able to preempt a four

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priority, for example.

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And when we say preempt,
just so we're clear,

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yes, this does mean that
if the other tunnel is,

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in fact, already up, we
will take that tunnel down

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for the higher priority tunnel.

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It's not just a matter
of preventing it

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from being established.

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But again, if a failure
occurs and we're suddenly

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short of bandwidth, it
will actually take down

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the lower priority tunnels.

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Now, the signaling
used for this--

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it's going to use
a path message.

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That's what's going to
be sent from the head end

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to the tail end.

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And this carries the explicit
path of the label switch

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routers that we're going
to be going through.

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The message itself will
follow that explicit path

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that we intend to use.

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This carries in it
the label request.

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We're requesting a
label from the tail end

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to know which label
to use for this LSP.

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Each intermediate router-- so
every router this has to pass

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through--

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is going to perform
admission control.

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This is where those
routers get to say, yes,

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I have enough bandwidth.

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Let's build this tunnel.

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Or no, I don't have enough
bandwidth available.

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We can't do this.

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That's the admission control
happening along the way.

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As long as the path message
gets all the way to the tail,

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we are then going to
send a reserve message

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from the tail back to the head.

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So this is, of
course, going to be

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sent along the reverse
path from the path message.

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This actually carries
the requested label

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that was generated by
the tail end router.

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And each intermediate
router then actually sets up

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the reservation.

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So far, it only agreed.

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Now it actually reserves
it and creates a label.

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Once this reserve message
gets back up to the head end,

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the tunnel is now up.

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That's what
constitutes the tunnel

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being up and ready to be used.

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So these are the main
communications that

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are going to happen in RSVP.

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Now, there are some other
messages that do get used.

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You want to be aware of them.

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But we don't necessarily have
to know the detail on them.

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So we have the path
teardown message.

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We have a reservation error
message if there's not

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enough reservation on the path.

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We have an actual
path error message

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if something goes
wrong with the path

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itself, A reservation
confirmation

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message, and a reservation
teardown message.

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So like I said, there are
several other message types

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that are used here.

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Just be aware that RSVP
is using these messages

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to do tunnel management.

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So in this video, we
took a closer look

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at RSVP, that protocol that
we borrowed from the IntServ

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model of QoS, and
how we're using

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it pretty much for
the same purpose

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that it was used in the
IntServ model, which

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is to reserve bandwidth for
a specific traffic flow.

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Just in this case, that
traffic flow is our tunnel.

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And of course, we
added the functionality

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of exchanging the
label that we're

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going to use into RSVP as well.

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Now, we also know that there
have been major changes

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to the routing protocols
as well, specifically

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the use of CSPF.

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And we need to take a
closer look at that as well.

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That's exactly what
we're going to do 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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