[MUSIC PLAYING]
So let's start off with a nice introduction
to some of the attributes of Label Distribution
Protocol, or LDP.
Now, as we've discussed, LDP is used
to exchange labels between our LSRs, the Label Switch Routers.
The reason, of course, we need to do this,
is that the labels generated on a router
are not used by that router.
Those labels are used by the neighbors
to send traffic to this LSR.
Therefore, we need a reliable method
to exchange those labels.
That's, of course, where LSR comes in.
So let's go ahead and jump in, and get
started by looking at some of its attributes.
So keeping in mind that those MPLS labels are locally
significant to the router, we have
to get the labels to our neighbors.
So first, keep in mind that a label, by default, is created
for every IP destination.
And again, for this, it's going to use the RIB, the Routing
Information Base, or the routing table.
Neighbors actually use these labels
to forward the traffic to their neighbors.
So in order to do that, these labels
are going to have to be known specifically by the directly
connected neighbors.
They don't have to go past that.
But we have to know all of the labels that are being generated
by our neighbors.
Because when we forward traffic to that neighbor going
to a specific destination, we need
to use its designated label.
This is, of course, where LDP comes into play.
It's used to exchange these labels,
specifically that we're using for IP forwarding.
We've discussed previously that there are different protocols
used for other applications, such as VPN and traffic
engineering.
But for IP forwarding, we use LDP.
Now, here's the interesting thing.
Routing protocols could have been used instead.
Why do I say that's such an interesting concept?
Well, think about this for a moment.
Our routing protocols already run between directly connected
neighbors.
So LDP is really just adding another protocol into the mix.
To a certain extent, it's one more thing
to break on our network, right?
But the reason that it was done, at least originally,
is it's a lot easier to make a new protocol to do a new task
than it is to modify all of these existing routing
protocols, get everybody's code up
to date on all of the updates, and all that kind of thing.
However, time has passed since all of this was done.
Now, it's obviously not our current topic.
But in this course, we will be covering segment routing.
And in segment routing, we, in fact,
do use the routing protocols to exchange the labels.
So just so we're clear, it can, of course, be done.
I just mentioned that.
But we're going to focus on LDP here first.
And I guess we could call it traditional MPLS forwarding.
And again, I just summarized this whole last point here,
that, yeah, a separate protocol was chosen in order to minimize
the changes needed to the IGPs.
And again, it wasn't necessarily that we didn't want to do it.
Because again, ultimately, we sort of did.
It's just, it's a lot quicker to create a new protocol
to do something when you're trying
to create a new technology.
Now, LDP is a standard.
It's referenced in RFC 3036.
This actually replaces Cisco's proprietary Tag Distribution
Protocol, or TDP.
Just as a quick note here.
Some devices do actually still support TDP.
It still works.
It still functions.
You could still use it.
They are not, however, compatible.
They operate on different port numbers, and so on.
They're not compatible with each other,
but they do an equivalent job.
Needless to say though, we should
be using the standard today.
Now, what's interesting about LDP
is it uses both UDP and TCP.
We'll see this in our traffic captures later.
Although, it does use the same port on both protocols.
And as we've mentioned, this is generally
used on directly connected neighbors.
In which case, it uses multicast.
However, it can be targeted to go to routed neighbors.
We can use this for a variety of reasons.
And we'll talk about some of those
as we move through this skill.
In this video, we just got an introduction to LDP,
or Label Distribution Protocol, which
is what we use to exchange labels between our routers
that are going to be used for IP forwarding.
Now, this is generally used, again,
directly between neighbors.
However, it can be targeted to get to a routed neighbor
if we need to.
Now we have to discuss neighborships.
And that's exactly what we get to do next.
Until then, I hope this has been informative for you,
and I'd like to thank you for viewing.