Now let's move on to the simplest of these packets,
the link state acknowledgement packet.
The format of the contents of the packet is the same as those
in the DBD packet types LSA header field.
This includes a list of the LSAs being acknowledged,
including the entry link state type, the link state id,
and the advertising device's router id.
Now as I noted, let's move again and take a look at this in the capture window.
Now again we showed this is the LSA update that has all this LSA information,
and then the last one is an acknowledgement packet.
So this is Core 1 talking back to Core 2,
and it's saying it doesn't show any reachability
information; it just shows this is in the same format as the
DBD packet at the end of the DBD packet.
So if we look at that DBD packet real quick.
The DBD packet,
this is that last field and part of the DBD packet which
shows this is the lookup triplet information,
and if we look at the acknowledgement packet,
it's the same format, and it's basically saying from Core 1 to Core 2,
I have received the information that you sent me
and the link state update packet, here's the triplet that I got from you,
just to make sure that you can sort of check that off your list and say,
I'm verifying that I received that information.
And this mechanism with this packet is what enables
this exchange to become reliable.
Now Core 2 is sure that the information that he sent to Core 1 was received
by Core 1 and re-verified what information was sent to them by sort of
sending that triplet information back to Core 2.
So now they've exchanged the information,
they've exchanged what they needed, they exchanged what they've requested,
and they're saying, okay, here's what I got,
just verify here's the triplet information again.
And if we broke this down and compressed all these again,
you would see that this would match that same list
that was inside that DBD packet.
In this case the way that we did that is since Core 1 doesn't know
about any of these networks and I reset this peer,
he doesn't know about any part of the network.
So that's why it gets this big dump of all these networks at once.
Normally, if you just bring up one device or you bring up an additional network,
then you would just show, you know, one of two of these entries in a packet.
So we went back down here a little bit.
See, there's an update from Core 1 to Core 2,
well from Core 1 to that network between it and Core 2,
and I'm just giving an update on this one.
I just found out about this network with this triplet
information and it's sending it over to Core 2.
So let's just perform a quick review.
The DBD or database descriptor packet is used to say to your
neighbors that I know about all this information and
reachability information for the network,
and I'm going to send you a triplet that says here's the
ones that I have that you can match,
and you can check your database and see if you have any matches.
The link state request packet is used by that receiving device to say okay,
I got your list, I don't have these,
please send me this LSAs so I get the link state
information that you know about that I don't.
The link state update packet is sent from the first guy who sent the DBD,
and it's the full contents of the LSA, all the networks that it's reachable from,
all the networks that it's advertising, and it sends that to that device saying,
okay, here's all the information; I know about that,
that you didn't have that you didn't have that entry.
He integrates all that information in his link state database,
and whenever he's done,
he'll send back an link state acknowledgement with that
triplet information again to say, okay,
I've received the information and here's just a
verification of the triplet that you sent me,
to sort of end that conversation to say,
okay, I received everything.
And that goes back and everything's reliable.
So with that,
we're going to wrap up this module and move into the next
module talking about the different communications blocks that are exchanged between devices.