Database descriptor, or DBD packets, are exchanged between devices when they
are establishing a neighborship during the exchange state.
They're used to relay a list of the contents of a device's link state database.
They do not actually contain any link state information itself. The first
field indicates the configured Interface MTU of the source.
Conflicting interface MTUs on common links can cause
odd OSPF neighborship behavior.
Next,
we have the Options field. This field has the same purpose in this packet
type as with the hello packet type. The Init, or I field, is used to
indicate the first database descriptor packet as multiple can be sent when
the database is large. And the More, or M field, is used to indicate whether
there are more DBD packets coming.
The Master Slave, or MS field, is used to indicate whether
the sender has been elected as the master.
Next, we have the Sequence Number field.
This is used to sequence the packets that are sent
between communicating devices.
The initial number will be unique and sent by the master.
This number will then be incremented as the database list
is exchanged. And the final field includes the headers of
the known LSAs of a device.
Enough information is exchanged that the receiving devices can
perform a comparison with their known LSAs,
then request any missing information.
Some of the common information includes the entry link state type,
link state ID, and the advertising device router ID.
So with this covered,
let's move back into the capture window and see
what a real DBD packet looks like.
And in this capture, I don't have any DBD packets,
so I'm going to go over to the lab real quick and I'm going to go on to core 1,
and I'm going to clear this OSPF process, and you're
going to see here it's going to force the exchange of
packets between the devices again.
Get it all good here.
And then we'll take a look at the specifics. Let's stop this capture over here.
Okay,
the database descriptor packet is used to send a list of the known information.
So if we look here a little bit closer.
As I said, in the database descriptor packet,
these bits that were set. So this is a message that comes
from core 1, and in this case he's been set as the maser.
There are more packets to come, and this is the first packet.
So if we look a little bit further down here,
same message was sent twice. It could be a larger one coming
up here. Now from core two again. Okay,
so this is from core 2. Actually, this initial DBD,
was it establishing a neighborship?
So we lost master out to core 2 here immediately after he sent that first thing,
and then whenever he sent the second packet he's no longer the master, plus
he doesn't have any routing information because he just reset all the peers,
so he doesn't have any network information.
So, he's going to get this whole list of information from core 2.
So this is coming from core 2, I don't have any additional information,
and this is not the first packet I'm sending. Core 2 is saying I'm the
master now and this is all the information.
This is just the header information for the link state.
So this is saying I had said previously in the slide that the
information that it really used as part of its index is the length state
type, the link state ID in the advertising router.
Now,
what goes in these two fields, the link state ID and the
advertising router field differ depending on what type of link
state advertisement is being sent.
And those specifics will be covered in a later module where we go over
specifically what this information is for and how it differs depending on
the different situation and how the connections work.
So this is a link state 1 it's sending as part of that last
LSA header field, and then this is a completely different type
of links state information pack.
And in this case, this looks a little bit different because this
doesn't look like a router ID, this looks actually like a
network number or a network IP address.
So these three pieces of information, this little triplet It is what is used by
these devices to determine if they have this entry in their link state database
and what they will use on their side to say okay,
I need the full LSA information for this traffic. And
how they do that will be covered next. But first, let's jump back to the slides and talk about the link state packets.