So now that we have covered the OSPF header,
let's move into the different types of OSPF packet,
beginning with the hello packet.
The hello packet is used by OSPF for two main
things; to establish a neighborship, and to maintain a neighborship.
Hello packets can be sent using both unicast and multicast,
depending on the network type.
The hello packet has a number of different fields so now let's review them.
First we have the network mask.
Since OSPF supports classless networking,
it is able to support any valid network mask.
This field will indicate the mask as configured on the sending device and is
used to verify that each device is in the same subnet.
Next we have the hello interval.
This indicates the amount of time that will go between sent hello packets.
This is used along with the dead interval to determine the status of a neighbor.
The dead interval gain specifies an amount of time that can go
by before a device will be considered down.
By default, the dead interval is four times the hello interval.
Next we have the options field.
This field is used to indicate the capabilities that
are supported by the sending device.
It is made up of several sub fields including the E bit,
MC bit, MP bit, EA bit, T bit,
DC bit that we'll talk about in a little bit more detail later.
We then have the priority field.
The use of the priority was discussed in a previous section and
alters the way that DR and BDR elections occur.
The next two fields are used to specify the link IP address of
the designated and backup designated router.
If the link doesn't implement a DR, then these values will be set to 0.0 .0 .0.
And finally we have the neighbor field.
This field is used to indicate the router id of any devices that have
sent out valid hello packets on the connecting link.
This field will extend based on the number of seen devices on the link.
So now let's jump back into the capture real quick and show an
example of what hello packet looks like.
So now moving back into the capture window again since we
were already looking at a hello packet, we'll just look at the same one again.
This top side is the hello header that we talked about,
and the second part is the hello packet that we just talked about here,
and as seen here,
the first field is the network mask and on this case the
network mask is set to 255.255 .255 .252,
which is also a /30.
The hello interval is set to 10 seconds as is the
dead interval is four times 10, or 40 by default.
The router priority is 1.
Designated router, this is a broadcast link,
it's an Ethernet link, though it has a DR implemented,
and as we briefly discussed earlier, Core 2 is the current DR,
Core 1 is the BDR,
and in this case there is only one neighbor on the
link from Core 1's perspective, which is Core 2.
Now if we look at a hello packet from Core 2, the reverse of this will be true.
This is a packet form Core 2, the same mask,
everything's the same,
except this last one where it shows my active neighbor
from Core 2's perspective is Core 1.
The option field and the different fields that we talked about
briefly in that are all covered here and several of these will be
covered in much more detail in a later section.
So let's move back to the slides for a second and take a look at the database descriptor packet.