So now that we have covered the different basics of
OSPF and its neighbor states, let's have a conversation about the OSPF packet.
OSPF runs over the internet protocol or IP and uses its own defined
protocol using IP protocol number 89 as specified in RFC 2328.
This standard also defines a number of different packet types that
are used by OSPF to communicate with its neighbors.
In this section we will review the different packet types that are defined,
what they are used for, and then move into the lab and show their use first hand.
Before we go into the specific types of OSPF packet,
we must first talk about the OSPF header.
The OSPF header is included in all OSPF
communications and is shown in the slide.
The version field is used to indicate the version of OSPF that is being used.
For IPv4 this is version 2.
Next we have the type field.
This field is used to indicate the type of OSPF
packet that will follow the header.
There are five different OSPF packet types: hello,
database descriptor, link state request,
link state update, and link state acknowledgement.
Each of these uses a number that indicates this
type as indicated in the callout.
The packet length field indicates the size of the OSPF protocol packet,
including the header in bytes.
Next we have the router ID field.
This field is used to indicate the router id of the source router.
Then we have the area id field.
Again, this will include the area of the source device.
Next we have a checksum.
A checksum is used to verify the integrity of the
entire content of the OSPF packet,
ensuring that it has not been altered in transit.
And finally we have two fields that are used for OSPF authentication.
The authentication type field is used to indicate the type
of authentication that is being used.
The supported options include no or null authentication,
simple password authentication, and cryptographic authentication.
The authentication field is used to hold the specific information
that will be used to authenticate the packet.
The specific contents will depend on the type of authentication used.
So now let's move into the lab briefly and show what this
header looks like in a network capture.
Okay,
so now let's move into the capture window here for a minute and just
take a little bit of a closer look at the OSPF header.
But let's look specifically at what's being transmitted,
in this case between Core 1 and Core,
well actually Core 1 and everybody on the link,
but in this case, only Core 2 is connected to this link.
So there's the version.
This is running on IPv4 version of OSPF so it's type 2.
This is a hello packet, which would be indicated.
It shows a 1 here, but if we pull up this a little bit,
we can say that it's actually 1 in the data.
Let's see here, 1, right there.
Packet length, source router.
This is the router id for Core 1,
the area is 00.00 .00 .00 or usually it's just written as 0,
but either is technically valid like Area 0 would be,
you can configure it either as the 00.00 .00 .00 area or as
Area 0 by itself without the periods.
The checksum, this verifies the integrity of the packet in transit.
Null authentication,
we're not currently using authentication on this link
and since there's no authentication, there's no information in this field.
So let's move back to the slide deck for a second where we're
going to talk about the next field real quick, and then we'll move back into the capture window and take a look at one of them.