So now that we have covered multi-area OSPF concepts,
let's go a bit further and look deeper into the different LSAs
that are used when utilizing multiple areas.
Let's start with a type 3 summary LSA.
As noted previously,
areas provide the ability to limit the communication between devices,
allowing for more efficient operation.
When not using stub areas,
this means taking the type 1 router and type 2 network LSAs from one area and
inserting them into other areas using a type 3 summary LSA.
This LSA is originated on the ABR that interconnects the different areas.
The structure of the type 3 summary LSA is shown here.
As you can see, this structure looks very similar to other LSAs.
Many of these fields,
including the OSPF header fields discussed in an earlier module,
are structured the same.
Actually the type 3 summary LSA shown here and the type 4
summary LSA that will be discussed next,
have the exact same structure, but the values are set differently.
When used as a type 3 summary LSA, the link state type is set to 3,
the link state id is set to the advertising IP network number,
and the advertising router is set to the advertising ABR's router id.
Next, let's jump to the network mask field,
which is used to indicate the network mask of the network being
advertised that is specified in the link state id field.
And finally we have the metric field.
This field is used to indicate the cost to reach the advertised destination.
This information will be taken from the router and network LSAs and
continue to accumulate until it reaches the last OSPF device. With this covered, let's take a look at it in the lab.