Now that we have run through OSPF basics and how OSPF neighborships form,
let's move into a deeper discussion of the process of exchanging information
between neighbors and within devices in an OSPF domain.
The indexing information used for the database descriptor
matching includes the link state type, the link state id,
and the advertising device's router id.
If a device determines that a neighbor has link state advertisement or LSA
entries in its link state database that it does not,
then it will request them from the neighbor using a link state request packet.
Once the neighbor receives this request,
it will return the requesting link state advertisement
entry using a link state update packet.
And finally, when the requesting device receives this update,
it will typically return a link state acknowledgement packet to the neighbor,
ensuring the exchange is reliable.
If the updating neighbor doesn't receive this packet,
then it will be resent until it is acknowledged.
So now that we have seen this,
including looking at how various LSA types are referenced,
we need to go into what these specific types are.
For this module we are going to focus on the three that
are commonly used with single area OSPF,
including the type 1 router LSA, the type 2 network LSA, and the type 5 external LSA.