In any routing protocol, scalability is an essential factor. When we split the OSPF routing domain, we are trying to reduce the size of each area's LSDB.
One of the other benefits of summarization is to make SPF calculation faster.
Keynote: Summarization occurs between areas on the ABRs. the reason is all routers within an area must maintain an identical copy of the LSDB Topology without summarization.
In this network topology, based on the metric, all traffic choose R2 to reach Area 0.
if the link between R1 and R2 fails, all routers in the area have to run SPF calculation, then LSA type 3 needs to be updated with a new path and then advertised into area 0.The routers in area 0 run an SPF calculation on affected prefixes.
Topology with summarization
in this topology, networks in area 1 being summarized at the ABR.
In the above topology, if the link between R1 and R2 fails, all the routers in area 1 still run the SPF calculation, but routers in area 0 are not affected.
We should always pay extra attention to IP address schemes and proper summarization.
In the next diagram, we do an overview of ospf summarizations.
Now time is for configuration and verification.
For better understanding, I created a video for it.
In the video, you can see how the summarization for inter area and external area configuration procedure and verification steps.
the topology for the lab is here: