When an ABR receive type 1 router and type 2 network LSAs within the area , it will only send the reachability information to another area. ABR is the choke point where topology information hides and only reachability information sends between the areas.
When ABR sends a summary type 3 LSAs into another area , it says I can reach network 192.168.1.0/24 , 192.168.2.0/24 etc and you can reach these networks through me. But ABR will not send with summary LSA , if you want to reach 192.168.1.0/24 send first to me and I will send packet to Router A , Router A will send packet to Router B and so on. Which mean is ABR will hide topology information.
Internal routers within area will believe what their ABR says , they cannot calculate end to end path since they dont know full topology. This is distance vector behavior. Is not the same with EIGRP ? At every hop EIGRP router calculate the best path which is feasible distance to destination and sends it to next router.
Here one thing is important ; calculate and then send !. First calculation is done so , other routers wait this router to finish its job and send the route with its the best path , receiving router puts these information from all the sending router to EIGRP topology database and then run DUAL.
But with link state protocols both OSPF and IS-IS , when they receive LSA , they first floods the LSAs to their neighbor and starts to run SPF, of course here we are talking about msec level.But within large environment with the lots of links these can be a scaling issue.
Likewise, if an ABR receives multiple Network Summary LSAs from other ABRs across the
backbone, the original ABR will choose the lowest cost advertised amongst them and create NEW summary LSA and send into attached areas. 2 things are important in this sentence :
1. It will create new summary LSA , because unlike Type 5 external LSA , Type 3 summary LSA is limited to Area , when ABR receive summary LSA from another ABR , It creates new summary and sends to attached areas , this can be scaling issue.
2.ABR needs to create new summary Type 3 LSA for every attached area. This is also can be a scaling issue. From here we could discuss How many ABR per area ? or how many router per OSPF domain but lets keep this for another blog post.
Lets create one scenerio where ABR connects Area0 backbone area and Area1 internal area. First lets talk when we have just one ABR and then redundant ABRs.
If there is only ABR and we cannot add second ABR or high availability is not a big concern at this time , hiding topology and reachability information is not a big issue. Only concern would be within Service Provider environment which if They put their PE into Area 1 and Area 1 does not have specific routes to exit from Area 1.
Because PEs needs to know each other loopback interface as /32 host route to create LSP for transport label. So the solution might be leaking PEs loopback into Area1. If ABR in this situation goes down , all internal Area 1 routers will lose their connectivity to the domain.
To better idea to put more than one ABR per area for high availibility. In this case still route leaking for special purpose is needed but also there are at least two more caveats.
First , if we hide reachability information ( By default we hide topology information ) internal Area 1 routers will chose the best path according to their cost to ABR but since those internal routes dont know the entire topology of OSPF domain, after ABR to the destination in the Ospf area 0 can be a sub optimal. This if from edge to core sub optimality consideration, it may or may not be an issue for your network and its application. But if you have tight latency requirements this may affect your design choice.
Second is blackholing could be a problem if we are also hiding reachability information from Area 1 to backbone area . In this case , from one of the ABR to one of thr internal Area 1 router link goes down , traffic still can reach for the network behind that internal router to the ABR which failed link. And traffic could be blackhole.
General idea and solution when we summarize from two ABR like in this situation , put the link between the ABRs and sends the routing information over that link. Here another big consideration is coming to play , Which area would you put that inter ABR link ? Area 0 ? Area 1 ? Lets keep this to nother blog post.