Discussioon - Ospf area types

[ITexpert]

Hello Guys, @Julio E. Moisa @Francesco Molino @Richard Burts @Joseph W. Doherty @paul driver  @Leo Laohoo @Georg Pauwen 

I go through the ospf area types and totally understand the technical overview of all of them.

1. Stub

2. T-stub

3. NSSA 

4. T-NSSA

It will be great if you guys helping me to know the real scenario's where each area type is consider as a solution.

Thanks

[balaji.bandi]

Cisco have good and easy document for reference :

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13703-8.html

[Joseph W. Doherty]

In the real world, much depends on how many routes would be injected into the non-zero area(s), how often they might change and the equipment involved. With modern Cisco (NB: Cisco's implementation of OSPF often scales better than other's vendors' OSPF implementation) equipment, you often are in the realm of very large topologies before you really need to worry about needing stub areas, and in such topologies, often you're working with other routing protocols and your design may not need to flood external routes (and/or all other area routes) into your OSPF topology. Understand, because of the possibility of using other routing protocols (where you control redistribution and/or what devices "know of a topology" and/or summarize routes leaving an OSPF area, it's difficult to provide real world example where you'll really need to use stub areas.

[ITexpert]

Hello @balaji.bandi @Joseph W. Doherty @Julio E. Moisa @Francesco Molino

I run lab to get some hands dirty , It answers my questions but new questions now.

I have R1, R2 -->(area 2) , R2, R3 --> (area0) , R3,R4 --> (area1) ,

external route connected with R1 is redistributed into ospf area 2  and I can see that O-E2 route types on all routers.

When i configure nssa , I see that O-N2 --> 0.0.0.0 default route on R1 and O-N2 --> external route on R2, which is perfect according to Nssa defination. 

I cannot reach external network from R3 and R4 , Now to reach external route from R4,Do  I need to change or substitute  O-N2 to O-E2 ?  

My question is,  whats the point of doing nssa if it was working with O-E2 without doing that ? 

Also, as R2 and R3 are ABR's, running command #sh ip ospf nei , i see one neighbor Full/BDR and other Full/DR.  My question is routes have to go where it has too.. then what these state's are making difference ?  

I know BDR -- means backup Designated router

            DR -- means Designated router

Last question,  what these commands making difference ?

               config# int xxx

                           ip ospf network point-to-point  

In lab they ask that network should be seen x.x.x.0/24 from x.x.x.x , I was thinking to summarize or running the commands 

     config# router ospf 1

                 network x.x.x.0 0.0.0255   (replacing these commands , network x.x.x.x 0.0.0.0)

Thanks