OSPF Area Advice Needed

Unanswered Question

Hi there, I'm studying for CCNP routing (BSCI) at the moment and I've read and re-read OSPF many times. I think I've pretty much got most of it sorted but one major area which still eludes me is that of OSPF areas.


From what I gather the point with the various area types

stub

totally stubby

not so stubby

NSSA totally stubby area


is to try and have the minimum required amount of LSAs in each area (and therefore reduced number of routes, processing, etc.).


Ok this seems fine to me but in all of the documentation I've read so far (including Cisco course notes), none of them actually give much information about these areas apart from which types of LSAs are allowed in.


Ok so on that basis why not just create every area that doesn't have an ASBR as a totally stubby area thus reducing LSAs, routes, etc? It seems to me that every area that's not area 0 only needs a default route out of the area into area 0, then area 0 would know how to reach all the other areas. Obviously it can't be as simple as this or there wouldn't be all the area types but I just can't seem to find any decent documentation that gives a simple explanation. I'm sure it's not difficult - I just need to find some documentation or have someone explain to me.


Please can anyone point me to any OSPF documentation that gives a good explanation of exactly what each area is and what is and why you'd need more than just a default route?


Thanks

JS.

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srue Fri, 06/22/2007 - 05:56
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All the stubby areas stop type 5 LSAs


any totally stub area stops type 3 LSAs


any NSSA allows creation of type 7 LSAs


memorize those three things and yo'ure good to go (as long as you know what each type of LSA is).


here's the OSPF design guide:

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml


that might answer some of your questions.


Wilson Samuel Fri, 06/22/2007 - 07:16
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Hi Sultan,


This is a question which even I have had struggled with for sometime and got some conclusions which I hope you will also find useful.


Anyways coming to the answer for your query, please condider the following:


"Ok so on that basis why not just create every area that doesn't have an ASBR as a totally stubby area thus reducing LSAs, routes, etc? It seems to me that every area that's not area 0 only needs a default route out of the area into area 0, then area 0 would know how to reach all the other areas. Obviously it can't be as simple as this or there wouldn't be all the area types but I just can't seem to find any decent documentation that gives a simple explanation. I'm sure it's not difficult - I just need to find some documentation or have someone explain to me."



The viewpoint set by you is true, by whatever information I have learnt thus far, infact if you make every Area a TSA or Totally Sub Area, you do send more and more packets to the Area 0 Routers for Route Lookups and hence introduce delay and degraded performance on those Routers, hence the idea was to keep minimum required awareness i.e. to reach all other areas (if its required) make it a normal Area.


However if a Stub Area is small and doesn't comprise too many Routers, then make it a Totally Stub Area, and let the more powerful routers in the Area 0 take care of the Route Lookups!


Finally, the NSSA, is as you have said yourself that is to have connectivity with a Non-OSPF Router which hosts an ASBR however needn't to be aware of all other Areas of the given AS.


Moreover, you may find this link quite useful


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a74.shtml#typesofospfareas



Hope that Helps,


Please rate if it helps.


Kind Regards,

Wilson Samuel


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