Introduction to OSPF Stub Areas

Lessons Discussion
1

This topic is to discuss the following lesson:

2

Very good lesson to understand OSPF Special Area Types

3

Hello. I am wondering why and when to use Stub Area instead of Totally Stubby Area and viceversa ?

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Hi Robert,

The stub area only blocks external prefixes, the totally stub area blocks external prefixes + inter-area routes.

Rene

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5

Yeah I am aware of that in 100 % procent. But in what practical cases use those two area ? I actually have configured stub area ( it has all router belonging to particular autonomus system and default route) and I have also configured totally stubby area It has routes belonging to one area with default route. So why does stub area need those additional routes ?

6

really good info
thanks very much

7

Excellent Info…
Thanks to the writer , such a cool explanation to perhaps a pretty confusing subject

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Hi Rene,

So when you configure stub and totally stub the LSA TYPE 5 are blocked but what about LSA TYPE 4? They are also external E1 routes.

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Hi Alfredo,

LSA type 4 won’t be flooded in the stub or totally stub area anymore, it’s not required since the routers in the stub/totally stub area don’t know about the different E1/E2 prefixes.

Rene

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10

so what is the different between these tow

  • <b>NSSA (not so stubby area)</b>
  • <b>Totally NSSA (totally not so stubby area)</b>
also what about LSA type 8 , 9 ?
11

Here you can find the difference between the two:

OSPF NSSA

OSPF Totally NSSA

LSA Type 8 and 9 are not used in OSPFv2 but I’ll explain these in OSPFv3.

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12

Hi Rene,

 

Very good explanation for a confusing topic. Great job!!!

My question:

So you meant to say with stub and totally stub you cannot have external prefixes ? if yes… can default origniate command be used to advertise a default route by the ASBR in the stub and totally stub area ?

Thank you and keep it up…Your lessons are really helpful.

 

Abhishek

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Hi Abhishek,

Glad to hear you like it!

With the stub and totally stub areas, you can’t have external prefixes in these areas. You’ll need to use a default route on the ABR for reachability.

Rene

1 Like
14

Hello Rene,

Can you explain me, what is the need of backbone areas in OSPF ?

 

Thanks,

Rajiv

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Hi Rajiv,

OSPF is a link-state routing protocol but it only uses SPF within an area. A router in one area doesn’t know what the topology looks like from another area.

Between areas, OSPF works similar to distance vector routing protocols like RIP or EIGRP. To ensure we have a loop-free topology, we have a hierarchical model. All areas have to be connected to the backbone and traffic from one area to another always has to go through the backbone area.

Hope this helps.

Rene

1 Like
16

Hi Rene,
I understood why do we create stubs and total stubs etc.
But if it is so beneficial, why dont we make all areas stubs ?

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It has to do with the design of the network in question. The benefits you see of an area becoming a stub (reduced LSDB size) comes at a cost, which is the loss of some routing information details. This translates into routers within a stub not having all the information necessary to make the best possible choices.

For example, suppose you have an area (which is non-zero, of course), that has multiple exit points. Now imagine that at each of those exit points there are separate external routing domains (say, EIGRP or BGP, etc). If this area is a stub, Type-5 LSAs are not allowed. This means that routers inside the stub won’t have the visibility to know the least cost path to get to external network X is through exit point 1, while getting to network Y, it is best to get through exit point 2.

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18

Thanks for making a video on this series. I am always thankful when I get to a video series as I read the book and website pages and just get worn out from studying 2-6 hours a day through the week. Not to mention on CCNP ROUTE I have taken to reading every single forum post as an added learning tool.

Sometimes I just want to lean back in chair put on headset and listen to video as it allows me to relax a bit when tired towards end of day so uch thanks for those videos. Anyway great lesson!

I am getting close to end of OSPF website lessons already finished the book lessons so woohoo on that!!! lol…

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Glad to hear you like it :slight_smile: Vimeo now also supports different playback speeds so you can set them to 1.5x or 2x if you like. I usually like videos when a topic is completely new to me or when I’m too tired to do anything else.

1 Like
20

So my understanding is stub area is configured to stop flooding of external traffic within one ospf area. Just using a single default route routers within this area can exit. But I am still not sure what is the use of NSSA?