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zillah
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The stub area types

Tue Dec 20, 2005 6:28 am

The stub area types can be summarized as follows:
Stub Type--------------LSAs-------------Default Injected
Stub-----------------------1,2,3,4---- ------------------YES
Totally Stubby------------1,2,default of 3-------------YES
NSSA----------------------1,2,3,4,7---------------------NO
Not-So-Totally-Stubby---1,2,default3,7---------------YES

1- Why do we need to inject two default routes (LSA3 and LSA7) into Not-So-Totally-Stubby, while we inject only one route (LSA3) into Totally Stubby ?

2- It is obvious under "Default injected" we have word "Yes" for both Totally Stubby and Not-So-Totally-Stubby,,,why do we have word "Yes" under Stub area ?

geko29
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Re: The stub area types

Mon Feb 06, 2006 5:51 pm

zillah wrote:1- Why do we need to inject two default routes (LSA3 and LSA7) into Not-So-Totally-Stubby, while we inject only one route (LSA3) into Totally Stubby ?


You're not injecting two DEFAULT routes. Any Default routes handed to a NSTSA or Totally stubby area by the ABR will be LSA 3, and that's what your diagram says. The type 7 LSA for NSTSA, like NSSA, is NOT a default route, rather it is an external route injected into the area by the ASBR as a type 7 summary. This will then be passed back to the backbone area as a type 5 summary.

zillah wrote:2- It is obvious under "Default injected" we have word "Yes" for both Totally Stubby and Not-So-Totally-Stubby,,,why do we have word "Yes" under Stub area ?


Because that's kind of the whole point of a stub area. You have nowhere to go but to the backbone, hence you get a default route.

zillah
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Tue Feb 07, 2006 1:53 am

1,2,default3,7

Here the phrase "default 3" means that word "default" is only for type 3 ,,,not for type 7.

My confusion was : my misunderstood that the word "default" for both types 3 and 7.

zillah
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Tue Feb 07, 2006 2:19 am

Could you please check the LSAs types and let me know if are corrects,,and if you are aware of any URL for it.

zillah
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Tue Feb 07, 2006 2:27 am

Because that's kind of the whole point of a stub area. You have nowhere to go but to the backbone, hence you get a default route.

With Totally Stubby and Not-So-Totally-Stubby, we have word "Yes" because default has been explicitly stated are type 3.

With Stub area what is the type number of the default route ?

geko29
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Fri Feb 10, 2006 2:42 pm

The summary for a stub is type 3, which is an O*IA route generated automatically by the ABR with a destination of 0.0.0.0

I'm not quite sure why some are notated and some are not. For Stub, Totally Stubby, and Not-So-Totally-Stubby networks, the ABR automatically generates the default, while for NSSA, you have to specify it manually with a command such as "area 5 nssa default-information originate". In this one case, the default route will be injected as a type 7. In every other case, including NSTSA, it will be a type 3.

See here for more info:

http://www.cisco.com/en/US/tech/tk365/t ... 4a74.shtml

zillah
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Sat Feb 11, 2006 2:09 am

I get confuse now:

Let me start form scratch:

Cisco curriculum says (6.7.1 Using stub and totally stubby areas ):

"When configuring a stub area, the ABR on the stub automatically propagates a 0.0.0.0/0 default route within the area ",,,,,,to my understanding there is not default router type 3 ,,,Am I right ?

Now if you look to the digram in the link that you have provided under "Stub and Totally Stub Areas" you can see that he used : Default route Type 3 LAS.

Do not you feel that there is contradiction between what cisco curriculum says and URL Link ?

geko29
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Sat Feb 11, 2006 9:03 am

Not at all. Stub areas support LSA types 3 and 4, even when they're not defaults. All routes/summaries that are local to the AS are forwarded to the stub, unless you use route maps or distribution lists to block them. The only thing stub areas don't get are external (LSA type 5 and 7) routes.

When it says it propagates a default/0.0.0.0 route, that's a type 3. It's ALWAYS a type 3, except for Normal Areas (type 5) and NSSA (type 7).

The best thing to do is just do some labs and watch the behavior. When you do show IP route, here's what the routes mean:

O Type 1 (Point-to-Point) or 2 (multiaccess)
O IA Type 3 or 4
O E1 Type 5
O E2 Type 5
O N1 Type 7
O N2 Type 7

Anything marked with an * is a candidate default route, and the difference between type 1 external (E1, N1) and type 2 external (E2, N2) is the metric. Type 1 increments for every link, type 2 does not.

zillah
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Sat Feb 11, 2006 4:10 pm

Stub areas support LSA types 3 and 4

Are you sure that Stub area support type 4,,could you please show me some document for that ? what cisco says is this :

((Remember that stub areas do not accept Type 5, external, LSAs.
Stub areas are typically created when using a hub-and-spoke topology, with the spokes configured as stub areas. The spokes could be the branch offices. In the case of a hub-and-spoke topology, the branch office may not need to know about every network at the headquarters site. It can instead use a default route to get there.

To further reduce the number of routes in a table, create a totally stubby area, which is a Cisco specific feature.

A totally stubby area is a stub area that blocks external Type 5 LSAs and summary, Type 3 and Type 4, LSAs from entering the area. This way, intra-area routes and the default of 0.0.0.0/0 are the only routes known to the stub area. ABRs inject the default summary link 0.0.0.0/0 into the totally stubby area)).


Cisco explicitly says that stub area does not accept type 5, but it does not mention explicity about type 3 and 4.



http://www.ietf.org/rfc/rfc3509.txt
Code: Select all
Note: Type 4
          summary-LSAs should not be generated if Area A has been
          configured as a stub area."

geko29
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Mon Feb 13, 2006 7:11 am

This was a tough one to find, but you're right, Type 4 LSAs are filtered from stub areas. I always assumed they weren't because every piece of documentation said stub blocks 5, while totally stubby blocks 3,4,5. It was a natural conclusion, methinks :) Anyway, after several hours of searching, I found this:

A stub area—This is an area that will not accept external summary routes. The LSAs blocked are Types 4 (summary link LSAs that are generated by the ABRs) and 5. The consequence is that the only way that a router within the stub area can see outside the autonomous system is via the configuration of a default route. Every router within the area can see every network within the area and the networks (summarized or not) within other areas. It is typically used in a hub-and-spoke network design.

A totally stubby area—This area does not accept summary LSAs from the other areas or the external summary LSAs from outside the autonomous system. The LSAs blocked are Types 3, 4, and 5. The only way out of the totally stubby area is via a configured default route. A default route is indicated as the network 0.0.0.0. This type of area is particularly useful for remote sites that have few networks and limited connectivity with the rest of the network. This is a proprietary solution offered only by Cisco. Cisco recommends this solution if you have a totally Cisco shop because it keeps the topological databases and routing tables as small as possible.

A not so stubby area (NSSA)—This area is used primarily to connect to ISPs, or when redistribution is required. In most respects, it is the same as the stub area. External routes are not propagated into or out of the area. It does not allow Type 4 or Type 5 LSAs. This area was designed as a special stub area for applications like an area with a few stub networks but with a connection to a router that runs only RIP, or an area with its own connection to an Internet resource needed only by a certain division.

A NSSA is an area that is seen as a stub area but that can receive external routes that it will not propagate into the backbone area, and thus the rest of the OSPF domain. Another LSA, Type 7, is created specifically for the NSSA. This LSA may be originated and communicated throughout the area, but it will not be propagated into other areas, including Area 0. If the information is to be propagated throughout the AS, it is translated into an LSA Type 5 at the NSSA ABR.

zillah
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Tue Feb 14, 2006 4:29 am

I always assumed they weren't because every piece of documentation said stub blocks 5, while totally stubby blocks 3,4,5. It was a natural conclusion

This is what happend to me as well.



after several hours of searching, I found this:

Could you please pass me the link for that.




I am going to raise another issue for discussion, share me your idea and comment please.



Cisco curriculum CCNP1 V3.0 Article 6.7.1 says:

((A stub area does not accept information about routes external to the AS. the ABR on the stub automatically propagates a 0.0.0.0/0 default route within the area)).


Cisco curriculum CCNP1 V3.0 Article 6.7.1 says:
(( A totally stubby area is a stub area that blocks external Type 5 LSAs and summary (Type 3 and Type 4) LSAs from entering the area.

This way, intra-area routes and the default of 0.0.0.0/0 are the only routes known to the stub area(should not it say “ to the totally stub area” instead of “to the stub area” ?). ABRs inject the default summary link 0.0.0.0/0 into the totally stubby area)).




My conclusion are these:
1- Both area (stub and totally stub) have got type 1 and type 2
2- Stub area has got type 3 only , whiles totally stub has not got that .
3- Both of them have got default route 0.0.0.0/0.
4- Now we can conclude that the difference between the stub and totally stub is only type 3,,,Am I right ?
5- Does the default route for stub area is of type 3 (why I am saying that see below) ? If so,,that means there is no difference between stub and totally stub!!! Any explanation
6- And suppose the default route is of type 3,,,what does that mean ?




If you look to the second figure in the link below
http://www.cisco.com/warp/public/104/21.html

It shows me that the default route that will be injected in Stub area and totally stubby area is type 3,,,,,,,,Is this typo ? because it looks different form what the Cisco curriculum states,,,,,,,Any comment ?


In another document (Internet work Expert Lab)

“A stub area blocks OSPF external routes (LSA 5) form entering the area. The ABR of a stub area automatically generates a default route (LSA 3) into the stub area “.

geko29
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Tue Feb 14, 2006 5:58 am

The info I found was actually from the BSCI Self-Study by Clare Gough:

http://www.ciscopress.com/articles/arti ... 31577&rl=1

As for your conclusions:

1. Right on.
2. Damn close. Stub has normal type 3 summaries, while totally stubby does not support that, but it DOES get the default route, which is propagated as a type 3 summary.
3. Yes
4. Yes
5. As I said above, regular summaries are supported in Stubs. These can be generated directly at the ABR, or elsewhere in the system. However...
6. Yes, the default route in both is type 3. It doesn't really mean anything, if you think about it, because all the filtering is done at the ABR.

When you put in "area x stub no-summary" on the ABR, those two keywords do two different things. First, the "stub" keyword tells it it has to filter LSAs of type 4 and 5, and create a default route pointing to itself and advertise it as a type 3 summary. Then the "no-summary" keyword tells it to filter all remaining type 3 LSAs. End result is you get only type 1 and 2 in the area, with the exception of the default, which is the only type 3 supported. The non-ABR routers in the area don't know any better, since as far as they're concerned the area is a stub (not totally stubby), so they don't know they're not supposed to get any type 3s. So they accept the default.

The last bits of your post are totally correct. This is not a typo, default routes are automatically injected into stub, totally stubby, and not-so-totally stubby networks as a type 3 LSA. The only exception to this is NSSA, in which the default is NOT automatic, and if configured, it is a type 7.

zillah
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Tue Feb 14, 2006 12:57 pm

I am going to clarify one by one just to make I understood well.

((A stub area does not accept information about routes external to the AS. the ABR on the stub automatically propagates a 0.0.0.0/0 default route within the area)).

6. the default route in both is type 3


2- Stub has normal type 3 summaries.
6- regular summaries are supported in Stubs


Correct my explaination , you are saying that default route for stub is type 3 and summary for stub area is type 3 as well!!!!,,,,Doesn't it make sense for me.

By the way you mean by summary (interarea summarization) which it will be implemented by using this statemnet : area area-id range address mask. ,,,don't you ?






and cisco curriculum says default route for stub is 0.0.0.0/0

I feel there is contradiction between what you are saying and what cisco curriculum saying, about default route for stub area. Now to avoid this contradiction, you are saying that the default of 0.0.0.0/0 (Cisco curriculum) is similar to type 3 for stub area ""When it says it propagates a default/0.0.0.0 route, that's a type 3"",,,,,,,,,,,Am I right ?


The rest of questions I leave them after I clarify the above.

geko29
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Tue Feb 14, 2006 2:23 pm

zillah wrote:ICorrect my explaination , you are saying that default route for stub is type 3 and summary for stub area is type 3 as well!!!!,,,,Doesn't it make sense for me.


Type 3 is a summary. The default route is a summary. Ergo, the default route is a type 3. But receiving a default route doesn't mean it doesn't also get other routes/summaries. For that, you need the "no-summary" keyword.

By the way you mean by summary (interarea summarization) which it will be implemented by using this statemnet : area area-id range address mask. ,,,don't you ?


Yes, that's the most common method to manually create a type 3 summary.

and cisco curriculum says default route for stub is 0.0.0.0/0


The default route is ALWAYS 0.0.0.0/0, no matter what the network layout, routing protocol, etc. This is the very definition of a default route or route of last resort.

I feel there is contradiction between what you are saying and what cisco curriculum saying, about default route for stub area. Now to avoid this contradiction, you are saying that the default of 0.0.0.0/0 (Cisco curriculum) is similar to type 3 for stub area ""When it says it propagates a default/0.0.0.0 route, that's a type 3"",,,,,,,,,,,Am I right ?


The default route isn't similar to a type 3 summary, it IS a type 3 summary. You could do the same thing manually to a normal area by typing "area x range 0.0.0.0 0.0.0.0"

zillah
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Tue Feb 14, 2006 3:56 pm

Cisco curriculum CCNP1 V3.0 Article 6.7.1 says:
(( A totally stubby area is a stub area that blocks external Type 5 LSAs and summary (Type 3 and Type 4) LSAs from entering the area.

This way, intra-area routes and the default of 0.0.0.0/0 are the only routes known to the stub area (should not it say “ to the totally stub area” instead of “to the stub area” ?). ABRs inject the default summary link 0.0.0.0/0 into the totally stubby area)).


I believe there was type in the statement above, look to my bolded correction and comment that please.

geko29
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Tue Feb 14, 2006 5:20 pm

Yes, That statement refers to a totally stubby area, which receives only intra-area routes and the default from the ABR. A stub area receives those plus Inter-area routes.

Definitely a typo.


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