OSPF area question for BSCI Certification Exam

Unanswered Question
Aug 23rd, 2009

Hi !

I currently studdying my BSCI Certification exams, now I'm in OSPF section of my studdy. In attach file a small diagram of my lab setup and also some output command from each router, my router are represented by simple circle with there specific name inside of it....

My router R3 have all of it's interfaces in area stub #23 which is a stub area.... I see if I check it's routing table a default route to network to router R2. But I see also route to Lo1 interface connected to R1 and in area stub #10, in this situation I had understand my router have not to know route are part of inside a different area then it self and also area0. If my router know about route inside all area this route, I'm question about the advantage to divide network in many area ? I had understand the necessity to segregate network in different area is to obtain smaller routing table.... but in this configuration all route exist in all router.... Was I misunderstanding something wrong ??

Also in the lab a question to which I not sure about the correct answer. "Why all router in this stub area should know this area is stub ?" My answer is Because router in stub area do not publish route to which the router is not directly connected... because this imply more then one router need to be directly connected to the backbone area which make the utilisation of the same area number on more then one router a curious design, also I'm not sure it this design will work correctly....

Also on R2, if I run sh ip ospf database the network, are appera to be directly connected to R2, but not on R3 where it is in reallity.... (Link ID ADV router (which is R2))

Thanks a lot in advanced !!

I have this problem too.
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Mohamed Sobair Sun, 08/23/2009 - 13:27

Hi Xine,

first question:

OSPF uses hirarchical design, Each Area has its own link state database. Dividing the OSPF Networks into Areas and having stub area has advantages:

1- Making the routing table smaller

2- some routers are not capable of having full topology information interms of memory and CPU resources.

3- Stub Areas Only needs a default route to reach other OSPF Areas instead of getting full OSPF topology info.

Having said that, the Area Border router should have sufficient CPU and Memory Resources.

Second Question:

why All routers in Stub should know this Area as Stub?

In order to have OSPF adjacency between neighbors , 3 criteria must match:

1- The Area

2- The Stub Flag

3- The Hello and Dead Intervals.

So, In the OSPF Hello Packets, those information are carried out, it it matches then OSPF adjacency performed, if not then NO OSPF adjacency is performed.

Third Question:

If you examine the Network , The router-link and Network Type 1 & Type 2 is generated by R3

The Summary Net is always generated by the ABR which is type-3 LSA.

Type-1 and 2 LSAs are converted at the ABR which will generate type-3 LSA based on it.

So, The Advertising router in Area0 for Network should be the ABR which is because its type-3 LSA.



xine xine Mon, 08/24/2009 - 17:23

Thanks you to both of you, you had answer to some of my question but also bring some new, also a lot of part of your messages appear nebulous to me... I think I will have to read all of my OSPF chapter first and read back it a second time to start to make this more clear...

At this time it's not easy to me differenciate correctly LSA type, zone type, and some other OSPF concept...

Giuseppe Larosa Sun, 08/23/2009 - 21:47

Hello Christian,

first of all both area 10 and area 23 are standard stub areas not totally stub areas so it is correct to see inter-area routes that are LSA type 3.

ABR nodes can make area 23 a totally stub area if you change this on R2 that is ABR (0,23) we will see the routes originated in area 10 or 0 disappearing from R3 OSPF DB.

To be noted that if you had external routes that are carried inside LSA type 5 these are blocked and not seen also in stub area.

To see this you can use a loopback interface and you can use redistribute connected on a router that connects to area 0 like R1 or R2.

This makes the device an ASBR injecting an external route in the OSPF domain.

This external route cannot enter any type of stub area.

So the savings on the DB size can be seen if you have several external routes or if you use totally stub areas.

>> Why all router in this stub area should know this area is stub ?

Mohamed has answered to this a stub flag exist in OSPF hello messages and routers have to agree on its value to build an OSPF adjacency.

ABR nodes only may use the totally stub option to hide also all inter area routes.

Hope to help


xine xine Mon, 08/24/2009 - 17:31

Hi Giuseppe,

Like I have response to Mohamed, I think I will have to read all my OSPF chapter and read it a second time to make it more clear to me.... LSA type and area type are always concept are'nt clear to me.... I had stop my reading on this weekend to make some lab (I think it was a good idea to better understanding, but today I not sure if this was so good moved !!

Thanks for your help !


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