Introduction to OSPF Stub Areas

(ALFREDO V) #8

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.

(Rene Molenaar) #9

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

(husam s) #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 ?
(Rene Molenaar) #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.

(Abhishek S) #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

(Rene Molenaar) #13

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

(Rajiv N) #14

Hello Rene,

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

 

Thanks,

Rajiv

(Rene Molenaar) #15

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

(Mansi B) #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 ?

(Andrew P) #17

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.

(Brian C) #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…

(Rene Molenaar) #19

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.

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(sumu s) #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?

(Lazaros Agapides) #21

Hello sumu

The NSSA area is an OSPF area where you know there are no other OSPF routers participating in OSPF beyond this interface, however, you know that there is an ASBR router found within that area. An ASBR is a router that connects to non-OSPF autonomous systems. No OSPF goes beyond this area, however, only other AS information should be relayed here. You can find out a more comprehensive explanation from Cisco at this Cisco documentation.

I hope this has been helpful!

Laz

(Radcliffe W) #22

So how does the Totally Stubby Area communicate with the rest of the OSPF network ?

(Lazaros Agapides) #23

Hello Radcliffe

Having a totally stubby area doesn’t mean that that area will not be able to communicate with other OSPF areas. Inter area communication for OSPF occurs via Area Border Routers (ABRs) and all OSPF non-zero areas must have them in order to function. Totally stubby areas can communicate their OSPF state using Type 1 and 2 LSAs, which are intercepted by the ABR and communicated to other OSPF areas.

Totally stubby areas cannot have Autonomous System Boundary Routers (ASBRs) because by definition, a totally stubby are has only a single point of communication with the outside world, specifically, the ABR.

I hope this has been helpful!

Laz

(Sahil S) #24

HI,
I have a question on this point.
Doesnt ABR just generate type3 LSA or type 4 to let us know of ASBR.
Now in TSA, the only allowed LSA’s are 1 & 2. So how does ABR help ?
if an ABR is configured with default originate always, my understanding is that route alwas appears as an O *E2 or Type 5 or External route in the downlink routers . But I am seeing it as O *IA in my setup, R1 – R2(ABR) – R3(TSA) , pls let me know why ?
Addtionally in show ip ospf dbs cmd on TSA router , I see only LSA1 and Sumary Net link States (LSA3) being shown, should not LSA 3 be blocked on this ?
Note:THis is a office setup, working since ages, just chekcing my output there.
Thanks

(Lazaros Agapides) #25

Hello Sahil

An ABR will generate type 3 LSAs in order to inform other areas of the routes that are found in a particular area. Type 3 LSAs are sometimes known as a summary LSA, that summarises all networks within an area. A type 4 LSA is used to inform other areas of the existence of an ASBR.

Yes, only types 1 and 2 are allowed within the TSA. The ABR doesn’t send any type 3, 4 or 5 LSAs into the TSA, however, it still does send type 3, 4 and 5 LSAs into the other areas it is connected to that are not TSAs, informing them of the networks within the TSA.

When you use the default-information originate command (either with or without the always keyword), the router will generate a type 5 LSA. This means that you should indeed have an O *E2 route in R1 of your topology. Can you verify where you are applying the default-information originate command and where you are seeing the O *IA route and let us know?

You will see only a single “Summary Net Link States” entry which is that of the ABR. This is necessary for the stub router to know how to get out of the area. No other entries will be there since no Type 3 LSAs will enter the area from other ABRs.

I hope this has been helpful!

Laz

(Sergei K) #26

Here are some of the rules when dealing with the stub and totally stub areas:

  • There should be at least one ABR in the area.
  • All routers in the stub area have to be configured as stub router.
  • There is no ASBR within the stub or totally stub area.
  • The backbone area cannot become stub or totally stub area.

What about totally NSSA and NSSA?
Do these few rules also apply to totally NSSA and NSSA?

(Lazaros Agapides) #27

Hello Sergei

The rules apply to NSSAs and totally NSSAs as well. When Rene refers to “stub” areas he means both stub and NSSA, while when he refers to “totally stub areas” he is referring to both totally stub and totally NSSAs.

So yes, the rules apply to all.

I hope this has been helpful!

Laz

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