Thank you for responding, I understand NSSA area doesn’t generate default route. My question here would be why only NSSA area behaves like this and doesn’t generate default route.
Hi Ravi,
There is a good reason for this. With a stub or totally stub area, there is only one way out of the area and that’s the ABR.
With an NSSA area, your ASBR could also advertise a default route.
Now imagine both your ABR and ASBR advertise a default route. Which of the two default routes are used then? OSPF prefers routes in this order:
* Intra area
* Inter area
* External
* NSSA external
So if the ABR would advertise a default route by default, then the default route from the ASBR would never be used. That’s why it is not enabled by default.
Hope this helps!
Rene
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Hi Rene,
Isn’t command should be “default information originate always” to advertise default route in NSSA ? as we do not already have default route in our topology …
Your config:
R2(config)#router ospf 1
R2(config-router)#area 1 nssa default-information-originateHello aniket
The command area 1 nssa default-information originate will cause the router to advertise the default route that exists in its routing table. If the router does not have a default router in its routing table, it will not advertise itself as the next hop for a default route.
The command area 1 nssa default-information originate always will do the same thing except that it will advertise itself as a next hop router even if it doesn’t have a default route itself.
This is the case whether you are configuring this for an NSSA as in this lesson, or as a normal area.
Looking at the configs, there is no default route configured in R2 so in order to get the same results shown in the lesson, you are right that you must add the always keyword. Otherwise, a static default route should exist in the configuration of R2. I will let Rene know about this so it can be modified appropriately.
Thanks for pointing this out!
Laz
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It’s also worth noting, if you manually generate a default route in an NSSA area then it will appear as a type 7 within the NSSA area.
Whereas the automatic default route in a Totally Stubby NSSA area will appear as a type 3 within the NSSA area.
But, when I have tested the ABR doesn’t require a default route, even without the “always” keyword, when creating default routes in any stub areas.
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hello rene,
I have a question 
1> In NSSA LSA5 is blocked but ASBR router we can use because it generate a LSA type 7 to provide another protocol prefixes , so why default route which is inserted by LSA 3 in stub area but its not inserted in NSSA area we have to insert dafult route manually in NSSA area please explain this ? WHY ?
2>sir when you adding manually default route in NSSA area on ABR router so why it is showing N2 for default route instead IA in router 3 routing table because in stub area it is showing in R3 router route IA ? why ?
R3#show ip route ospf
O IA 192.168.12.0/24 [110/2] via 192.168.23.2, 00:06:21, FastEthernet0/0
O*N2 0.0.0.0/0 [110/1] via 192.168.23.2, 00:00:54, FastEthernet0/0Hello Harshit
For your first question, take a look at Rene’s answer in this post:
For question 2, the default route shows up as N2 in the routing table of R3 because it was learned from an ABR (R2) that has been configured to make AREA1 an NSSA. So the default route learned is due to the fact that the NSSA has been configured. Notice that no default route has actually been configured in R2 but it is forced to send default route information to R3 stating that it is the next hop for the default route. Because this was created due to the NSSA configuration, the default route info has a designation of N2. Now the route to 192.168.12.0/24 was created in R3 via an Type 3 LSA also known as an inter-area or summary LSA, from Area 0 that has been sent as a summary of all the networks available in Area 0. That is why the designation there is IA.
I hope this has been helpful!
Laz
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Hi all,
I’ve tried this command on a c7200 router IOS 15.2 in GNS3. It doesn’t work for me, it doesn’t accept the Always parameter, it’s invalid … Anyone have any idea why?
Thank you,
Vlad
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Hello Vlad
You are correct, this command doesn’t have the always keyword available, according to the following Cisco IOS command line reference:
I also took a look at the lab topology and verified that this is the case. The always keyword is only available for the default-information originate command, which is not configured under a particular area, as seen in this command reference:
I will let @ReneMolenaar know to make the clarification…
Thanks for pointing that out!
Laz
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Hi Laz,
You’re welcome, thanks for the prompt reply. As a workaround I installed a default static route and used the command without always. Is that the best way or do you know of another way?
BR,
Vlad
Hi Vlad,
I think the behavior of the NSSA has changed. When I wrote this post I used IOS 12.4 (2013…long time ago). I just tried it on IOS 15:
R1#show version | include IOS
Cisco IOS Software, IOSv Software (VIOS-ADVENTERPRISEK9-M), Version 15.6(3)M2, RELEASE SOFTWARE (fc2)Even without a default route on R2, I get the default route on R3:
R3#show ip route ospf | include 0.0.0.0/0
O*N2 0.0.0.0/0 [110/1] via 192.168.23.2, 00:11:12, GigabitEthernet0/1I updated the lesson to get rid of the always parameter.
Rene
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Hi,
What is the use case of NSSA ?
Thanks
Hello Sims
The purpose of an NSSA OSPF area is to allow an OSPF stub area to carry external routes (routes learned from other protocols like EIGRP) while still allowing them to maintain their stub characteristic as far as OSFP is concerned. This enables an NSSA to filter out tye 3 and type 5 LSAs, but it allows the use of an ASBR, and thus a type 7 LSA to be advertised.
So you have the benefit of avoiding the advertising of routes found within the NSSA to other areas such as the backbone area, but you can still advertise routes learned via other protocols using a type 7 LSA, thus allowing the NSSA to be a route to destinations outside of the OSPF domain.
I hope this has been helpful!
Laz
Hi Laz,
Here I did not understand how R1 getting external routes b/c as we know R3 in nssa and as per nssa area rule it does not allow ospf to advertise LSA 5 then how R1 still getting LSA 5 which external route?
please detail this process?
Hello Pradyumna
It is true that an NSSA doesn’t allow any Type 5 LSAs, but it does allow the use of an ASBR. In this lesson, R3 plays the role of an ASBR that exists within a stub area. In order to get its external route (3.3.3.3) advertised, the stub must be configured as an NSSA. This will result in the advertising of the external route to the backbone area, and to R1.
So how does the ASBR do this if a type 5 LSA is not allowed? It creates a type 7 LSA which remains within the NSSA. Once the ABR (R2) receives this, it converts it to a normal Type 5 LSA and sends it to R1. And this is how R1 learns of the external route. This process is further described in the OSPF LSA Types Explained lesson. This diagram will also help to visualize this:
I hope this has been helpful!
Laz
Hi Laz,
Ok got it. please let me know why we always get external route as type 2 like E2 or N2 by default. why are we not getting E1 and N1?
Please share actual mean of E1&E2, N1&N2, what should I answer if interviewer ask what is E1 and E2 routes in OSPF?
Hello Pradyumna
Take a look at this lesson. You should find all the answers to your questions about these route types and their priorities.
I hope this has been helpful!
Laz
Hey Laz,
Sorry to bother you, I understand your above post was dated two years ago. Many things probably had changed, by the time I read your forum.
But I really like your awesome forum, nevertheless !
I had done a bit of research, and I had found the below Cisco document. The last paragraph said:
"NSSA ABR can generate a default route with or without a default route in its own routing table".
In your lesson, R2 was the NSSA ABR, therefore Rene need not add the “always” keyword. Rene also did not need to add a static default route.
I hope I had interpreted the Cisco doc correctly. Many thanks in advance for your very good knowledge 
Hello Jenny
First of all, there was a typo in my post which I have now fixed. I had forgotten the keyword always in the second command I was describing.
However, you are correct, that in this particular case of the lesson, the always keyword is not necessary because of the fact that the ABR will advertise itself as the default route even if it doesn’t have a default route in its routing table.
Thanks for sharing this clarification, it’s always helpful! And by the way, it’s no bother, that’s what we’re here for!
I hope this has been helpful!
Laz
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Got it !! Thank you awesome Laz !
Very happy with your networking lessons, giving me the needed confidence to interpret Cisco document ! Thanking you gratefully ! 
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