How to configure OSPF NSSA (Not So Stubby) Area

This topic is to discuss the following lesson:

When we look at router Susan you’ll see network 192.168.12.0 /24 as inter-area (LSA Type 3) and 1.1.1.0 /24 as external type 2 (LSA Type 5).:???

That’s how it is supposed to be right? Network 192.168.12.0 /24 is inter-area for router Susan (LSA Type 3) and 1.1.1.0 /24 is an external prefix (LSA Type 5).

If Susan in configured as area 1 stub, don’t you think LSA 4&5 will get blocked. Output of Susan shows correctly that it will allow LSA Type 3 for Inter area routes and a default route by the ABR Donna. Kindly check and confirm.

Thanks Hemant, the text didn’t match with the output…just fixed it.

*Mar 1 00:20:02.495: %OSPF-4-BADLSATYPE: Invalid lsa: Bad LSA chksum Type 7, LS
ID 3.3.3.3 from 3.3.3.3, 192.168.23.3, FastEthernet0/1 – receiving error on Donna router
plz reply

Hi Jeevan,

That is a really interesting error. When did you get this?

Rene

Rene just a question, why is that cisco never put a default route in NSSA? why did you need to configure it manually? does is have a reason why they didn’t put it in default?

Hi John,

I think there is a good reason for this. Take a look at this post first:

Now imagine that the ABR and the ASBR both advertise a default route, when this is the case, OSPF will never use the default route from the ASBR since information of inter-area will always be used over NSSA external routes.

Cisco probably disabled the default route on the ABR for this reason. There’s no need to do this for stub or totally stub areas since they don’t support an ASBR.

Rene

Hi Rene,

Could you please help in explaining about N/P bits used in NSSA?

I like to know more about role of N/P bits and what all the cases p bit will be set and the cases where it will not be set.

Sure, take a look here:

OSPF P-bit NSSA explained

Dear Rene,

Great tutorial, clear and concise…

I have one questions …

Why we will use Stub & NSSA where as Totally stub & Totally Nssa perform the same and reduce Routing table entry.I think we can completely use Totally Stub rather than Stub and Totally Nssa rather than NSSA.Please do clear Is there any reason to use Stub & NSSA ??

br//
zaman

Zaman,
The answer lies in the complexity of the network you are dealing with. In a lab / learning environment, you often see OSPF stub examples represented with just a single router connecting the stub area to the rest of the network. In real life, however, you might be dealing with multiple exit points out of the stub (note, this does not mean the stub is a transit area–by definition it can’t be). In this case, it might not be optimal to have all traffic leaving the stub area to flow through just one ABR. In order for routing decisions to be made as to which ABR should be used for a particular destination, it would be necessary to have type 3 LSAs. As you know, a Totally Stubby Area doesn’t allow Type 3 LSAs (other than the ABR generated default route).

A similar argument can be made for NSSA vs NSSA Totally Stubby Area

Hi Rene,
I have a question , What is the significance of forward address in ospf ??? .

br//zaman

Hello Mohammad

A forwarding address in OSPF allows extra hops to be avoided in certain routing situations because it allows a router to specify another router’s IP address as the forwarding or next hop address address. So essentially, a R1 can tell R2 that the next hop router for a destination is R3.

You can find a detailed description of forwarding addresses in OSPF and its uses at this Cisco documentation.

I hope this has been helpful!

Laz

Hi Laz,
Now got the answer . Many Thanks

br//zaman

1 Like

hI,

Finally we can see 3.3.3.0 on R1. But that is E2 route type right ?
E2 means that came from type 5. How Type 5 pass through R2 which is NSSA ?

R1#show ip route ospf 
     3.0.0.0/24 is subnetted, 1 subnets
O E2    3.3.3.0 [110/20] via 192.168.12.2, 00:07:25, FastEthernet0/0
O IA 192.168.23.0/24 [110/2] via 192.168.12.2, 00:07:41, FastEthernet0/0

===================================================
I think R3 will create Type 7 request and R2 will convert back to Type 5. So R1 can update 3.3.3.0/24 route as E2 :slight_smile:

why do we need area 1 default-information-originate command in NSSA area only?

Hi Hasindu,

That’s right. Within a stub area type, LSA type 5 is not allowed. R3 creates a LSA type 7 which is translated into LSA type 5 by R2.

Hi Ravi,

By default, no default route is generated for the NSSA area. If you want your routers in the NSSA to be able to reach other areas, you’ll need a default route.

Rene