Regarding NSSA in OSPF

[jain.manish94]

Hello Team,

I hope you all are fine.

I have one question plz help me with answer.

When we don use STUB in OSPF that time also we can see all LSA5 routes right?

When we enable NSSA that time also we can see all LSA 5 routes right ?

Then why we are using NSSA ? 

Thanks

Manish

[balaji.bandi]

More explained here clear :

https://packetlife.net/blog/2008/jun/24/ospf-area-types/

[paul driver]

Hello

NSSA 

no type 4-5 lsa

allow type 1,2,3,7 lsa

NSSA + default-information originate

no type 4-5 lsa

allow type 1,2,3,7 + default

NSSA no summary 

no type 3,4-5 lsa

allow type 1,2,7 + default 

[paul driver]

Hello

forgot to answer your question appropriately-

you would use a nssa when you have an ospf stub area that has a requirement to advertise external routes and you still want the filtering of LSA types.

[jain.manish94]

I am not able to configure both Stup and NSSA together. 

is this possible ?

see my diagram. 

Area 51 is in Stub and Area - 678 is in NSSA. not working as expected. 

[paul driver]

Hello

---

@jain.manish94 wrote:

I am not able to configure both Stup and NSSA together. 

No -  all rtrs in a specific stub area need to be either a stub or nssa stub they cannot be mixed

[MHM Cisco World]

what you expect ? what issue here ?

When we don use STUB in OSPF that time also we can see all LSA5 routes right?

in STUB area there is NO ASBR so there is no LSA5/LSA7

When we enable NSSA that time also we can see all LSA 5 routes right ?

in NSSA there is ASBR BUT there is no LSA5 inside area there is LSA7 inside area, when the LSA7 reach the ABR connect NSSA area to Area0 the LSA7 convert to LSA5.

[MHM Cisco World]

My Note

[MHM Cisco World]

@paul driver Hi, 

""The typical design question is this: Where can these areas be used and why? 

The basic standard answer is this: It depends on the requirements and topology.

For instance, if no requirement specifies which path a route must take to reach external networks such as an extranet or the Internet, you can use the “totally NSSA” area type to simplify the design. For example, the scenario in Figure 2-16 is one of the most common design models that use OSPF NSSA. In this design model, the border area that interconnects the campus or data center network with the WAN or Internet edge devices can be deployed as totally NSSA. This deployment assumes that no requirement dictates which path should be used [15]. Furthermore, in the case of NSSA and multiple ABRs, OSPF selects one ABR to perform the translation from LSA type 7 to LSA type 5 and flood it into area 0 (normally the router with the highest router ID, as described in RFC 1587). This behavior can affect the design if the optimal path is required.""

from <<CCDE Study Guide.pdf>>

[Meddane]

Stub area does not allow to inject a Type-5 LSA for external routes, so for some scenarios where your stub area is connected to an external domain routing and you want to redistribute the external routes into OSPF through the stub area. It's not possible since a Stub area block LSA Type-5 and this area does not accept an ASBR, (ASBR is a special router that redistributes external routes into OSPF domain). So to overcome this issue and allow you to redistribute external routes, another concept of stub area is added, it's called NSSA Area or Not-So-Stubby-Area, it's like the stuby area but not really full stubby, this is why we call it -Not-So-Stubby-. This NSSA area will allow to configure an ASBR and inject external routes. But wait, the NSSA Area has some similitaries with Stub area, both dont accept the LSA Type-5. Therefore the developper was forced to define a dedicated LSA for external routes. This is called Type-7 LSA. This LSA has the same fields as the Type-5 LSA. But it has the NSSA area flooding scope so it does not traverse the NSSA area. And to allow other routers located in a regular area where only LSA Type-5 is accepted. The developpers provides the responsability to an NSSA ABR to translate this Type-7 LSA received through an NSSA AREA and convert it into a Type-5 LSA, so that other routers in regular areas will have the reachability to external routes.

Best regards

MEDDANE

[Georg Pauwen]

Hello,

I assume R6 is the ABR between area 0 and NSSA area 678 ? What is not working as expected ? Can you post the running config of R6 ?