I'm not clear on why OSPF has the NSSA stub area type. From the looks of it, in both the text and my test lab, it is the same as a non-stub area, except that external networks are flooded with a type 7 LSA rather than a type 5. Does anyone have any insight as to what conditions would make this is helpful, or if there's something else that happens that I'm not aware of?
With no stub, if you do "show ip route ospf" it will gave all routes to other area. By definition NSSA http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a88.shtml it is can be "middle man area", making similar to default gateway functionality.
Therefore once you enable "area 1stub", "area 1 stub no-summary" it will summarize routes and simplify routing table, still yet allowing you options for "fine-tuning"- which route can be present or not in NSSA routing table.
Simply to put it will do close to same result as default route in static routing command.
See attach, where STUB was used to summiraze route in GNS3. There is some comments,I put for myslef as reminder.
I understand your thinking about NSSA, It seems to be the same unless you have an ASBR into the NSSA area, In a nutshell, the point of this Area type is to have the improvements given by the Stub feature while having the ability to get external routes thru the ASBR into the NSSA and therefore in the OSPF domain, that otherwise will be not allowed. So in fact is a feature that gives you more design flexibilty while keeping the OSPF routing information concise thanks to the stub feature.
You are correct, in that STUB and NSSA are very similar. Likewise Totally STUB and Totally NSSA are very similar. The main difference is that NSSA and Totally NSSA areas allow the OSPF domain to connect to external networks via an ASBR. STUB and Totally STUB areas only connect back to Area 0. Go to the "documents" tab under the CCNP Study Group and there are a couple of documents that may help you understand the different OSPF areas and the LSA types for each.
I really appreciate everyone's input on this. After reading everyone's notes, and doing some more testing, I think the simple answer is as follows:
NSSAs exist because we can't redistribute external routes into a non-backbone, non-NSSA area.
I tested this theory using the following setup (forgive the crude rendering):
EIGRP ---redistribute---> OSPF AREA 1 --------- OSPF AREA 0 <---- redistribute--- EIGRP
I found that when Area 1 is a regular, non-stub area, I see Type 5 LSAs from the backbone, but not from the EIGRP redistribution into Area 1. When I convert Area 1 into a NSSA, I see Type 7 LSAs from both the direct redistribution and from the backbone area.
I take it you never heard of Totally NSSA? I first heard this term back in 2004 when studying for my CCNP. The correct term is NSSA Totally Stub, but I like the shortened version, Totally NSSA. This keeps things similar to the STUB and Totally STUB terminology already in use. What is the difference between a STUB and Totally STUB? The Totally STUB includes the "no summary" keywords on the OSPF command. Example,
router ospf 100
area 1 stub
rouiter ospf 100
area 1 stub no summary
Likewise the difference between NSSA and Totally NSSA (or NSSA Totally STUB if you prefer) is that Totally NSSA includes the "no summary" keyword.
router ospf 100
area 2 nssa
Totally NSSA (or NSSA Totally Stub)
router ospf 100
area 2 nssa no summary
Please read the attached Cisco PDF on OSPF NSSA. You can even Google Totally NSSA and read several posts. Cisco even uses the term on their CCIE R&S v3.0 Written Blue Print.
1.Standard OSPF area
3.Totally stub area
6.Link State Advertisement (LSA) types
7.Adjacency on a point-to-point and on a multi-access (broadcast)
8.OSPF graceful restart
9.Troubleshooting failing adjacency formation to fail
10.Troubleshooting of external route installation in the RIB
The two terms are equal or interchangable, so if you hear it in the future from someone else, you will know what they "really" mean.
Yes, you are on the right track. Here is what Cisco says about NSSA areas:
"Not-so-stubby areas (NSSAs) are an extension of OSPF stub areas. Like stub areas, they prevent the
flooding of AS-external link-state advertisements (LSAs) into NSSAs, relying instead on default
routing to external destinations. As a result, NSSAs (like stub areas) must be placed at the edge of an
OSPF routing domain. NSSAs are more flexible than stub areas in that an NSSA can import external
routes into the OSPF routing domain, thereby providing transit service to small routing domains that
are not part of the OSPF routing domain."
Like stub areas, they prevent the flooding of AS-external link-state advertisements (LSAs) into NSSAs, relying instead on default
routing to external destinations.
Ok, the part about the default route is where I am confused. This is not the behavior I am seeing in my test gear (a pair of 2650XM routers running 12.4-19 and a 2650 running 12.3-21).
Referring back to my basic diagram:
EIGRP site A ---redist---> OSPF AREA 1 ------ OSPF BACKBONE <---redist--- EIGRP site B
When Area 1 is a regular, non-stub area, I receive Type 5 and Type 3 LSAs from the backbone, and I generate a Type 5 LSA for the EIGRP site A route.
When I make Area 1 a stub, I lose my Type 5 LSAs and receive a default route summary LSA from the backbone (matches expected behavior).
When I make Area 1 Totally Stubby I also lose my Type 3 LSAs and only have my default route summary LSA from the backbone (matches expected behavior).
When I make Area 1 NSSA, I receive Type 7 and Type 3 LSAs from the backbone, and I generate a Type 7 LSA for the EIGRP site A route (does not match expected behavior).
Notice how the NSSA behavior is identical to the regular area behavior except for the Type 5 becoming a Type 7? There is no default summary LSA as Cisco mentions in the text. This is why I am so confused.
It looks like the diagram (attached to the thread linked below) reflects what I am seeing, which brings me back to the original question - since the only difference between a NSSA and a non-stub area is the use of Type 7 LSAs, what is the point of the NSSA area type?
In response to your post#9, and referring to your basic diagram, you have the following:
External AS (eigrp) <--> OSPF Area 1 <--> OSPF Area 0 <--> External AS (eigrp)
This means you have an ASBR in both Area 0 and Area 1, which is ok. However, Area 1 should be configured as NSSA. By default NSSA does not inject a default route into Area 1. Area 1 will only see LSA type 1,2,3,and 7. The LSA type 7 will be changed to LSA type 5 at the ABR between Area 1 and Area 0. This is exactly the behavior you should see. Please refer to the attached OSPF Area and LSA diagram. This explains the different OSPF areas and the LSA allowed in each area. To get a default route into Area 1 you will have to use one of three configurations.
router ospf 100
area 1 nssa no summary
router ospf 100
area 1 nssa default-information originate
router ospf 100
area 1 nssa default-information originate always
The last one will inject a default route 0.0.0.0 even when no default route exists in the routing table. Here are a couple of other documents regarding OSPF default routes.
How OSPF generates default routes
How OSPF injects default route in to NSSA
How OSPF injects default route in to Normal Area
Check these out and hopefully you find what your looking for.
In response to your post #10, the point of the NSSA is to allow the injection of external routes from an external AS (eigrp in you case). The NSSA area was created just for this purpose. With the original STUB area you cannot redistribute routes that were external, it was not allowed. Hence, we have NSSA, which allows the redistribution of external routes from outside the OSPF domain. Here is another Cisco PDF on OSPF NSSA.
I really appreciate all the info, but I'm still seeing the exact same thing. We all agree that stub areas exist to help control the size of the routing table for routers internal to the area. This is accomplished by automatically replacing Type 5 LSAs with a default route LSA (both stubby and totally stubby areas) or by replacing both Types 3 and 5 LSAs with a default route LSA (totally stubby areas only).
This automatic behavior does not happen with NSSAs. As you noted earlier, NSSAs require the ABR to explicitly forward default information, using the default information-originate command. The same functionality can also be accomplished with regular OSPF areas. The only automatic behavior with a NSSA is the use of Type 7 LSAs as opposed to Type 5.
So, maybe a better way to frame this question is this - can anyone provide me with an example in which you would need to use a NSSA as opposed to a regular non-stub OSPF area in order to gain some kind of benefit?
Just to clarify, because I think it may have become muddled in all the discussion - I understand the difference between NSSAs and stub areas. I'm concerned specifically with the difference between NSSAs and regular areas, and what benefit, if any, the use of a NSSA provides over the use of a regular area.
Once again, I really appreciate the input. I don't work with anyone that I can swap this kind of info with, so I was thrilled to find this group