OSPF NSSA and Normal Area

[rakeshvelagala]

Dear All,

Please advise on the below.

If we configure an area as an NSSA, we will not see type 5 and Type4. Instead we see type 7 and also other external routes if redistributed from other other areas as type 7 inside nssa area database.

In Normal area, we will see type 4 and Type5.

Question:

Why there is a need for NSSA then? I mean if I have a external network connected, can't I just configure it as a normal area?

I understand NSSA converts the Type 7 to Type 5 if the P bit is set. But I am sure I am missing something.

Thanks for the advise.

[Peter Paluch · Accepted solution]

Hi Rakesh,

Why there is a need for NSSA then? I mean if I have a external network connected, can't I just configure it as a normal area?

If you configured the area as a "normal" area then you would also see external networks coming in from the backbone and from other areas behind the backbone. With a NSSA area, the only external networks you see are those redistributed in the same NSSA area.

A NSSA area can inject external networks, and these external networks will be "uploaded" to the backbone and propagated further according to normal OSPF flooding rules. However, external networks injected in other areas will not be "downloaded" to a NSSA area. This is the difference to a regular area that can both inject its own external networks which will then be seen in other areas, as well as see external networks that have been injected in other areas.

The NSSA area is arguably the most flexible and yet conservative area type, as it prevents the area from seeing external networks in other areas, yet it allows that area to introduce its own external networks if that is necessary, and allow other areas to know about them.

Best regards,
Peter

[rakeshvelagala]

Hi Peter,

Many thanks for your reply.Please advise on the below. I am sure I am doing something wrong.

Please see my topology. I cannot attach to this. Please download from my dropbox.

https://www.dropbox.com/s/hqlmfpvd33khj4j/OSPF_Images.zip?dl=0

Image8--Topology (Sorry not very Descriptive)

Image1-->Router 13 (Running RIP) having IP45.32.x.x range. NSSA ASBR redistributing these ranges  is Router 9-->Image2.

#show ip ospf database on router 9-->Image 3.

In the ospf databse, I can see the type 7 for 45.32.x.x and in addition to that, I also see  37.x.x.x and 12.x.x.x (Actually these two are again re disributed from other router as LSA type 5 but learnt by the Area 50 as type 7).

"If you configured the area as a "normal" area then you would also see external networks coming in from the backbone and from other areas behind the backbone. With a NSSA area, the only external networks you see are those redistributed in the same NSSA area."

I am still seeing other external routes, which does not belong to this. Please advise.

                Type-7 AS External Link States (Area 50)

Link ID         ADV Router      Age         Seq#       Checksum Tag
12.12.12.0      11.11.11.11     550         0x80000002 0x00CC80 0
31.31.31.0      11.11.11.11     550         0x80000002 0x001EF5 0
31.31.32.0      11.11.11.11     550         0x80000002 0x0013FF 0
31.31.33.0      11.11.11.11     550         0x80000002 0x00080A 0
37.0.0.0          11.11.11.11     550         0x80000002 0x009BB0 0
42.65.45.0      3.6.24.7        869         0x80000001 0x004F1E 0
45.0.0.0          3.6.24.7        866         0x80000001 0x0028B0 0

I have attached the required screenshots.

Image 4-->Router 12 running only RIP.

Image 5 -->Router 1 doing the redistribution.

Image 6&7-->Ospf database of router 1

Please let me know what other info you need.

Thanks

[rakeshvelagala]

Hi Peter,

Sorry I have checked and verified, previously I am redistributing another from the same NSSA ABR. 

Thanks

[Rajabov]

Explained very well. Thank you Peter!