How to configure OSPF Totally NSSA Area

OSPF has different special area types, I covered these before:

Now it’s time to demonstrate the totally NSSA area. Here is the topology we will use:

ospf stub area

And here’s the configuration. We will turn OSPF Area 1 into a stub area:

R1(config)#router ospf 1
R1(config-router)#network area 0
R1(config-router)#redistribute connected subnets
R2(config)#router ospf 1
R2(config-router)#area 1 stub
R2(config-router)#network area 0 
R2(config-router)#network area 1
R3(config)#router ospf 1
R3(config-router)#area 1 stub
R3(config-router)#network area 1

Let’s see what the routing table of R3 looks like:

R3#show ip route ospf 
O IA [110/2] via, 00:00:42, FastEthernet0/0
O*IA [110/2] via, 00:00:42, FastEthernet0/0

When we look at R3 you’ll see network /24 as inter-area (LSA Type 3) and /24 as external type 2 (LSA Type 5). First we’ll create a loopback interface on R3 and redistribute it into OSPF to show you the NSSA area.

R3(config)#interface loopback 0
R3(config-if)#ip address
R3(config)#router ospf 1
R3(config-router)#redistribute connected subnets

I’ll add a new loopback interface and try to redistribute it into OSPF on R3, here’s what you’ll see:

R3# %OSPF-4-ASBR_WITHOUT_VALID_AREA: Router is currently an ASBR while having only one area which is a stub area

The stub and totally stub area block LSA Type 5 so it’s impossible to have an ASBR within these areas. First I’ll change area 1 into an NSSA to allow this ASBR:

We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You've Ever Spent on Your Cisco Career!
  • Full Access to our 660 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)

510 Sign Ups in the last 30 days

100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!


Forum Replies

  1. It has to do with the design of the network in question. The benefits you see of an area becoming a stub (reduced LSDB size) comes at a cost, which is the loss of some routing information details. This translates into routers within a stub not having all the information necessary to make the best possible choices.

    For example, suppose you have an area (which is non-zero, of course), that has multiple exit points. Now imagine that at each of those exit points there are separate external routing domains (say, EIGRP or BGP, etc). If this area is a stub, Type-5

    ... Continue reading in our forum

  2. Thanks for making a video on this series. I am always thankful when I get to a video series as I read the book and website pages and just get worn out from studying 2-6 hours a day through the week. Not to mention on CCNP ROUTE I have taken to reading every single forum post as an added learning tool.

    Sometimes I just want to lean back in chair put on headset and listen to video as it allows me to relax a bit when tired towards end of day so uch thanks for those videos. Anyway great lesson!

    I am getting close to end of OSPF website lessons already finished

    ... Continue reading in our forum

  3. Hello sumu

    The NSSA area is an OSPF area where you know there are no other OSPF routers participating in OSPF beyond this interface, however, you know that there is an ASBR router found within that area. An ASBR is a router that connects to non-OSPF autonomous systems. No OSPF goes beyond this area, however, only other AS information should be relayed here. You can find out a more comprehensive explanation from Cisco at this Cisco documentation.

    I hope this has been helpful!


  4. HI,
    I have a question on this point.
    Doesnt ABR just generate type3 LSA or type 4 to let us know of ASBR.
    Now in TSA, the only allowed LSA’s are 1 & 2. So how does ABR help ?
    if an ABR is configured with default originate always, my understanding is that route alwas appears as an O *E2 or Type 5 or External route in the downlink routers . But I am seeing it as O *IA in my setup, R1 – R2(ABR) – R3(TSA) , pls let me know why ?
    Addtionally in show ip ospf dbs cmd on TSA router , I see only LSA1 and Sumary Net link States (LSA3) being shown, should not LSA 3 be b

    ... Continue reading in our forum

  5. Hello Sahil

    An ABR will generate type 3 LSAs in order to inform other areas of the routes that are found in a particular area. Type 3 LSAs are sometimes known as a summary LSA, that summarises all networks within an area. A type 4 LSA is used to inform other areas of the existence of an ASBR.

    ... Continue reading in our forum

34 more replies! Ask a question or join the discussion by visiting our Community Forum