OSPF NSSA (Not So Stubby) Area

In the first lesson, I introduced the OSPF stub areas. I also covered the stub area and totally stub area. Now we get to see the OSPF NSSA (Not So Stubby) area. Here’s 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

The loopback 0 interface of R1 is redistributed into OSPF. I did this to test the stub area since it blocks LSA type 5. This is what the routing table of R3 looks like now:

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 and a default route as inter-area (LSA Type 3). Now to demonstrate the NSSA, I will create a new loopback interface on R3:

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Forum Replies

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

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

  3. 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.

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

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

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