OSPF nssa

Not-so-stubby-area

OSPF nssa is the area that prohibits the LSA type 5 into the area but instead allow LSA type 7 to the area for the external routes.

There are options added on to the nssa area
 - area x nssa no-summary
     This area does not allow also the LSA type 3 or above into the area.
 - area x nssa no-redistribute
     This command is used in the the router that acts both ABR and ASBR. This command will not allow redistributed routes LSA type 7 into the nssa area but still allow the type 5 LSA in the OSPF process in area 0 ( backbone area )

Example for the NSSA no-redistribute

From the network diagram above, R2 is working as both ABR and ASBR. It redistributes the network 2.2.2.2/32 into the OSFP domian. However we do not want the router to advertise LSA type 7 in to the nssa area. but it is acceptable to have LSA type 5 in the OSPF domain. Here is the configuration on the R2.


interface Loopback0
 ip address 2.2.2.2 255.255.255.255

router ospf 1
 log-adjacency-changes
 area 1 nssa no-redistribution
 redistribute connected subnets
 network 192.168.1.2 0.0.0.0 area 0
 network 192.168.2.1 0.0.0.0 area 1

Looking at the R1 routing table, we can see the 2.2.2.2/32 network in it.

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     2.0.0.0/32 is subnetted, 1 subnets
O E2    2.2.2.2 [110/20] via 192.168.1.2, 00:21:56, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
O IA 192.168.2.0/24 [110/20] via 192.168.1.2, 00:34:03, FastEthernet0/0
O IA 192.168.3.0/24 [110/30] via 192.168.1.2, 00:27:32, FastEthernet0/0

However R4 doesn't see the 2.2.2.2/32 network in its database and its routing table.

R4#sh ip route

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

       E1 - OSPF external type 1, E2 - OSPF external type 2

       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

       ia - IS-IS inter area, * - candidate default, U - per-user static route

       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     4.0.0.0/32 is subnetted, 1 subnets

C       4.4.4.4 is directly connected, Loopback0

O IA 192.168.1.0/24 [110/30] via 192.168.3.1, 00:29:46, FastEthernet0/0

O    192.168.2.0/24 [110/20] via 192.168.3.1, 00:29:46, FastEthernet0/0

C    192.168.3.0/24 is directly connected, FastEthernet0/0

So this illustrate how the nssa no-redistribute works.