OSPF NSSA-NSSTSA

The general rule with OSPF is that you can't make an ASBR  a stub router but it is possible by bending the rules with a Not So Stubby Area. At this point the above configuration has been configured. What if we needed R3 to be an ASBR by redistributing another ospf route 14.14.14.14, area 34 could not be a stub area anymore because of the stub network rules. NSSA uses Type 7 LSAs to allow this breakage of the rules.


R3(config)#int lo14

R3(config-if)#ip address 14.14.14.14 255.255.255.255
R3(config-if)#router ospf 1
R3(config-router)#redistribute connected subnets

R1#show ip route ospf

     2.0.0.0/32 is subnetted, 1 subnets

O IA    2.2.2.2 [110/65] via 172.12.123.2, 00:01:02, Serial0/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/65] via 172.12.123.3, 00:01:02, Serial0/0

     172.34.0.0/24 is subnetted, 1 subnets

O IA    172.34.34.0 [110/65] via 172.12.123.3, 00:01:02, Serial0/0

     14.0.0.0/32 is subnetted, 1 subnets

O E2    14.14.14.14 [110/20] via 172.12.123.3, 00:01:02, Serial0/0

R1#show ip route rip  

R    5.0.0.0/8 [120/1] via 15.0.0.5, 00:00:21, FastEthernet0/0

R    6.0.0.0/8 [120/1] via 15.0.0.5, 00:00:21, FastEthernet0/0

R    7.0.0.0/8 [120/1] via 15.0.0.5, 00:00:21, FastEthernet0/0

R2#show ip route ospf 

     1.0.0.0/32 is subnetted, 1 subnets

O IA    1.1.1.1 [110/65] via 172.12.123.1, 00:06:53, Serial0/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/65] via 172.12.123.3, 00:06:53, Serial0/0

O E2 5.0.0.0/8 [110/20] via 172.12.123.1, 00:00:41, Serial0/0

O E2 6.0.0.0/8 [110/20] via 172.12.123.1, 00:00:41, Serial0/0

     172.34.0.0/24 is subnetted, 1 subnets

O IA    172.34.34.0 [110/65] via 172.12.123.3, 00:06:53, Serial0/0

O E2 7.0.0.0/8 [110/20] via 172.12.123.1, 00:00:41, Serial0/0

O E2 15.0.0.0/8 [110/20] via 172.12.123.1, 00:00:31, Serial0/0

R3#show ip route ospf 

     1.0.0.0/32 is subnetted, 1 subnets

O IA    1.1.1.1 [110/65] via 172.12.123.1, 00:07:28, Serial0/0

     2.0.0.0/32 is subnetted, 1 subnets

O IA    2.2.2.2 [110/65] via 172.12.123.2, 00:07:28, Serial0/0

O E2 5.0.0.0/8 [110/20] via 172.12.123.1, 00:01:16, Serial0/0

O E2 6.0.0.0/8 [110/20] via 172.12.123.1, 00:01:16, Serial0/0

O E2 7.0.0.0/8 [110/20] via 172.12.123.1, 00:01:16, Serial0/0

O E2 15.0.0.0/8 [110/20] via 172.12.123.1, 00:01:06, Serial0/0

R4#show ip route ospf 

     1.0.0.0/32 is subnetted, 1 subnets

O IA    1.1.1.1 [110/66] via 172.34.34.3, 00:10:16, FastEthernet0/0

     2.0.0.0/32 is subnetted, 1 subnets

O IA    2.2.2.2 [110/66] via 172.34.34.3, 00:10:16, FastEthernet0/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/2] via 172.34.34.3, 00:10:16, FastEthernet0/0

O E2 5.0.0.0/8 [110/20] via 172.34.34.3, 00:00:34, FastEthernet0/0

O E2 6.0.0.0/8 [110/20] via 172.34.34.3, 00:00:34, FastEthernet0/0

     172.12.0.0/24 is subnetted, 1 subnets

O IA    172.12.123.0 [110/65] via 172.34.34.3, 00:10:16, FastEthernet0/0

O E2 7.0.0.0/8 [110/20] via 172.34.34.3, 00:00:34, FastEthernet0/0

O E2 15.0.0.0/8 [110/20] via 172.34.34.3, 00:00:24, FastEthernet0/0

R5#show ip route rip

     1.0.0.0/32 is subnetted, 1 subnets

R       1.1.1.1 [120/2] via 15.0.0.1, 00:00:06, FastEthernet0/0

     2.0.0.0/32 is subnetted, 1 subnets

R       2.2.2.2 [120/2] via 15.0.0.1, 00:00:06, FastEthernet0/0

     3.0.0.0/32 is subnetted, 1 subnets

R       3.3.3.3 [120/2] via 15.0.0.1, 00:00:06, FastEthernet0/0

     172.12.0.0/24 is subnetted, 1 subnets

R       172.12.123.0 [120/2] via 15.0.0.1, 00:00:06, FastEthernet0/0

     172.34.0.0/24 is subnetted, 1 subnets

R       172.34.34.0 [120/2] via 15.0.0.1, 00:00:06, FastEthernet0/0

Now we want to make our routing tables complete yet concise and realize that R4 next hop address are all through area 34 to R3. We want to create a stub area but we can't because R3 is ASBR but with the NSSA command we still can. 

R3(config-router)#area 34 nssa    

R3(config-router)#

*Mar  1 00:47:59.031: %OSPF-5-ADJCHG: Process 1, Nbr 172.34.34.4 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Adjacency forced to reset

R4(config-router)#area 34 nssa

R4(config-router)#

*Mar  1 00:48:15.069: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Adjacency forced to reset

*Mar  1 00:48:18.711: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on FastEthernet0/0 from LOADING to FULL, Loading Done

R4 now has all external routes removed from the table but we can still make it smaller by using the no summary command, which is called a not so stubby total stub area.

R4#show ip route ospf 

     1.0.0.0/32 is subnetted, 1 subnets

O IA    1.1.1.1 [110/66] via 172.34.34.3, 00:00:24, FastEthernet0/0

     2.0.0.0/32 is subnetted, 1 subnets

O IA    2.2.2.2 [110/66] via 172.34.34.3, 00:00:24, FastEthernet0/0

     3.0.0.0/32 is subnetted, 1 subnets

O IA    3.3.3.3 [110/2] via 172.34.34.3, 00:00:24, FastEthernet0/0

     172.12.0.0/24 is subnetted, 1 subnets

O IA    172.12.123.0 [110/65] via 172.34.34.3, 00:00:24, FastEthernet0/0

     14.0.0.0/32 is subnetted, 1 subnets

O N2    14.14.14.14 [110/20] via 172.34.34.3, 00:00:24, FastEthernet0/0

R3(config-router)#area 34 nssa no-summary 

R4(config-router)#area 34 nssa no-summary

R4 has a default no summary route now like we had before with a total stub area.

R4#show ip route ospf 

     14.0.0.0/32 is subnetted, 1 subnets

O N2    14.14.14.14 [110/20] via 172.34.34.3, 00:00:23, FastEthernet0/0

O*IA 0.0.0.0/0 [110/2] via 172.34.34.3, 00:00:28, FastEthernet0/0