Configuring OSPF - NSSA (Not-So-Stubby-Areas)

In this blogtorial we are going to talk about OSPF Not-So-Stubby-Area (NSSA). So we learned from my previous blogtorial that we can turn an Area into a stubby area to reduce the size of the database, but what if that Area is also connected to another domain such as an EIGRP? Well that's where NSSA comes in.


We will be building on this topology. Similar to the topology from my previous blogtorial but slightly different. Note that the topology shows more than what we really need so just pay attention to Area 0, Area 1 and eigrp domain 20. Alright let's get started.

Relevant configurations are posted below. 

R12#sh run | sec eigrp ** Just configuring good ole EIGRP 20 **

router eigrp 20

 network 10.4.4.0 0.0.0.255

 no auto-summary

R5#sh run | sec eigrp

router eigrp 20

 redistribute ospf 1 ** This is so we can redistribute OSPF **

 network 10.4.4.0 0.0.0.255

 default-metric 100000 1 0 1 1500 ** If you do not set the seed metric none of the routes from ospf will make it into EIGRP 20 **

 no auto-summary

R5#sh run | sec ospf

router ospf 1

 log-adjacency-changes

 area 1 nssa ** States that this is a 'not-so-stubby-area' **

 redistribute eigrp 20 subnets ** redistribute eigrp subnets ** 

 network 10.1.1.0 0.0.0.255 area 1

 network 10.3.3.0 0.0.0.255 area 2

 network 172.16.5.0 0.0.0.255 area 1

R4#sh run | sec ospf

router ospf 1

 log-adjacency-changes

 area 1 nssa default-information-originate ** this is so a default route also gets pushed to area 1 as well state that area 1 is 'not-so-stubby-area' **

 network 10.0.0.0 0.0.0.255 area 0

 network 10.1.1.0 0.0.0.255 area 1

 network 192.168.1.0 0.0.0.255 area 0

Notice how I can ping R12 interface from R4 in Area 0 and check R5 RIB. 

R4#ping 10.4.4.12

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.4.4.12, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1

R5#sh ip route

     172.16.0.0/24 is subnetted, 1 subnets

C       172.16.5.0 is directly connected, Loopback0

     10.0.0.0/24 is subnetted, 3 subnets

C       10.4.4.0 is directly connected, FastEthernet2/0

C       10.1.1.0 is directly connected, FastEthernet1/0

O IA    10.0.0.0 [110/2] via 10.1.1.4, 01:18:01, FastEthernet1/0

O*N2 0.0.0.0/0 [110/1] via 10.1.1.4, 01:18:01, FastEthernet1/0

R5#sh ip ospf 

<-- output shortened --> 

 Area 1

        Number of interfaces in this area is 2 (1 loopback)

        It is a NSSA area

        Area has no authentication

        SPF algorithm last executed 01:22:14.696 ago

        SPF algorithm executed 9 times

        Area ranges are

        Number of LSA 6. Checksum Sum 0x0841FF

        Number of opaque link LSA 0. Checksum Sum 0x000000

        Number of DCbitless LSA 0

        Number of indication LSA 0

        Number of DoNotAge LSA 0

        Flood list length 0

Remember in a NSSA all IGRP routes will be advertised as Type 7 LSA inside NSSA, however they will be converted into Type 5 LSA by the ABR (so R4 in this case) and will be leaked into the OSPF domain. NSSA will allow Type 3 LSAs. 

Many more articles to come so stay tuned!! If you like my posts please subscribe!!