Wednesday, August 3, 2011

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
 no auto-summary

R5#sh run | sec eigrp
router eigrp 20
 redistribute ospf 1 ** This is so we can redistribute OSPF **
 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
 area 1 nssa ** States that this is a 'not-so-stubby-area' **
 redistribute eigrp 20 subnets ** redistribute eigrp subnets ** 
 network area 1
 network area 2
 network area 1

R4#sh run | sec ospf
router ospf 1
 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 area 0
 network area 1
 network area 0

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

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 1

R5#sh ip route is subnetted, 1 subnets
C is directly connected, Loopback0 is subnetted, 3 subnets
C is directly connected, FastEthernet2/0
C is directly connected, FastEthernet1/0
O IA [110/2] via, 01:18:01, FastEthernet1/0
O*N2 [110/1] via, 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!!