Configuring OSPF Stub Areas in Cisco

Prerequisites: OSPF Implementation and Open Shortest Path First (OSPF) protocol States

Open Shortest Path First (OSPF) is a link-state routing protocol used to find the best path between the source and destination router using its own shortest path first). OSPF was developed by the Internet Engineering Task Force (IETF) as one of the Internal Gateway Protocols (IGP), i.e., a protocol aimed at moving packets within a large autonomous system or routing domain. . It is a network layer protocol that operates on protocol number 89 and uses the AD value 110. OSPF uses the multicast address 224.0.0.5 for normal communication and 224.0.0.6 for updates to the Designated Router (DR)/ Backup Designated Router (BDR).

OSPF Regular Area: 

Areas that don’t perform any automatic filtering on the type of accepted information are called regular areas.

OSPF Stubby Areas:

• To reduce the overhead with several variations of rules in each area, OSPF makes use of a concept called a stubby area.
• These types of areas take advantage of the fact that not all areas in an actual network topology need to have knowledge of individual external networks.
• Doesn’t contain an ASBR and thus does not meditate external connectivity to an entire OSPF domain.
• Such areas can benefit from filtering out type 4 and 5 LSAs.
• The stubby area can contain one or more ABRs.

If the area containing ABR is configured as a stubby area, the following things will happen : 

1. ABRs will stop advertising type 4 & type 5 LSAs in this area.
2. Every internal router in the stubby area will ignore any received type 5 LSAs.
3. Internal routers will also not originate any such LSAs.
4. ABRs will automatically inject a default route into the area as a type 3 LSA.
5. Internal routers will be able to reach external networks but their LSDBs will be sparser.

OSPF Stubby Area Types:

Type	Stops injection of  type 4/5 LSAs	Stops injection of  type 3 LSA	Internal routers originate  type 7 LSA
Stubby	Yes	No	No
Totally stubby (TS)	Yes	Yes	No
Not-So-Stubby-Area (NSSA)	Yes	No	Yes
Totally NSSA (NSSA-TS)	Yes	Yes	Yes

Different types of stubby area configuration commands: 

Type	Router OSPF subcommand
Stubby	area <area-id> stub
Totally stubby	area <area-id> stub no-summary
NSSA	area <area-id> nssa
Totally NSSA	area <area-id> nssa no-summary

Configuring OSPF Stub Areas:

Step 1: Create the physical topology in GNS3 as shown in the image below :

Step 2: Configure the IPv4 address on the physical interface on the routers :

On R1 :

R1(config)#interface FastEthernet0/0
R1(config-if)#ip address 10.1.1.1 255.255.255.0

On R2 :

R2(config)#interface FastEthernet0/0
R2(config-if)#ip address 10.1.1.2 255.255.255.0

R2(config-if)#interface FastEthernet1/0
R2(config-if)#ip address 20.1.1.1 255.255.255.0

On R3 :

R3(config)#interface FastEthernet1/0
R3(config-if)#ip address 20.1.1.2 255.255.255.0

Step 3: Configure a loopback interface on R1 with an IPv4 address :

R1(config)#interface Loopback0
R1(config-if)#ip address 2.2.2.2 255.255.255.255

Step 4: Configuring the OSPF process on all three routers with the command router OSPF <process-id> :

R1(config)#router ospf 1
R1(config-router)#exit

R2(config)#router ospf 1
R2(config-router)#exit

R3(config)#router ospf 1
R3(config-router)#exit

Step 5: Enabling OSPF on the interfaces with the interface level sub-command IP OSPF <process-id> area <area-id> :

R1(config)#interface FastEthernet0/0
R1(config-if)#ip ospf 1 area 0
R1(config)#interface Loopback0
R1(config-if)#ip ospf 1 area 0

R2(config)#interface FastEthernet0/0
R2(config-if)#ip ospf 1 area 0
R2(config-if)#interface FastEthernet1/0
R2(config-if)#ip ospf 1 area 1

R3(config)#interface FastEthernet1/0
R3(config-if)#ip ospf 1 area 1

Step 6: Configuring area 2 as a stub area with the router sub-command area <area-id> stub

R2(config)#router ospf 1
R2(config-router)#area 1 stub
R2(config-router)#exit

R3(config)#router ospf 1
R3(config-router)#area 1 stub
R3(config-router)#exit

Step 7: Verifying OSPF configuration on the routers :

R1#show ip protocol

R2#show ip protocol

R3#show ip protocol

Step 8: Checking the IPv4 routing table on the routers and seeing OSPF route entry :

R1#show ip route

R2#show ip route

R3#show ip route