OSPF IPv4 - Not So Stubby Area
Not So Stubby Area
In OSPF (Open Shortest Path First) for IPv4, a Not-So-Stubby Area (NSSA) is an OSPF area type that allows for the injection of external routes, including Autonomous System External Routes (ASER), into the OSPF domain without the full complexity of a standard OSPF Autonomous System. NSSAs are often employed when an OSPF network has connections to external networks, such as the internet, and it is desirable to maintain a degree of summarization within the OSPF domain. In an NSSA, the Area Border Router (ABR) injects a default route along with external routes, encapsulated in Type 7 LSAs (Link State Advertisements), which are then converted into Type 5 LSAs as they traverse the OSPF domain. This enables routers within the NSSA to reach external destinations without burdening the OSPF backbone with detailed external route information. NSSAs strike a balance between the simplicity of Stub Areas and the need to convey external routing information in OSPF networks.
Lab:
Task 1: Configure OSPF Not So Stubby Area (NSSA)
Step 1: In the configuration mode of router configure loopbacks & redistribute these connected network in OSPF process:
R1:
interface loopback 50
ip address 50.0.0.1 255.255.255.0
exit
R1:
router ospf 1
redistribute connected subnets
exit
R2#show ip route
! (Shows router’s routing table and IPv4 routes entries)
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
11.0.0.0/32 is subnetted, 3 subnets
O 11.0.1.1 [110/65] via 12.0.0.1, 01:20:34, Serial2/0
O 11.0.2.1 [110/65] via 12.0.0.1, 01:20:34, Serial2/0
O 11.0.3.1 [110/65] via 12.0.0.1, 01:20:34, Serial2/0
12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 12.0.0.0/24 is directly connected, Serial2/0
L 12.0.0.2/32 is directly connected, Serial2/0
22.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
C 22.0.1.0/24 is directly connected, Loopback1
L 22.0.1.2/32 is directly connected, Loopback1
C 22.0.2.0/24 is directly connected, Loopback2
L 22.0.2.2/32 is directly connected, Loopback2
C 22.0.3.0/24 is directly connected, Loopback3
L 22.0.3.2/32 is directly connected, Loopback3
23.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
Step 2: Configure OSPF Not So Stubby Area
! (When loop50 is redistributed in stub area, external routes LSA 5 will no propagate in stub area and hence cannot be seen in any other router’s routing table. To make seen this redistributed route to all other routers, replace stub area to Not So Stubby Area.)
R1:
router ospf 1
area 1 nssa
exit
R2:
router ospf 1
area 1 nssa
exit
Task 2: Verification:
Step 1: Verify routes on neighbor router routing table:
R2#show ip route
! (Shows router’s routing table and IPv4 routes entries)
Codes: L - local, 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, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
11.0.0.0/32 is subnetted, 3 subnets
O 11.0.1.1 [110/65] via 12.0.0.1, 00:00:14, Serial2/0
O 11.0.2.1 [110/65] via 12.0.0.1, 00:00:14, Serial2/0
O 11.0.3.1 [110/65] via 12.0.0.1, 00:00:14, Serial2/0
12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 12.0.0.0/24 is directly connected, Serial2/0
L 12.0.0.2/32 is directly connected, Serial2/0
22.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
C 22.0.1.0/24 is directly connected, Loopback1
L 22.0.1.2/32 is directly connected, Loopback1
C 22.0.2.0/24 is directly connected, Loopback2
L 22.0.2.2/32 is directly connected, Loopback2
C 22.0.3.0/24 is directly connected, Loopback3
L 22.0.3.2/32 is directly connected, Loopback3
23.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 23.0.0.0/24 is directly connected, Ethernet0/0
L 23.0.0.2/32 is directly connected, Ethernet0/0
33.0.0.0/32 is subnetted, 3 subnets
O 33.0.1.3 [110/11] via 23.0.0.3, 00:00:29, Ethernet0/0
O 33.0.2.3 [110/11] via 23.0.0.3, 00:00:29, Ethernet0/0
O 33.0.3.3 [110/11] via 23.0.0.3, 00:00:29, Ethernet0/0
34.0.0.0/24 is subnetted, 1 subnets
O IA 34.0.0.0 [110/74] via 23.0.0.3, 00:00:29, Ethernet0/0
44.0.0.0/32 is subnetted, 3 subnets
O IA 44.0.1.4 [110/75] via 23.0.0.3, 00:00:29, Ethernet0/0
O IA 44.0.2.4 [110/75] via 23.0.0.3, 00:00:29, Ethernet0/0
O IA 44.0.3.4 [110/75] via 23.0.0.3, 00:00:29, Ethernet0/0
50.0.0.0/24 is subnetted, 1 subnets
O N2 50.0.0.0 [110/20] via 12.0.0.1, 00:00:14, Serial2/0
(R2 receives external routes E2 through Not so Stubby Area (NSSA) and marked as N2.)
R3#show ip route
! (Shows router’s routing table and IPv4 routes entries)
Codes: L - local, 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, + - replicated route
Gateway of last resort is not set
11.0.0.0/32 is subnetted, 3 subnets
O IA 11.0.1.1 [110/66] via 23.0.0.2, 00:11:01, FastEthernet0/0
O IA 11.0.2.1 [110/66] via 23.0.0.2, 00:11:01, FastEthernet0/0
O IA 11.0.3.1 [110/66] via 23.0.0.2, 00:11:01, FastEthernet0/0
12.0.0.0/24 is subnetted, 1 subnets
O IA 12.0.0.0 [110/65] via 23.0.0.2, 00:58:33, FastEthernet0/0
22.0.0.0/32 is subnetted, 3 subnets
O 22.0.1.2 [110/2] via 23.0.0.2, 00:58:33, FastEthernet0/0
O 22.0.2.2 [110/2] via 23.0.0.2, 00:58:33, FastEthernet0/0
O 22.0.3.2 [110/2] via 23.0.0.2, 00:58:33, FastEthernet0/0
23.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 23.0.0.0/24 is directly connected, FastEthernet0/0
L 23.0.0.3/32 is directly connected, FastEthernet0/0
33.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
C 33.0.1.0/24 is directly connected, Loopback1
L 33.0.1.3/32 is directly connected, Loopback1
C 33.0.2.0/24 is directly connected, Loopback2
L 33.0.2.3/32 is directly connected, Loopback2
C 33.0.3.0/24 is directly connected, Loopback3
L 33.0.3.3/32 is directly connected, Loopback3
34.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 34.0.0.0/24 is directly connected, Serial1/0
L 34.0.0.3/32 is directly connected, Serial1/0
44.0.0.0/32 is subnetted, 3 subnets
O 44.0.1.4 [110/65] via 34.0.0.4, 00:56:11, Serial1/0
O 44.0.2.4 [110/65] via 34.0.0.4, 00:56:01, Serial1/0
O 44.0.3.4 [110/65] via 34.0.0.4, 00:56:01, Serial1/0
50.0.0.0/24 is subnetted, 1 subnets
O E2 50.0.0.0 [110/20] via 23.0.0.2, 00:11:01, FastEthernet0/0
(R3 receives external routes E2 through Not so Stubby Area (NSSA) forwarded by ABR R2 and marked as E2.)