Earlier I looked at stub areas. One problem we found was that you can’t have an ASBR in a stub area — no “redistribute static” on any external links. What a pity!
Not so stubby areas get around this by allowing the ASBR to exist and propagate LSAs. The problem is that stub areas can’t have type 5 LSAs (external), so NSSAs use a type 7 LSA which is converted back to a type 5 on the ABR as the LSA is flooded to the backbone. Because of this functionality, all routers need to be configured as a NSSA.
R4#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#router ospf 1
R4(config-router)#area 2 nssa
OSPF: Area is configured as stub area already
R4(config-router)#no area 2 stub
R4(config-router)#area 2 nssa
And on the ABR:
r3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
r3(config)#router ospf 1
r3(config-router)#no area 2 stub
r3(config-router)#area 2 nssa
Like the stub area, we have lost our external route to 1.1.1.1:
R4#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
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
Gateway of last resort is not set
4.0.0.0/24 is subnetted, 1 subnets
S 4.4.4.0 is directly connected, Null0
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C 10.0.2.0/24 is directly connected, FastEthernet0/0
O IA 10.0.0.0/24 [110/3] via 10.0.2.3, 00:00:27, FastEthernet0/0
O IA 10.0.1.0/24 [110/2] via 10.0.2.3, 00:00:27, FastEthernet0/0
O IA 10.0.33.1/32 [110/4] via 10.0.2.3, 00:00:27, FastEthernet0/0
Note there is no static route injected by the ABR! Another option is needed:
r3(config)#router ospf 1
r3(config-router)#area 2 nssa ?
default-information-originate Originate Type 7 default into NSSA area
no-redistribution No redistribution into this NSSA area
no-summary Do not send summary LSA into NSSA
r3(config-router)#area 2 nssa default-information-originate
Now, R4 has a default route:
O*N2 0.0.0.0/0 [110/1] via 10.0.2.3, 00:00:49, FastEthernet0/0
Note it is N2, meaning it is a type 7 (pseudo type 5) LSA.
R4#show ip ospf database nssa-external
OSPF Router with ID (10.0.2.4) (Process ID 1)
Type-7 AS External Link States (Area 2)
Routing Bit Set on this LSA
LS age: 102
Options: (No TOS-capability, No Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 10.0.2.3
LS Seq Number: 80000001
Checksum: 0x454E
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 10.0.2.3
External Route Tag: 0
Remarkably similar to the default route we saw in the stub area!
Now, to make use of the nssa features.
R4(config)#router ospf 1
R4(config-router)#redistribute static subnets
With redistribute static subnets on, R4 generates type 7 LSAs for the 4.4.4.0 prefix:
R4#show ip ospf database nssa-external 4.4.4.0
OSPF Router with ID (10.0.2.4) (Process ID 1)
Type-7 AS External Link States (Area 2)
LS age: 79
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 4.4.4.0 (External Network Number )
Advertising Router: 10.0.2.4
LS Seq Number: 80000001
Checksum: 0x367
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 10.0.2.4
External Route Tag: 0
And R3 sees it as a N2 route:
r3#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
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
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
O E2 1.1.1.0 [110/10] via 10.0.1.2, 00:02:13, FastEthernet0/0
4.0.0.0/24 is subnetted, 1 subnets
O N2 4.4.4.0 [110/20] via 10.0.2.4, 00:00:08, FastEthernet1/0
9.0.0.0/24 is subnetted, 1 subnets
S 9.9.9.0 is directly connected, Null0
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C 10.0.2.0/24 is directly connected, FastEthernet1/0
O IA 10.0.0.0/24 [110/2] via 10.0.1.2, 00:02:13, FastEthernet0/0
C 10.0.1.0/24 is directly connected, FastEthernet0/0
O IA 10.0.33.1/32 [110/3] via 10.0.1.2, 00:02:13, FastEthernet0/0
r3#
And way over on R1, it looks like a regular old external route:
r1>show ip route 4.4.4.0
Routing entry for 4.4.4.0/24
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 3
Last update from 10.0.0.2 on FastEthernet0/0, 00:01:11 ago
Routing Descriptor Blocks:
* 10.0.0.2, from 10.0.2.3, 00:01:11 ago, via FastEthernet0/0
Route metric is 20, traffic share count is 1
r1>show ip ospf data
r1>show ip ospf database e
r1>show ip ospf database external 4.4.4.0
OSPF Router with ID (10.0.33.1) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 87
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 4.4.4.0 (External Network Number )
Advertising Router: 10.0.2.3
LS Seq Number: 80000001
Checksum: 0x9DD7
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 10.0.2.4
External Route Tag: 0
There is also a "no-summary" option to creating an NSSA, which does exactly the same thing as in stub areas.
How Does OSPF Generate Default Routes?
OSPF Not-So-Stubby Area (NSSA)
Content Copyright Sean Walberg
Not So Stubby Areas