OSPF: Stub, totally stubby, not-so-stubby areas

Stub area: filter type 4 and 5 LSA.


R3#sh ip route

1.0.0.0/32 is subnetted, 1 subnets
O IA    1.1.1.1 [110/66] via 10.32.0.2, 00:00:42, Serial1/0
2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/65] via 10.32.0.2, 00:00:42, Serial1/0
3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 2 subnets
C       10.32.0.0 is directly connected, Serial1/0
O IA    10.100.0.0 [110/65] via 10.32.0.2, 00:00:42, Serial1/0
O*IA 0.0.0.0/0 [110/65] via 10.32.0.2, 00:00:42, Serial1/0

R3’s serial1/0 is configured as a stub network. R2’s Se1/0 is also configured as stub network, from the routing table type 3 LSA still propagates in Area 32.

To reach network outside of Area 32, R3 only needs to know how to go through R2 hence knowing type 3 network summary LSA is not necessary. To further filter away type 3 LSA, Cisco has its own proprietary Totally-stubby area.

R2(config-router)#area 32 stub no-summary

Totally stubby area command is as above, this command is applied on R2’s se1/0 interface, this will make R2 filter away type 3 LSA as well. Look at the routing table of R3 again:

R3#sh ip route
3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 1 subnets
C       10.32.0.0 is directly connected, Serial1/0
O*IA 0.0.0.0/0 [110/65] via 10.32.0.2, 00:00:57, Serial1/0

Only the type 3 LSA with a default route entry is received, type 3 to type 5 LSAs are all filtered by R2 of Area 32, this save a lot of resources for R3 in Area 32.

Not-so-stubby-area, is a stub area which needs to distribute external routes into OSPF, however if a stub area is unable to distribute external route into a stub area because type 4 and 5 LSAs are filtered, hence for a stub area to be able to redistribute external route and still maintain a stubby area status Not-so-stubby-area is configured.

NSSA generates type 7 NSSA external LSA and will only propagate within NSSA, if the Hello packet option has a P-bit set to 1 ABR of NSSA will translate type 7 to type 5 LSA and propagate to other areas including the backbone area. If Hello packet of option has a P-bit set to 0 then ABR of NSSA will drop type 7 LSA.

R4’s serial 1/0 which is in Area 45 must also configured as NSSA otherwise adjacency will not be formed with R4’s serial 1/0 interface. In addition to configure R4’s serial 1/0 as NSSA, totally-stubby is also configured to filter away type 3, 4 and 5 LSA coming into Area 45.

Before area 45 nssa no-summary command is configured, R5’s routing table contains type 3 LSA, again R5 does not need to know route to other areas it only needs to know how to route through R4 to get to other areas:

R5#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA    1.1.1.1 [110/66] via 10.45.0.4, 00:01:28, Serial1/0
2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/66] via 10.45.0.4, 00:01:28, Serial1/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/130] via 10.45.0.4, 00:01:28, Serial1/0
4.0.0.0/32 is subnetted, 1 subnets
O IA    4.4.4.4 [110/65] via 10.45.0.4, 00:01:28, Serial1/0
172.30.0.0/16 is variably subnetted, 5 subnets, 2 masks
O       172.30.0.0/22 is a summary, 00:01:32, Null0
10.0.0.0/24 is subnetted, 3 subnets
O IA    10.32.0.0 [110/129] via 10.45.0.4, 00:01:28, Serial1/0
O IA    10.100.0.0 [110/65] via 10.45.0.4, 00:01:28, Serial1/0


After area 45 nssa no-summary is configured in R4’s serial 1/0 interface, R5’s routing table is reduced:

R5(config-router)#do sh ip route ospf
172.30.0.0/16 is variably subnetted, 5 subnets, 2 masks
O       172.30.0.0/22 is a summary, 00:00:37, Null0
O*IA 0.0.0.0/0 [110/65] via 10.45.0.4, 00:00:23, Serial1/0

R4#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/2] via 10.100.0.1, 00:16:23, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/2] via 10.100.0.2, 00:16:23, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/66] via 10.100.0.2, 00:15:26, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O       5.5.5.5 [110/65] via 10.45.0.5, 00:15:26, Serial1/0
172.30.0.0/22 is subnetted, 1 subnets
O N2    172.30.0.0 [110/200] via 10.45.0.5, 00:15:26, Serial1/0
10.0.0.0/24 is subnetted, 3 subnets
O IA    10.32.0.0 [110/65] via 10.100.0.2, 00:15:26, FastEthernet0/0

R4’s routing table contains N2 route which is from type 7 LSA.

R1#sh ip route ospf
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/2] via 10.100.0.2, 00:17:33, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/66] via 10.100.0.2, 00:17:33, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/2] via 10.100.0.4, 00:17:33, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O IA    5.5.5.5 [110/66] via 10.100.0.4, 00:16:36, FastEthernet0/0
172.30.0.0/22 is subnetted, 1 subnets
O E2    172.30.0.0 [110/200] via 10.100.0.4, 00:16:31, FastEthernet0/0
10.0.0.0/24 is subnetted, 3 subnets
O IA    10.45.0.0 [110/65] via 10.100.0.4, 00:17:33, FastEthernet0/0
O IA    10.32.0.0 [110/65] via 10.100.0.2, 00:17:33, FastEthernet0/0

As highlighted in R1’s routing table, R1 received external route which is from type 5 AS external LSA.

R2#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/2] via 10.100.0.1, 00:18:45, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/65] via 10.32.0.3, 01:17:13, Serial1/0
4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/2] via 10.100.0.4, 00:18:45, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O IA    5.5.5.5 [110/66] via 10.100.0.4, 00:17:48, FastEthernet0/0
172.30.0.0/22 is subnetted, 1 subnets
O E2    172.30.0.0 [110/200] via 10.100.0.4, 00:17:43, FastEthernet0/0
10.0.0.0/24 is subnetted, 3 subnets
O IA    10.45.0.0 [110/65] via 10.100.0.4, 00:18:45, FastEthernet0/0

Area 78 is discontiguous, OSPF need all areas to be connected to backbone area (area 0), Area 78 is not directly connected to Area 0, hence Area 78 route will not propagate to other areas nor can Area 78 receive LSA from other areas.

R8#sh ip route
8.0.0.0/32 is subnetted, 1 subnets
C       8.8.8.8 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 1 subnets
C       10.78.1.0 is directly connected, Serial1/1

There’s no OSPF route in R8’s routing table.

R8#sh ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
7.7.7.7           0   FULL/  –        00:00:37    10.78.1.7       Serial1/1

R8 however has established full adjacency with R7 in Area 78. Area 78 is totally isolated.

R7#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA    1.1.1.1 [110/66] via 10.67.1.6, 00:03:12, Serial1/0
2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/66] via 10.67.1.6, 00:03:12, Serial1/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/130] via 10.67.1.6, 00:03:12, Serial1/0
4.0.0.0/32 is subnetted, 1 subnets
O IA    4.4.4.4 [110/66] via 10.67.1.6, 00:03:12, Serial1/0
5.0.0.0/32 is subnetted, 1 subnets
O IA    5.5.5.5 [110/130] via 10.67.1.6, 00:03:12, Serial1/0
6.0.0.0/32 is subnetted, 1 subnets
O IA    6.6.6.6 [110/65] via 10.67.1.6, 00:03:12, Serial1/0
172.30.0.0/22 is subnetted, 1 subnets
O E2    172.30.0.0 [110/200] via 10.67.1.6, 00:03:12, Serial1/0
8.0.0.0/32 is subnetted, 1 subnets
O       8.8.8.8 [110/65] via 10.78.1.8, 00:03:12, Serial1/1
10.0.0.0/24 is subnetted, 5 subnets
O IA    10.45.0.0 [110/129] via 10.67.1.6, 00:03:12, Serial1/0
O IA    10.32.0.0 [110/129] via 10.67.1.6, 00:03:12, Serial1/0
O IA    10.100.0.0 [110/65] via 10.67.1.6, 00:03:12, Serial1/0

R7 however is able to receive all OSPF routes in its routing table, because R7 has two interfaces in Area 67 (which connects to Area 0) and Area 78 (which does not connect to Area 0), the OSPF route actually is learned from Area 67 as Area 67 receive LSAs from other areas.

To resolve this temporary, virtual link should be configured. Virtual link is like a tunnel, tunnels through Area 67 to connect to Area 0.

R7(config)#router ospf 1
R7(config-router)#area 67 virtual-link 6.6.6.6

R6(config)#router ospf 1
R6(config-router)#area 67 virtual-link 7.7.7.7

After virtual link is configured, R8 received OSPF routes in its routing table:

R8#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA    1.1.1.1 [110/130] via 10.78.1.7, 00:00:42, Serial1/1
2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/130] via 10.78.1.7, 00:00:42, Serial1/1
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/194] via 10.78.1.7, 00:00:42, Serial1/1
4.0.0.0/32 is subnetted, 1 subnets
O IA    4.4.4.4 [110/130] via 10.78.1.7, 00:00:42, Serial1/1
5.0.0.0/32 is subnetted, 1 subnets
O IA    5.5.5.5 [110/194] via 10.78.1.7, 00:00:42, Serial1/1
6.0.0.0/32 is subnetted, 1 subnets
O IA    6.6.6.6 [110/129] via 10.78.1.7, 00:00:42, Serial1/1
172.30.0.0/22 is subnetted, 1 subnets
O E2    172.30.0.0 [110/200] via 10.78.1.7, 00:00:37, Serial1/1
7.0.0.0/32 is subnetted, 1 subnets
O IA    7.7.7.7 [110/65] via 10.78.1.7, 00:02:02, Serial1/1
10.0.0.0/24 is subnetted, 5 subnets
O IA    10.45.0.0 [110/193] via 10.78.1.7, 00:00:42, Serial1/1
O IA    10.32.0.0 [110/193] via 10.78.1.7, 00:00:42, Serial1/1
O IA    10.67.1.0 [110/128] via 10.78.1.7, 00:02:02, Serial1/1
O IA    10.100.0.0 [110/129] via 10.78.1.7, 00:00:42, Serial1/1

R1 in backbone area (area 0) also knows the existence of R8 and Area 78:

R1#sh ip route ospf
2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/2] via 10.100.0.2, 00:01:58, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/66] via 10.100.0.2, 00:01:58, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/2] via 10.100.0.4, 00:01:58, FastEthernet0/0
5.0.0.0/32 is subnetted, 1 subnets
O IA    5.5.5.5 [110/66] via 10.100.0.4, 00:01:58, FastEthernet0/0
6.0.0.0/32 is subnetted, 1 subnets
O       6.6.6.6 [110/2] via 10.100.0.6, 00:01:58, FastEthernet0/0
172.30.0.0/22 is subnetted, 1 subnets
O E2    172.30.0.0 [110/200] via 10.100.0.4, 00:01:58, FastEthernet0/0
7.0.0.0/32 is subnetted, 1 subnets
O IA    7.7.7.7 [110/66] via 10.100.0.6, 00:01:58, FastEthernet0/0
8.0.0.0/32 is subnetted, 1 subnets
O IA    8.8.8.8 [110/130] via 10.100.0.6, 00:01:58, FastEthernet0/0
10.0.0.0/24 is subnetted, 5 subnets
O IA    10.45.0.0 [110/65] via 10.100.0.4, 00:01:58, FastEthernet0/0
O IA    10.32.0.0 [110/65] via 10.100.0.2, 00:01:58, FastEthernet0/0
O IA    10.78.1.0 [110/129] via 10.100.0.6, 00:01:58, FastEthernet0/0
O IA    10.67.1.0 [110/65] via 10.100.0.6, 00:01:58, FastEthernet0/0

The GNS3 file appears to be corrupted, R10 and R11 had no data in the file. I deleted R11 to save time as I think udp port used by R11 clashed with other unknown applications that could be using it.

R2#sh ip route
1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/2] via 10.100.0.1, 00:00:41, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback0
3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/65] via 10.32.0.3, 00:05:55, Serial1/0
4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/2] via 10.100.0.4, 00:00:41, FastEthernet0/0
6.0.0.0/32 is subnetted, 1 subnets
O       6.6.6.6 [110/2] via 10.100.0.6, 00:00:41, FastEthernet0/0
7.0.0.0/32 is subnetted, 1 subnets
O IA    7.7.7.7 [110/66] via 10.100.0.6, 00:00:41, FastEthernet0/0
8.0.0.0/32 is subnetted, 1 subnets
O IA    8.8.8.8 [110/130] via 10.100.0.6, 00:00:42, FastEthernet0/0
10.0.0.0/24 is subnetted, 5 subnets
O IA    10.45.0.0 [110/65] via 10.100.0.4, 00:00:42, FastEthernet0/0
C       10.32.0.0 is directly connected, Serial1/0
O IA    10.78.1.0 [110/129] via 10.100.0.6, 00:00:42, FastEthernet0/0
O IA    10.67.1.0 [110/65] via 10.100.0.6, 00:00:42, FastEthernet0/0
C       10.100.0.0 is directly connected, FastEthernet0/0
O E2 192.168.0.0/23 [110/10] via 10.100.0.1, 00:00:43, FastEthernet0/0

RIP route shown in R2.

R8#sh ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O IA    1.1.1.1 [110/130] via 10.78.1.7, 00:07:24, Serial1/1
2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/130] via 10.78.1.7, 00:07:21, Serial1/1
3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/194] via 10.78.1.7, 00:07:21, Serial1/1
4.0.0.0/32 is subnetted, 1 subnets
O IA    4.4.4.4 [110/130] via 10.78.1.7, 00:07:24, Serial1/1
6.0.0.0/32 is subnetted, 1 subnets
O IA    6.6.6.6 [110/129] via 10.78.1.7, 00:07:24, Serial1/1
7.0.0.0/32 is subnetted, 1 subnets
O IA    7.7.7.7 [110/65] via 10.78.1.7, 00:07:24, Serial1/1
10.0.0.0/24 is subnetted, 5 subnets
O IA    10.45.0.0 [110/193] via 10.78.1.7, 00:07:24, Serial1/1
O IA    10.32.0.0 [110/193] via 10.78.1.7, 00:07:21, Serial1/1
O IA    10.67.1.0 [110/128] via 10.78.1.7, 00:07:24, Serial1/1
O IA    10.100.0.0 [110/129] via 10.78.1.7, 00:07:24, Serial1/1
O E2 192.168.0.0/23 [110/10] via 10.78.1.7, 00:02:01, Serial1/1

Because Area 78 is a non-stub area it receives the type 5 LSA and hence in routing table contains RIPv2 route as well.

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