OSPF Stub Areas

There are four types of stub areas

• stub
• totally-stubby
• not-so-stubby areas (NSSA)
• totally not-so-stubby areas
• All stub areas do not allow Type 5 (external) LSAs (ABR always filters them)
• For the totally stub areas the ABR filters the Type 3 LSAs.

Stub areas

R1(config-router)# area 1 stub
R1(config-router)# area 1 default-metric (metric)

• stub has to be defined on both routers (Neighbor goes down with mismatch)
• ABR (R2) creates a default route into the stub area
  • O*IA 0.0.0.0/0 [110/11] via 10.10.10.2, 00:33:10, FastEthernet0/0
    
• inter area routes are allowed

R1#sh ip route
     1.0.0.0/24 is subnetted, 2 subnets
C       1.1.0.0 is directly connected, Loopback2
C       1.1.1.0 is directly connected, Loopback1
     2.0.0.0/24 is subnetted, 1 subnets
O IA    2.2.2.0 [110/20] via 10.10.10.2, 00:43:52, FastEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/21] via 10.10.10.2, 00:43:52, FastEthernet0/0
     20.0.0.0/32 is subnetted, 1 subnets
O       20.20.20.20 [110/11] via 10.10.10.2, 00:43:52, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.10.10.0 is directly connected, FastEthernet0/0
O*IA 0.0.0.0/0 [110/11] via 10.10.10.2, 00:43:54, FastEthernet0/0

Open Shortest Path First  (STUB ROUTER HELLO)
OSPF Header
OSPF Hello Packet
Network Mask: 255.255.255.0
Hello Interval [sec]: 10
Options: 0x10 ((L) LLS Data block)
0... .... = DN: Not set
.0.. .... = O: Not set
..0. .... = (DC) Demand Circuits: Not supported
...1 .... = (L) LLS Data block: Present
.... 0... = (N) NSSA: Not supported
.... .0.. = (MC) Multicast: Not capable
.... ..0. = (E) External Routing: Not capable
.... ...0 = (MT) Multi-Topology Routing: No
Router Priority: 1
Router Dead Interval [sec]: 40
Designated Router: 10.10.10.2
Backup Designated Router: 10.10.10.1
Active Neighbor: 20.20.20.2

Totally Stubby areas

R2(config-router)# area 1 stub no-summary

• Only has to be defined on the ABR (R2)
• Type 3 summery LSA are not allowed and are not displayed in the route table

R1#sh ip route  
1.0.0.0/24 is subnetted, 2 subnets
C 1.1.0.0 is directly connected, Loopback2
C 1.1.1.0 is directly connected, Loopback1
 20.0.0.0/32 is subnetted, 1 subnets
O 20.20.20.20 [110/11] via 10.10.10.2, 00:46:05, FastEthernet0/0
 10.0.0.0/24 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, FastEthernet0/0
O*IA 0.0.0.0/0 [110/11] via 10.10.10.2, 00:43:54, FastEthernet0/0

sh ip ospf
sh ip ospf database
sh ip ospf database database-summary

Not-so-stubby area (NSSA)

R1(config-router)# area 1 nssa

• external routes cannot be injected to stub/totally stubby. Routes within the area can’t redistribute
• NSSA are used for ASBR within a stub area to redistribute.
• ASBR within a NSSA creates a Type 7 NSSA-External LSA.
• If an ABR receives a Type 7 NSSA-External LSA it forwards a Type 5 LSA.
• ABRs Type 3 LSA are still advertized (same as stub area).
• ABRs will not create a default route (different than stub area).
  • R1(config-router)# area 1 nssa default-information-originate

R1#sh ip ospf database nssa-external

     OSPF Router with ID (1.1.1.1) (Process ID 1)
     Type-7 AS External Link States (Area 1)
LS age: 29
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 1.1.1.0 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0x3656
Length: 36
Network Mask: /24
      Metric Type: 2 (Larger than any link state path)
     TOS: 0
     Metric: 20
     Forward Address: 1.1.0.1
     External Route Tag: 0


R2# sh ip route
 1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O N2 1.1.1.0/24 [110/20] via 10.10.10.1, 00:03:16, FastEthernet0/0
O 1.1.0.1/32 [110/11] via 10.10.10.1, 00:08:22, FastEthernet0/0
 2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, FastEthernet0/1
 3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/11] via 2.2.2.1, 00:09:29, FastEthernet0/1
 20.0.0.0/24 is subnetted, 1 subnets
C 20.20.20.0 is directly connected, Loopback0
 10.0.0.0/24 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, FastEthernet0/0

Totally Not-so-stubby area (NSSA)

R1(config-router)# area 1 nssa no-summy

• ABR doesnt inject Type 3 LSAs in de area (same as totally stubby)