How to configure OSPF Stub Area

Lessons Discussion
41

Hello @lagapidis
Thanks for your answer. I’m using the same topology, but On R2 I’m getting LSA type5. On R3 I stop to get it and I see the route 0.0.0.0/0
I will share my topology

Captura de pantalla 2025-02-06 a la(s) 9.23.23
Captura de pantalla 2025-02-06 a la(s) 9.23.231222×438 39.6 KB

Mi configuration en R1 is this:

interface Loopback0
 ip address 1.1.1.1 255.255.255.0
!
interface GigabitEthernet0/0
 ip address 192.168.12.1 255.255.255.0
 ip ospf 1 area 0
 duplex auto
 speed auto
 media-type rj45
!
!
router ospf 1
 router-id 0.0.0.1
 redistribute connected subnets
!

Mi configuration on R2 is this:

!
interface GigabitEthernet0/0
 ip address 192.168.12.2 255.255.255.0
 ip ospf 1 area 0
 duplex auto
 speed auto
 media-type rj45
!
interface GigabitEthernet0/1
 ip address 192.168.23.2 255.255.255.0
 ip ospf 1 area 1
 duplex auto
 speed auto
 media-type rj45
!
router ospf 1
 router-id 0.0.0.2
 area 1 stub

The configuration on R3 is this:

!
router ospf 1
 router-id 0.0.0.2
 area 1 stub
!
router ospf 1
 router-id 0.0.0.3
 area 1 stub

Now the database and routing table on R3 look like this:

R3#show ip ospf database 

            OSPF Router with ID (0.0.0.3) (Process ID 1)

Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
0.0.0.2         0.0.0.2         1392        0x80000054 0x00379F 1
0.0.0.3         0.0.0.3         104         0x80000054 0x0032A2 1

Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.23.3    0.0.0.3         104         0x80000051 0x0077E8

Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         0.0.0.2         1392        0x8000004E 0x000BE3
192.168.12.0    0.0.0.2         1392        0x8000004F 0x00D2A5
R3# show ip route
O*IA  0.0.0.0/0 [110/2] via 192.168.23.2, 1d19h, GigabitEthernet0/0
O IA  192.168.12.0/24 [110/2] via 192.168.23.2, 1d19h, GigabitEthernet0/0
      192.168.23.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.23.0/24 is directly connected, GigabitEthernet0/0
L        192.168.23.3/32 is directly connected, GigabitEthernet0/0

Now the big question is about R2, cause I still se LSA type 5 and I’m not sure if that is a normal behavior

Link ID         ADV Router      Age         Seq#       Checksum Link count
0.0.0.1         0.0.0.1         236         0x80000054 0x0021CC 1
0.0.0.2         0.0.0.2         40          0x80000054 0x001CCF 1

Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.12.2    0.0.0.2         40          0x8000004F 0x00CEA0

Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.23.0    0.0.0.2         40          0x8000004F 0x003B30
Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
0.0.0.2         0.0.0.2         1536        0x80000054 0x00379F 1
0.0.0.3         0.0.0.3         250         0x80000054 0x0032A2 1

Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.23.3    0.0.0.3         250         0x80000051 0x0077E8

Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         0.0.0.2         1536        0x8000004E 0x000BE3
192.168.12.0    0.0.0.2         1536        0x8000004F 0x00D2A5

Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
1.1.1.0         0.0.0.1         236         0x8000004F 0x00212D 0

R2#show ip route

Gateway of last resort is not set

      1.0.0.0/24 is subnetted, 1 subnets
O E2     1.1.1.0 [110/20] via 192.168.12.1, 1d19h, GigabitEthernet0/0
      192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.12.0/24 is directly connected, GigabitEthernet0/0
L        192.168.12.2/32 is directly connected, GigabitEthernet0/0
      192.168.23.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.23.0/24 is directly connected, GigabitEthernet0/1
L        192.168.23.2/32 is directly connected, GigabitEthernet0/1
42

Hello Daniel

Yes, this is normal behavior. R2 should see type 5 LSAs because it is an ABR. It is part of both Area 0 and Area 1. Area 0 is not a stub and cannot be a stub, so the Gi0/0 interface on R2 which is in area 0 will still receive type 5 LSAs, and R2 will place them in the OSPF database. But it will not propagate them into Area 1 which is a stub area. That’s why you don’t see them on R3.

So what you are seeing is normal behavior. Remember, the ABR (R3 in this topology) acts as the “wall” that will not allow Type 5 LSAs to pass, but the router itself receives and processes them normally, as part of the OSPF Area 0.

I hope this has been helpful!

Laz

1 Like
43

Thank you so much @lagapidis

1 Like
44

Hi,

Does the default route injected by the ABR in the stub area start with a metric of 2?