Why OSPF and EIGRP route in routing table

[jared.j01]

Hello,

I am currently studying my CCNA and I have my own lab equipment at home.

I am practicing the below Lab which requires;

- RIP v2 to be setup and confirm that all routes are present and everything can ping each other

- Then setup OSPF and confirm the RIP routes have been replaced by OSP routes (due to the administative distance being lower)

- Finally setup EIGRP and confirm that the OSPF routes have been replaced by the EIGRP routes (again due to the administrative distance being lower)

Everything is ok except for some reason in R1 and R3's routing table, I have one OSPF route as shown below.

Below are the networks I advertised on R2. I would appreciate it if anyone could tell me why the 1 OSPF route appears in R1 and R2's routing table and why it also has a /32 mask.

Thank you very much!

networks I advertised on R2

OSPF

10.1.1.0 0.0.0.3 area 0

10.2.2.0 0.0.0.3 area 0

10.12.12.0 0.0.0.255 area 0

192.168.20.0 0.0.0.255 area 0

209.165.200.160 0.0.0.31 area 0

EIGRP

10.1.1.0/30

10.2.2.0/30

10.12.12.0/24

192.168.20.0

209.165.200.160/27

R1's Routing Table

Gateway of last resort is not set

D    192.168.30.0/24 [90/30720] via 10.13.13.3, 00:34:35, FastEthernet0/1.13

C    192.168.10.0/24 is directly connected, FastEthernet0/1.10

     209.165.200.0/24 is variably subnetted, 2 subnets, 2 masks

D       209.165.200.160/27

           [90/156160] via 10.12.12.2, 00:34:31, FastEthernet0/1.12

O       209.165.200.161/32

           [110/2] via 10.12.12.2, 00:34:21, FastEthernet0/1.12

D    192.168.20.0/24 [90/30720] via 10.12.12.2, 00:34:31, FastEthernet0/1.12

     10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

C       10.13.13.0/24 is directly connected, FastEthernet0/1.13

C       10.12.12.0/24 is directly connected, FastEthernet0/1.12

D       10.2.2.0/30 [90/2172416] via 10.13.13.3, 00:34:35, FastEthernet0/1.13

                    [90/2172416] via 10.12.12.2, 00:34:35, FastEthernet0/1.12

C       10.1.1.0/30 is directly connected, Serial0/0/0

[cadet alain · Accepted solution]

Hi,

OSPF treats loopback as  stub and so advertises as /32( so as prefix-length is different,neither metric nor AD are involved and it is installed in the routing table), to change this behaviour then configure under the interface:

ip ospf network point-to-point

It will get advertised with correct mask and if EIGRP advertises the same prefix then it will disappear from the routing table being replaced by the EIGRP prefix.

Regards

Alain

Don't forget to rate helpful posts.

[jared.j01]

Hello Cadet Alain,

Thank you for your reply and description. It now makes sense to me.

Appreciated!