OSPF with multi area

[libra_ali786]

I am a CCNA student. I created one simple topology with three routes , each router have 2 loopback interfaces represent the different networks. Router 0 have Area Backbone 0 , Router 1 have Area 1 and Router 2 have area 2.

Problem is When i changed the AS number at Router 2 for the Area 2 , then those networks does not made the Adjancies. Why ?

As per my understanding we can change the AS numbers ? on each router in the same topology?

Below is the my configuration :

Router 0 have :

router ospf 1
log-adjacency-changes
network 192.168.1.0 0.0.0.255 area 0
network 192.168.2.0 0.0.0.255 area 0
network 20.20.20.0 0.0.0.255 area 0
network 10.10.10.0 0.0.0.255 area 0

Router 1 have :

router ospf 1
log-adjacency-changes
network 50.50.50.0 0.0.0.255 area 1
network 60.60.60.0 0.0.0.255 area 1
network 192.168.1.0 0.0.0.255 area 0

Router 2 have :

router ospf 1
log-adjacency-changes
network 192.168.2.0 0.0.0.255 area 0
!
router ospf 2
log-adjacency-changes
network 40.40.40.0 0.0.0.255 area 2
network 30.30.30.0 0.0.0.255 area 2

[Georg Pauwen]

Hello,

Router 2 needs to have an interface in area 0, also, the OSPF process on all routers needs to match.

When you advertise 'networks' you are actually telling OSPF which interfaces belong to which area. The IP address of any network that falls within the advertised network range is part of the specified area.

The below will work:

router ospf 1
log-adjacency-changes
network 192.168.2.0 0.0.0.255 area 0

network 40.40.40.0 0.0.0.255 area 2
network 30.30.30.0 0.0.0.255 area 2

Does that make sense ?

[Julio E. Moisa · Accepted solution]

Hi

I agree with Georg, you need include the network under the interface with the Router 0 and assign the area 0.

Now if you created 2 OSPF instance like seen on the configuration:

router ospf 1
log-adjacency-changes
network 192.168.2.0 0.0.0.255 area 0
!
router ospf 2
log-adjacency-changes
network 40.40.40.0 0.0.0.255 area 2
network 30.30.30.0 0.0.0.255 area 2

The networks included into the OSPF 2 will not be advertised to other router because it does not have adjacency just OSPF 1, it could work if you create mutual redistribution between the OSPF instances.

Hope it is useful

:-)

[libra_ali786]

Some topics are not included in CCNA v.3 (excluding authentication, filtering, manual summarization, redistribution, stub, virtual-link, and LSAs)

Please explain the below point.

I am not understanding this reply statement  "the OSPF process on all routers needs to match."

I just changed the AS number at router 2 and advertised the networks . It worked fine.

router ospf 2

log-adjacency-changes

network 192.168.2.0 0.0.0.255 area 0

network 40.40.40.0 0.0.0.255 area 2

network 30.30.30.0 0.0.0.255 area 2

[Julio E. Moisa]

Hi

The OSPF AS is not required to match like in EIGRP where all the routers must have the same autonomous system, you can use different AS on the OSPF routers and it will work fine, now is important to include into the OSPF the network used to interconnect the routers in order to create adjacency and the same area. 

In order to create OSPF adjacency it must accomplished:

- Same OSPF timers

- Same MTU value

- Same Network and mask

- Same Network type

- Same area

- Same password.

- Same area type

You can have different AS and it will work fine but the domain ID could be vary but it will not affect the functionability. I recommend to use the same AS to keep a standard.

Hope it is useful

:-)

[libra_ali786]

Thanks for the reply.

[Julio E. Moisa]

You are welcome, 

Have a great day

:-)