OSPF config

hshah12

Hi guys,

Just couple of quick questions regarding OSPF configuration:

Is there any diff b/w the following two configs:

router ospf 1
network 1.1.1.1 0.0.0.0 area 0
network 2.2.2.2 0.0.0.0 area 0

and

router ospf 1
network 2.2.2.2 0.0.0.0 area 0
network 1.1.1.1 0.0.0.0 area 0

and another question regarding the same topic

When configuring OSPF in exam, can we just use:

The Interface IPs and 0.0.0.0 as wild card(eg:172.1.2.3 0.0.0.0), rather than the subnet no and working out the correct wild card mask(eq: 172.1.2.0 0.0.0.3)?

Iam asking this cus obviously chances of getting it wrong is greater in the second method than the first one.

Thanks guys.

BubbaJ

hshah12 wrote:

Is there any diff b/w the following two configs:

router ospf 1
network 1.1.1.1 0.0.0.0 area 0
network 2.2.2.2 0.0.0.0 area 0

and

router ospf 1
network 2.2.2.2 0.0.0.0 area 0
network 1.1.1.1 0.0.0.0 area 0

They are the same thing.

hshah12 wrote:

When configuring OSPF in exam, can we just use:

The Interface IPs and 0.0.0.0 as wild card(eg:172.1.2.3 0.0.0.0), rather than the subnet no and working out the correct wild card mask(eq: 172.1.2.0 0.0.0.3)?

It depends on how the question is worded.

hshah12

Thanks for your quick reply.

Ok, here is the example:

There are three routers, named abc, def, and xyz used by abc.com. All are Cisco routers. All have been configured with the Single Area OSPF routing protocol.
You are required to correct the configuration on the abc router. "abc" has been installed and configured, however, connectivity is not complete since the routing tables are not updated properly.

E0: 192.168.33.1/24
S0: 192.168.100.5/30

As it turns out there is no OSPF config on this router at all, so we have to config OSPF from scratch.

Can we use either method here? Under what circumstances we should/should not use those methods?

Thanks.

BubbaJ

That question doesn't say so I think you could use either one. The least work is to do it like this:

network 192.168.0.0 0.0.255.255 area 0

ceejay3001

i found you do need to enter the subnet address and appropiate wild card mask. otherwise what networks will share info if they dont relate to the configured interface...

BubbaJ

ceejay3001 wrote:

i found you do need to enter the subnet address and appropiate wild card mask. otherwise what networks will share info if they dont relate to the configured interface...

No. In an IGP, the network command is really just telling the IGP which interfaces will participate in the routing protocol. The interfaces' addresses tell the IGP which networks to use. You can be as specific as you like (down to the exact interface address), and the IGP will get the correct network address from the interface address.

On the other hand, BGP needs the exact network address and mask, and they must match exactly what is in the routing table.

ceejay3001

oh, i remember when carried out the ospf question -- i worked out the correct sub net address and configured the network - with appropiate wildcard mask - as this is how i was taught to do this -- and i could tell from my overall mark i didnt get the question wrong

Danman32

I too wasn't clear about addressing networks with OSPF. The Cisco coursebook went by the mask, yet you'd see the interface address itself indicated by the wildcard mask.

I asked my instructor about this after flubbing the exam. One thing to note that this is a link state protocol. All the router does is broadcast (multicast really) what links are up and which are down. All it needs to know are what links (interfaces) are participating in what area.
It was even said that you could have an IP and mask cover several interfaces.

BubbaJ

Danman32 wrote:

It was even said that you could have an IP and mask cover several interfaces.

That was the example that I gave him:

BubbaJ wrote:

network 192.168.0.0 0.0.255.255 area 0

You wouldn't want to do this if the two interfaces were going to be in different areas.

I think I like putting the OSPF area directly on the interface better than using the network command under the routing process. You have a finer control, and you don't have to worry about network address/wildcard mask.

ceejay3001

cheers - looks like i learnt something new!