OSPF - IP Summarisation on each router **I'm confused**

lon21

When configuring OSPF within Area 0 do we summarise all the ip address which are connected to the routers? Or is this only done in Area 1?

Because in CBT video:

Area 0 contains the following routers (exclude router 1) and Jeremy writes the following network cmd (below routers)


Router 2 - int fa0/0 192.168.1.2
Router 2 - int fa0/1 192.168.2.1
Router 2 - int fa0/2 192.168.10.1
Router 2 - int fa0/3 192.168.20.1

'network 192.168.0.0 0.0.255.255 area 0'

Router 3 - int fa0/0 10.168.2.1
Router 3 - int fa0/1 10.168.2.2

''network 10.168.2.0 0.0.0.255 area 0'

But in Area 1

One router contains the following ip address:

172.30.0.0
172.30.1.0
172.30.2.0
172.30.3.0
172.30.4.0
172.30.5.0
172.30.6.0
172.30.7.0

But his network command is 'network 172.30.0.0 0.0.7.255 area 1'

I can clearly see that Area 1 cmd shows that it summarise ip address up .7.0 but is this not the case for area 0. Where area 0 summarise more ip address than it is actually using?

E.g.
Router 2 cmd is:
'network 192.168.0.0 0.0.255.255 area 0'

should it not be

'network 192.168.0.0 0.0.31.3 area 0' given a more specific summarisation?


Thanks.

techie2012

lon21 wrote: »

When configuring OSPF within Area 0 do we summarise all the ip address which are connected to the routers? Or is this only done in Area 1?

Because in CBT video:

Area 0 contains the following routers (exclude router 1) and Jeremy writes the following network cmd (below routers)


Router 2 - int fa0/0 192.168.1.2
Router 2 - int fa0/1 192.168.2.1
Router 2 - int fa0/2 192.168.10.1
Router 2 - int fa0/3 192.168.20.1

'network 192.168.0.0 0.0.255.255 area 0'

Router 3 - int fa0/0 192.168.2.1
Router 3 - int fa0/1 192.168.2.2

''network 192.168.2.0 0.0.0.255 area 0'

But in Area 1

One router contains the following ip address:

172.30.0.0
172.30.1.0
172.30.2.0
172.30.3.0
172.30.4.0
172.30.5.0
172.30.6.0
172.30.7.0

But his network command is 'network 172.30.0.0 0.0.7.255 area 1'

I can clearly see that Area 1 cmd shows that it summarise ip address up .7.0 but is this not the case for area 0. Where area 0 summarise more ip address than it is actually using?

E.g.
Router 2 cmd is:
'network 192.168.0.0 0.0.255.255 area 0'

should it not be

'network 192.168.0.0 0.0.31.3 area 0' given a more specific summarisation?


Thanks.

First off, I am going to say, don't take my explanations as absolute as I am in the learning process myself but from my studies I believe that I can tell you this.

Network 192.168.0.0 0.0.255.255 is saying to route all traffic destined for 192.168.0.0-192.168.255.255 so that covers all of the interfaces on router 2 and if you wanted it to be more specific, providing that the network you are using is a /19 it would be 192.168.0.0 0.0.31.255 area 0. The .31 would be for the subnet and is always 1 less than the block size and in the fourth octet for the hosts it would be 255. Also, Im not sure why the interface on router 2 and router 3 have the same ip. Not a very good explanation but I am at the point of my studies where I understand the concept but am not good at explaining it yet. Hope this helps some

Taking a closer look however. It seems that the interfaces on Router2 are all separate networks on a 192.168.0.0/16 scheme so that is why it is 0.0.255.255 There are no subnets.

networker050184

The network command isn't going to summarize anything. Its only there to tell the router what interface to run OSPF on.

lon21

networker050184 wrote: »

The network command isn't going to summarize anything. Its only there to tell the router what interface to run OSPF on.

I thought that was what the wild card it used for?

networker050184

lon21 wrote: »

I thought that was what the wild card it used for?

Nope. That is just there to match on IP addresses configured on interfaces. The mask configured on the interface controls what network is advertised.

lon21

networker050184 wrote: »

Nope. That is just there to match on IP addresses configured on interfaces. The mask configured on the interface controls what network is advertised.

Sorry what do you mean by mask?

For the wild card surely using a 0.0.31.3 wild card is better than using 0.0.255.255 for router 2?

0.0.31.3 advertises the exact number if ip address where as 0.0.255.255 advertises more ip address.

networker050184

lon21 wrote: »

Sorry what do you mean by mask?

For the wild card surely using a 0.0.31.3 wild card is better than using 0.0.255.255 for router 2?

0.0.31.3 advertises the exact number if ip address where as 0.0.255.255 advertises more ip address.

Again, the network mask in the OSPF network statement does NOT influence what mask is advertised. All the network statement does is match the IP address configured on the interface and run OSPF on it. Then OSPF uses the IP and mask configured on the interface to form the updates.

lon21

networker050184 wrote: »

Again, the network mask in the OSPF network statement does NOT influence what mask is advertised. All the network statement does is match the IP address configured on the interface and run OSPF on it. Then OSPF uses the IP and mask configured on the interface to form the updates.

Oh, now I understand lol

Therefore using 0.0.31.3 or 0.0.255.255 in the network statement would not make a difference?

Thanks

networker050184

lon21 wrote: »

Oh, now I understand lol

Therefore using 0.0.31.3 or 0.0.255.255 in the network statement would not make a difference?

Thanks

Exactly. You can use 0.0.0.0 and just the exact IP of the interface or just 0.0.0.0 255.255.255.255 to run OSFP on all interfaces.

techie2012

networker050184 wrote: »

Exactly. You can use 0.0.0.0 and just the exact IP of the interface or just 0.0.0.0 255.255.255.255 to run OSFP on all interfaces.

I am glad that I read this, my understanding was off on the subject