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.