OSPF Area

[Danilo Dy]

Hi,

If i have the following configuration;

router ospf 1

network 10.10.0.0 0.0.63.255 area 10

network 10.10.63.0 0.0.0.3 area 0

What would be the implication of using ip address from area 10 to area 0?

Any docs refering to this? I search cisco Web Site but I can't find anything similar to this.

Thanks in advance.

[thisisshanky]

Hi,

I am not understanding your question correctly. In your configuration IP addresses are not used to configure OSPF.Both statements have network addresses.

Use of IP addresses in OSPF network masks come to use when you do not want to calculate complicated OSPF masks.If you have an interface with IP address 10.10.10.1 / 24 you can do either of the following to enable OSPF on this interface.

1. router ospf 10

network 10.10.10.0 0.0.0.255 area 0

or

2. router ospf 10

network 10.10.10.1 0.0.0.0 area 0

In case 1 I have used a network address, while in case 2 I have used an IP address.

I am not sure whether I did answer your question or not..

[Danilo Dy]

sorry, i use ip address in my question but my example is very clear. let me rephrase the question ok?

I have the following configuration;

router ospf 1

network 10.10.0.0 0.0.63.255 area 10

then i happen to use a subnet in the above area 10 to area 0;

router ospf 1

network 10.10.0.0 0.0.63.255 area 10

network 10.10.0.0 0.0.0.3 area 0

you see that 10.10.0.0/30 in area 0 comes from area 10. what would be the implication of this?

is this clear enough for you?

thanks.

[thisisshanky]

If a network address covering a broad range of IP addresses are used first in the OSPF configuration, and then a specific subnet is used later, the first command takes precedence. I bet, the interface would with network address 10.10.0.0/30 will still be in area 10 and not area 0...

[Danilo Dy]

I have this error after i configure it;

%OSPF-4-ERRRCV: Received invalid packet: mismatch area ID, from backbone area must be virtual-link b

ut not found from 10.10.63.2, Ethernet1

Reason i ask this question is that, existing networks using static routing could have this problem when their WAN link ip addresses are using subnets from other area once they migrate their routing from static to ospf.

I need some documentation that i can use to backup my theory because old network administrator (die harder) could insist retaining those ip address in their wan link. Because they have not plan properly their ip addressing when they still using static routing.

thanks a lot.

[rdeguzman75]

You received the error because of an OSPF misconfiguration. In this case, the overlapping networks is the misconfiguration. What you can do is use a different subnet for area 10 that does not also contain the subnet in area 0.

example:

router ospf 1

network 10.10.64.0 0.0.63.255 area 10

network 10.10.0.0 0.0.0.3 area 0

Hope this helps.