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Community Member

OSPF Connected networks

In a 7513 router (IOS c7200-is-mz.113-5.T) there are two areas

network 10.0.0.0 0.31.255.255 area 0.0.0.0

network 10.31.0.0 0.31.255.255 area 1.0.0.0

I want to collapse area 0 IP address space from 10.0.0.0 to 10.1.255.255 and

change area 1 to 10.2.0.0 to 10.63.255.255. I entered the following after deleting the above statements

network 10.0.0.0 0.1.255.255 area 0.0.0.0

network 10.2.0.0 0.63.255.255 area 1.0.0.0.

When "show run" I see the following in the router ospf field:-

network 10.0.0.0 0.1.255.255 area 0.0.0.0

network 10.0.0.0 0.63.255.255 area 1.0.0.0

(My area 1.0.0.0 doesn't start at 10.2.0.0 as input? it starts at 10.0.0.0?

Connected interfaces seem to be getting put into the correct area eg 10.31.31.254 is under area 1.0.0.0 with the new statement and area 0.0.0.0 with the original.

Why doesnt the router display what was configured.

2 REPLIES
Cisco Employee

Re: OSPF Connected networks

Because your mask is 0.63.255.255 which means you have a /26 network, and 10.2.0.0 is not a valid subnet for /26

valid subnets for /26 are

10.0.0.0/26 <---- 1st subnet

10.64.0.0/26 <---- 2nd subnet

10.128.0.0/26 <---- 3rd subnet

10.192.0.0/26 <---- 4th subnet

So when you put an invalid 2nd octet (10.2.0.0), it replaces it with the first subnet.

HTH

R/Yusuf

Community Member

Re: OSPF Connected networks

Further to the first reply by R Yusuf, the configuration as it now stands, while working, could cause some problems in the future. The lines that determine which areas interfaces are in are:

network 10.0.0.0 0.1.255.255 area 0.0.0.0

network 10.0.0.0 0.63.255.255 area 1.0.0.0

If, during future configuration editing, you manage to swap the lines around to give:

network 10.0.0.0 0.63.255.255 area 1.0.0.0

network 10.0.0.0 0.1.255.255 area 0.0.0.0

you will find that all your area 0 interfaces are now in area 1.0.0.0, and you've lost connectivity to the backbone. This is because interfaces are allocated to areas by working down the network list from top to bottom and finding the first match for each interface, not the closest match.

The point to remember is that you are not specifying which IP address ranges lie within each area, but which interfaces on the router are within a given area. This may be a good time to specify smaller, non-overlapping, ranges to ensure that there is only one area match for any given interface, even going down to host address level if you have a small enough number of interfaces.

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