OSPF SETUP

Unanswered Question
Feb 14th, 2007

Hi all i have seen people setting up networks using ospf with addresses 172.16.10.1 0.0.0.0 area 0, what does the 0.0.0.0 do in this case ?

I have this problem too.
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dominic.caron Wed, 02/14/2007 - 09:00

0.0.0.0 is a wildcard mask. In this case, it means that an interface with IP 172.16.10.1 is part of area 0. (0.0.0.0 is a exact match)

glen.grant Wed, 02/14/2007 - 09:50

The way you have it that is not a specific network it is a single address , the mask is a match for that whole address not a network . If you wanted to advertise a network it would be 172.16.10.0 0.0.0.255 .

Richard Burts Wed, 02/14/2007 - 12:49

actually the mask used on the OSPF network statement has nothing to do with what subnet and subnet mask will be advertised. The network statement has only an indirect relationship to what is advertised.

The purpose of the network statement is to tell OSPF what interfaces to include in the protocol. OSPF takes the network statement (including mask) and scans the available interfaces on the router to see what to include. Once OSPF has selected the interfaces it will advertise based on the subnet mask configured on the interface, not on the mask in the network statement.

The reason to specify a mask in the network statement is to give us flexibility about choosing interfaces. You might configure network 172.16.0.0 0.0.255.255 area 0. If you did that any interface in 172.16.0.0 would be included. But what if you wanted 172.16.10.0 to be in area 0 and wanted 172.16.20.0 to be in area 2? By using a more specific mask you can select only 172.16.10.0 for area 0 and select 172.16.20.0 for area 2.

Many of us consider using the 0.0.0.0 mask to be a best practice in configuring OSPF because it gives us the greatest control over what happens in the router.

HTH

Rick

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