Be smart when using the OSPF network statement

For whatever reason, a lot of people have the impression that the wildcard bits in the OSPF network statement have to be the inverse of the interface subnet mask. For example, if you have configured ip address 192.168.1.2 255.255.255.240 on an interface, they would enter network 192.168.1.2 0.0.0.15 in the OSPF configuration ... and obviously use one network statement per interface.

In reality, the network statements work like simple IP access-list: whenever an interface IP address matches the network statement, the interface is put into the selected area. The IOS is also pretty helpful recently: the network statements are automatically sorted from most-specific to least-specific and (like with the access lists) the first match stops the search.

In my network implementations, I use the network statements in three different ways:

  • If I have to assign a specific interface into an area, I would always use network x.y.z.w 0.0.0.0 area n;
  • If the area address ranges are nicely assigned (which also helps immensely when you have to start summarizing), you can use a single network statement to cover the whole area. If, for example, area 3 has address range 10.1.16.0/20, use network 10.1.16.0 0.0.15.255 area 3;
  • If the router has all interfaces in a single area, I would almost always use network 0.0.0.0 255.255.255.255 area area-id (unless there is an extremely good reason that some interfaces should not be seen by the OSPF process).

10 comments:

  1. Nice articule but with one mistake. When you configure ospf , the network statement work, as you said, as an ACL but... from the most-specific to the less specific. To use Cisco terms "the longest,the better".

    Thanks anyway for this post
    Alex

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  2. Ivan Pepelnjak22 July, 2010 09:16

    Thank you! Fixed.

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  3. You say "unless there is an extremely good reason that some interfaces should not be seen by the OSPF process ", can you give some examples of when this might be the case?

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  4. Ivan Pepelnjak03 April, 2012 08:26

    No ... 8-)

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  5. "If the area address ranges are nicely assigned (which also helps immensely when you have to start summarizing), you can use a single network statement to cover the whole area. If, for example, area 3 has address range 10.1.16.0/20, use network 10.1.16.0 0.0.15.255 area 3; "

    But doesn't the router-lsa see what is on the physical interface and assign accordingly, I have attempted to use a summary but the router-lsa will always advertise what mask you have configured

    Steve

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    Replies
    1. The "network" command specifies on which interfaces OSPF runs (and which subnets it includes in the LSA), not what subnet mask it should use.

      For OSPF summarization, look up "area summary" command, and remember that it only works across area boundaries. You cannot summarize within an OSPF area.

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    2. I agree on using area summary on the ABR for summaries, but what im trying to understand is you're point on an address range of 10.1.16.0/20 if the interface has the same mask then it's ok as the LSA type 1 will have the correct mask details if however you tried to summize to a /19 then the LSA would still show the /20 as configured on the interface

      Steve

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    3. I guess my point being what's the difference in using a .255 over a .0 in the wildcard when the genuine mask is /24 and will be propagated in the LSA 1 as such

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  6. I think you should re-read my comment. The "network" statement has nothing to do with the way OSPF advertises subnets and all to do with which subnets (interfaces) are advertised.

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    Replies
    1. Agree, so why you're point 2 "use network 10.1.16.0 0.0.15.255 area 3" when using a mask of 0.0.0.0 will achieve the same thing

      Steve

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.