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Configure OSPF on unnumbered interfaces

When we've been assigning router interfaces in OSPF areas with the network router configuration command, it was impossible to start OSPF only on some unnumbered interfaces and not on others (or place the unnumbered interfaces in different areas). These restrictions are removed if you use the ip ospf area interface configuration command. For example, to put the loopback interface into another area than the WAN links using its IP address, use the following configuration commands:

router ospf 1
interface Loopback0
 ip address
 ip ospf 1 area 0
interface Serial1/0
 ip unnumbered Loopback0
 ip ospf 1 area 1
interface Serial1/1
 ip unnumbered Loopback0
 ip ospf 1 area 2


  1. What could be the real world scenario using this trick?

  2. There are mixed opinions about whether you should use unnumbered interfaces or not.

    If you decide to use them and have multiple areas configured on the router, previously you needed a numbered interface in each area. Now you can have a single numbered interface serving unnumbered interfaces in multiple areas.

    Sounds confusing? It probably is :)

  3. Really interesting. Thanks for letting me know the existence and usage of "ip ospf area".

  4. Thank you so much!! I'm so happy to find this solution, we do use ip unnumbered and I'm implementing different areas!


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Ivan Pepelnjak, CCIE#1354 Emeritus, is an independent network architect. 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.