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 10.0.0.3 255.255.255.255
 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

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4 comments:

  1. soulache 20 February 2008 15:54
    What could be the real world scenario using this trick?
  2. Ivan Pepelnjak 20 February 2008 18:05
    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. Ludovic Kuty 06 June 2009 11:44
    Really interesting. Thanks for letting me know the existence and usage of "ip ospf area".
  4. Mariya 22 August 2012 09:29
    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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