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The CCNA book seems to imply only 2 levels possible under OSPF and IS-IS; I am curious as to if that was all you could do or if you could have further sublevels.

Can link state routing protocols support more than a 2 level hierarchy? Can an area connected to the backbone have its own subordinate area?

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    Do you really mean all link state protocols in general, or are you asking specifically about OSPF? Because those are two different answers.
    – Brett Lykins
    Feb 1, 2014 at 13:07

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Well, both OSPF and IS-IS use two-level hierarchy and that's it (for single process) - you have backbone area (Level 2 in IS-IS, Area 0 in OSPF), and "leafs" - Level 1 in IS-IS and Areas !0 for OSPF. All areas connected to leafs need to somehow connect to area 0 also to avoid routing loops.

Is this design/configuration question or just theoretical question? You can have multi-level hierarchy by (for example) putting different instances on different levels of hierarchy and do redistribution between them.

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  • Just theoretical. Putting notes together and writing down definitions, and when the book defined IS-IS L1/L2 and OSPF Area 0/Area !0, I wondered if more than 2 levels could be implemented, connecting to the backbone through another layer or area.
    – gr0k
    Feb 1, 2014 at 19:34
  • Virtual links(linking a remote area to the backbone through a non-backbone area) can be used to reach the backbone, but this is generally regarded as a temporary workaround, for instance, when making network changes to accomodate a corporate acquisition.
    – Dave in Ohio
    Feb 1, 2014 at 20:56

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