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Highest accumulative OSPF Metric allowed

  • 1.  Highest accumulative OSPF Metric allowed

    Posted 12-01-2020 17:49

    Hi guys! it suddenly became a question today when I was troubleshooting an ospf network where metrics are done manually! what is the maximum metric allowed on ospf, something equivalent to RIP 16hop limitation but in terms of ospf. here is a simple topology:

    R1 <--link1--> R2 <--link2-->R3 <--link3--> R4 where we run OSPF on links 1-2-3  with a metric of 65000 on all routers for testing purposes. 
    R1 also has its loopback part of Ospf, 1.1.1.1/32.

    R2 sees lo0 /32 with metric of 65k, R3 sees it with metric of 130k and R4 sees it at 195k! is there a maximum value that ospf honors? whats the highest number allowed?  

    Thanks everyone! 



  • 2.  RE: Highest accumulative OSPF Metric allowed

    Posted 12-01-2020 18:44
    Hi ali.taheri,

    I think you mean LSInfinity which is:

    16-bit  ( 65535 ) for Router LSA
    24-bit ( 16777215 ) for Summary LSA and External LSA

    We can find from RFC3137:

    To address both problems, router X announces its router-LSA to the
       neighbors as follows.
    
          o  costs of all non-stub links (links of the types other than 3)
             are set to LSInfinity (16-bit value 0xFFFF, rather than 24-bit
             value 0xFFFFFF used in summary and AS-external LSAs).
    
          o  costs of stub links (type 3) are set to the interface output
             cost.
    



    ------------------------------
    Regards,
    Elchin
    ------------------------------



  • 3.  RE: Highest accumulative OSPF Metric allowed

    Posted 12-01-2020 19:08
    Hi Elchin! 
    Thanks for the answer! Briefly skimming over the RFC it sounds like this is a workaround to premature aging on external and summary LSA's. how does go about a flat single area domain?


  • 4.  RE: Highest accumulative OSPF Metric allowed

    Posted 12-02-2020 09:57
    Yes you're correct, for single-area domain I don't actually remember something for accumulate OSPF metric restriction :( Calculation actually performed correctly by adding path costs, but don't have any idea what is the maximum value that is allowed...

    ------------------------------
    Regards,
    Elchin
    ------------------------------



  • 5.  RE: Highest accumulative OSPF Metric allowed

    Posted 12-03-2020 09:59

    Thanks Elchin! hopefully someone knows the answer to this question