why "suppress-fa" not the default behaviour



today I was reading about suppress-fa commnad in OSPF,


if this command used to suppress forwarding address and put instead for external routes redistributed into NSSA areas.

then why this is not the default behaviour ?? I guess it always should be




  • JoeMJoeM ✭✭✭

    Hi Oudmaster,

    Someone else may be better, to explain WHY this is not the default behavior. But here is my thinking.

    First, where is this command used?  on the ABR, not the original ASBR

    There is an LSA Type 4 that tells the rest of the network about the location of an ASBR. That would be the default Forwarding-Address.

    It does seem easier to just link the ABR to the forwarding-address -- when there is only one ABR.


    But I am guessing that there is an attempt to maintain a link-state protocol, versus distance-vector behavior ("routing by rumour").

    Also, I suppose that it is better to know the best metric to the true ASBR  -- when there are multiple ABR's into an NSSA area.


    @anyone, please feel free to correct me or add to it.  I am just making a guess at WHY the protocol is designed this way.




  • By default the ABR does NOT suppress the FA when it does the 7/5 LSA translation, because you want all devices receiving the LSA type-5 to perform best cost computation against the FA address, thus towards the exit of the network from the NSSA area, and NOT towards the ABR (which is what happens if the ABR suppresses the FA and sets it to, With the default behaviour and FA set, when a router performs the cost calculation for LSA type-5 it takes into consideration also the link costs inside of the NSSA area (known from LSA Type3). 

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