External LSA Forwarding Address Issue-OSPF

I am quite confusing about the forwarding address filed of LSA 7 that what IP address should it relate with,

I have a topolgy in which NSSA ASBR generates an external route but stick with one of my interface(of stub network) as forwarding address ,

your understanding will be highly appreciated.




  • Hi

    according to Jeff Doyle : "
    Forwarding Address


    , if the network between the NSSA ASBR and the adjacent autonomous system is

    advertised as an internal route, is the next hop address on the network. If the network is not

    advertised as an internal route, the forwarding address will be the NSSA ASBR's Router ID "

    get it ? or wanna more explanation?




  • Hello,


    As far as i observed in specific topology, if the network between NSSA ASBR and adjacent autonomas system is advertised as an internal route then it is the forwarding address of external route. But if i didnt advertise then forwardin address becomes the highest physical interface address or of loopback,even though i have hard coded router-id.

    The process is still confusing :)

  • why it is confusing,  if you dont know how to reach the actual address of the advertising router (Adjacent AS router that advertised the route) , u should give ur packet to someone who knows a way to that. (it is ur NSSA ASBR ). 

    why it is the router ID not the IP address of the interface of the ASBR on NSSA network (area) ?

    I agree it would make sense to use IP address of NSSA ASBR Router's interface(on NSSA area) as Forwarding address but actually no problem will occure in using NSSA's Router ID , because Type 4 LSAs (ASBR Summary LSA) are allowed in NSSA areas and routers inside area already know the ASBR and its Router ID and its interface address on the NSSA network. So they can reach it easily, even if they have its router ID as forwarding address.



  • Hello,

    i am 100% agreed with your argument that with in OSPF domain, it is no issue to trace router-id sourced path,

    i am just confusing about the the statement of Doyle book(which you also gave) that if connecting segment of ASBR and adjacent AS is not advertised, then forwarding address will be router-id, unfortunately this is not happening in my case.

    When i took off segment advertisement, then highest physical interface IP address of ASBR becomes forwarding address (even though i have hard coded router-id and have cleard OSPF process)



  • test the Loopback interface , create one and set the highest IP address of all on that, then check what willl happen.

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