Question about OSPF LSA 5 and 7

Hi guys! one short question.. just a little doubt that's spinning in my head:

 

Why LSA 5 does not follow LSA 7 rules regarding the Forwarding Address? I mean, in LSA 7, there's always an IP Address in the Forwarding Address field, and it's the highest IP among the OSPF enabled loopbacks, and if none, highest OSPF enabled Interface.

 

But, LSA 5 gets an Forwarding Address of 0.0.0.0 if the External route resolves to a non-OSPF interface, so then it needs an LSA 4 so ABRs can show the world how to get to this route. How come it cant get a Forward address just like a LSA 7 (and translated LSA5)?

 

What am I missing :S?

Comments

  • Hi,

    With a single NSSA translator for 7-to-5 you are most likely to need
    additional "hint" for optimal routing toward the original ASBR. Type-5
    LSAs in contrary are originated by ASBR itself, and FA "hint" may only be
    needed in a few certain cases to help selecting optimal path. In general,
    routing toward ASBR for external routes is optimal and safest path.

    HTH,
    Petr

    ----- Original Message -----
    From: "omarmontes"
    Sent: Fri, September 11, 2009 9:27
    Subject:[CCIE R&S] Question about OSPF LSA 5 and 7


    Hi guys! one short question.. just a little doubt that's spinning in my head:



    Why LSA 5 does not follow LSA 7 rules regarding the Forwarding Address? I
    mean, in
    LSA 7, there's always an IP Address in the Forwarding Address field, and
    it's the
    highest IP among the OSPF enabled loopbacks, and if none, highest OSPF
    enabled
    Interface.



    But, LSA 5 gets an Forwarding Address of 0.0.0.0 if the External route
    resolves to a
    non-OSPF interface, so then it needs an LSA 4 so ABRs can show the world
    how to get
    to this route. How come it cant get a Forward address just like a LSA 7 (and
    translated LSA5)?



    What am I missing :S?

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