Section 1 Task 4.12 OSPF Domain-ID

Hi all !! this is my first post in the community.

I was working on this task and I noticed 2 weird things about the explanations provided :

  1.     After completing all redistribution, R1 should see two paths to reach, one via backdoor link (XR2) and one via MPLS L3VPN, and the provided output compares the generated LSA's.  But with my understanding in this situation, R1, being an ABR, should never be able to calculate an inter-area path through a non-backbone area, even if it knows about the related summary LSA's.  If you disable the MPLS VPN part (the backdoor link being the only option) , you should not see the inter-area route appear on the routing table (at least that's what happened in my case).  So for me the reason why R1 prefers the MPLS VPN link is not related to the cost but to the global design mixing non-backbone areas.
  1.     According to solution, the external route ( is prefered via the backdoor link (on R1) because the PE (R2) would not reoriginate a type-5 LSA if someone else in the area is already originating it, but is this point correct even if the router-ids are different?? (forwarding addressess being both set to For me , what is happening is simply that R2 , when redistributing ospf external route in BGP, will have 2 MBGP routes for the same prefix one coming from redistribution, so the weight is set to 32768 (default behaviour) , and one coming from MPLS VPN (weight to 0). The weight attribute would play and R2 would prefer the path via R1, so no reason to generate a type-5 LSA. --> Maybe that was the initial idea of the provided explanation, but I didn't get it that way..

any thoughts??





  • Hi,

    that task has only a couple of purposes:

    1) to introduce the student to the OSPF sham-link solution 

    2) to identify and underline a natural OSPF behaviour that is to prefer intra-area over inter-area over external routes or more precisely:


    O wins over OIA which wins over E1 which wins over E2 which wins over N1 and finally N2.

    As you said,

    there is nothing to cost or metric per say. What really matters is to understand that the above mentioned OSPF behaviour applies regardless any other parameter.

    At last, the downward bit is what you want to read about on the RFC indicated on the WB and video #19 of the SP series. 


    Keep in mind that this info is one of the most important of the entire CCIE studies.

    Maybe this is another helpful piece of info:



    Good Luck with your studies and welcome to this blog !!!  [;)]




    PS: this is also a good video which Brian did in the R&S(v4) series about this topic:

    MPLS Layer 3 VPN OSPF PE-CE Routing Overview


  • Hi Alessio, 

    thanks very much for your answer!!

    I have the feeling that you answered thinking my question was about task 4.11 MPLS L3VPN and OSPF Sham-Links?? (your first point states one of the objectives was to introduce to sham-link solution). 

    But the subject was actually task 4.12 about Domain-id, and in this task I didn't see mention of sham-link ;) , or I missed something...

    Thank you for links and welcoming :)




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