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Forums » Internetwork Expert Products » CCIE R&S Core Knowledge Simulation » QID 123 ospf path selection
Latest post 07-06-2009 1:22 PM by Scott Morris. 8 replies.
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  • 05-11-2009 6:04 AM

    QID 123 ospf path selection

    Reply Contact

    can someone please explain  to me why in this case  NSSA Type 1 is preferred?

    The 'more information' link did not help me.

    thanks in advance!

    Matthias
    • Post Points: 35

  • 05-11-2009 7:15 AM In reply to

    Re: QID 123 ospf path selection

    Reply Contact

    I am going to have the author write up some Instructor Tips for these two questions - I will post them here later today as well.
    • Post Points: 20

  • 05-11-2009 7:25 AM In reply to

    Re: QID 123 ospf path selection

    Reply Contact

    thanks for the quick reply!
    • Post Points: 5

  • 05-11-2009 10:42 AM In reply to

    Re: QID 123 ospf path selection

    Reply Contact
    Generally, because the RFC says so.  :)    The p-bit (if it's the question I'm thinking of, it's one I wrote) on an N1 LSA will supercede the others per the requirements in the RFC.  No other reason! 

     


    Scott Morris, CCIEx4 (R&S/ISP-Dial/Security/Service Provider) #4713,

    JNCIE-M #153, JNCIS-ER, CISSP, et al.

    CCSI #21903, JNCI-M, JNCI-ER

    smorris@internetworkexpert.com


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    Matthias wrote:

    can someone please explain  to me why in this case  NSSA Type 1 is preferred?

    The 'more information' link did not help me.

    thanks in advance!

    Matthias




    Internetwork Expert - The Industry Leader in CCIE Preparation
    http://www.internetworkexpert.com

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    • Post Points: 20

  • 06-15-2009 7:56 AM In reply to

    Re: QID 123 ospf path selection

    Reply Contact

    The answer to QID 123 is OSPF External Type 1, not NSSA Type 1.  Is this post in regards to QID 124 and not QID 123?
    • Post Points: 20

  • 06-15-2009 8:39 AM In reply to

    Re: QID 123 ospf path selection

    Reply Contact
    Pretend I don't have the database....   What's the question/problem.  :)

    As for NSSA N1 route over E1, per RFC 3101, an N1 route with the P-bit set is selected BEFORE an E1 is.

     


    Scott Morris, CCIEx4 (R&S/ISP-Dial/Security/Service Provider) #4713,

    JNCIE-M #153, JNCIS-ER, CISSP, et al.

    JNCI-M, JNCI-ER

    evil@ine.com


    Internetwork Expert, Inc.

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    Knowledge is power.

    Power corrupts.

    Study hard and be Eeeeviiiil......



    PinGorilla wrote:

    The answer to QID 123 is OSPF External Type 1, not NSSA Type 1.  Is this post in regards to QID 124 and not QID 123?




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    • Post Points: 20

  • 07-03-2009 12:48 AM In reply to

    Re: QID 123 ospf path selection

    Reply Contact

    Hi

    I think that this it's not true,  I tested it in lab and it's my conclusion:

    1. Path with lower cost win (regardless it's E1 or N1)
    2. If equal cost than path E1

    Page with tests:

    http://blog.ccie.pl/index.php/garfield/2009/07/03/path-selection-between-n1-and-e1
    • Post Points: 20

  • 07-03-2009 8:39 AM In reply to

    Re: QID 123 ospf path selection

    Reply Contact

    Hi,

    Scott is right as per RFC3101. The thing is Cisco routers do not have RFC3101 fully implemented. They use RFC1587 which states as follows:

    When a type-5 LSA and a type-7 LSA are found to have the
    same type and an equal distance, the following priorities
    apply (listed from highest to lowest) for breaking the tie.

                     a. Any type 5 LSA.
                     b. A type-7 LSA with the P-bit set and the forwarding
                        address non-zero.
                     c. Any other type-7 LSA.

    http://www.ietf.org/rfc/rfc1587.txt

    Regards

    Piotr
    • Post Points: 20

  • 07-06-2009 1:22 PM In reply to

    Re: QID 123 ospf path selection

    Reply Contact
    So we should reword the questions to "What is the anticipated path if you were on a router that actually paid attention to the current RFC's following the receipt of the following LSAs"  ?

    ;)

     


    Scott Morris, CCIEx4 (R&S/ISP-Dial/Security/Service Provider) #4713,

    JNCIE-M #153, JNCIS-ER, CISSP, et al.

    JNCI-M, JNCI-ER

    evil@ine.com


    Internetwork Expert, Inc.

    http://www.InternetworkExpert.com

    Toll Free: 877-224-8987

    Outside US: 775-826-4344


    Knowledge is power.

    Power corrupts.

    Study hard and be Eeeeviiiil......



    pijablon wrote:

    Hi,

    Scott is right as per RFC3101. The thing is Cisco routers do not have RFC3101 fully implemented. They use RFC1587 which states as follows:

    When a type-5 LSA and a type-7 LSA are found to have the
    same type and an equal distance, the following priorities
    apply (listed from highest to lowest) for breaking the tie.

                     a. Any type 5 LSA.
                 b. A type-7 LSA with the P-bit set and the forwarding
                    address non-zero.
                 c. Any other type-7 LSA.

http://www.ietf.org/rfc/rfc1587.txt

Regards

Piotr



    Internetwork Expert - The Industry Leader in CCIE Preparation
    http://www.internetworkexpert.com

    Subscription information may be found at:
    http://www.ieoc.com/forums/ForumSubscriptions.aspx
    • Post Points: 5
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