[OSPF] RFC 3101 NSSA/External Route preference clarification

"Balaji Ganesh (balagane)" <balagane@cisco.com> Tue, 13 June 2017 08:13 UTC

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From: "Balaji Ganesh (balagane)" <balagane@cisco.com>
To: OSPF WG List <ospf@ietf.org>
Thread-Topic: RFC 3101 NSSA/External Route preference clarification
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Date: Tue, 13 Jun 2017 08:13:23 +0000
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Subject: [OSPF] RFC 3101 NSSA/External Route preference clarification
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Hi all,

When the metrics are same, RFC 3101 specifies the preference for NSSA/External routes as follows.
In the section 2.5<https://tools.ietf.org/html/rfc3101#section-2.5> Calculating Type-7 AS External Routes - 2.5.6.(e), it says..

          (e) If the current LSA is functionally the same as an
              installed LSA (i.e., same destination, cost and non-zero
              forwarding address) then apply the following priorities in
              deciding which LSA is preferred:

                 1. A Type-7 LSA with the P-bit set.

                 2. A Type-5 LSA.

                 3. The LSA with the higher router ID.


Points 1 and 2 are clear..

However Point 3 specifies preference of an LSA with a higher router ID. Why is it so?


-          Should we not install ECMP paths in this case?

-          Is point 3 actually intended for NSSA translators to prefer a Type 7 LSA which needs to be used for translation to Type 5?

Considering the above 2 points, I guess point 3 needs to be modified in the RFC to probably say..

                    3. Preference is same, install ECMP paths.
                       Additionally if the router is an NSSA translator, prefer the LSA with higher router ID for Type 7-Type 5 translation.

Please let know any views/comments on the same.

Regards,
Balaji