Re: [OSPF] RFC 3101 NSSA/External Route preference clarification

"Acee Lindem (acee)" <> Wed, 05 July 2017 17:36 UTC

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From: "Acee Lindem (acee)" <>
To: Chao Fu <>, "Balaji Ganesh (balagane)" <>, OSPF WG List <>
Thread-Topic: [OSPF] RFC 3101 NSSA/External Route preference clarification
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Date: Wed, 05 Jul 2017 17:36:18 +0000
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Subject: Re: [OSPF] RFC 3101 NSSA/External Route preference clarification
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Hi Chao,

I think rules (e) 1 and (e) 2 should remain to handle the case of an NSSA that receives both the intra-NSSA LSA and a translated AS External LSA (via a backbone path). I only think that rule (e) 3 needs to be relaxed. If we were doing another NSSA BIS, I’d remove it completely but since we are just talking about an Errata, I think we should just make the Router ID tie-breaker optional.


From: Chao Fu <<>>
Date: Wednesday, July 5, 2017 at 3:02 AM
To: "Balaji Ganesh (balagane)" <<>>, OSPF WG List <<>>, Acee Lindem <<>>
Subject: RE: [OSPF] RFC 3101 NSSA/External Route preference clarification

Hi all,

In my opinion rule (e) should be removed. ( ).
If not, it should be clarified more including removing “2. A Type-5 LSA.”

Chao Fu

From: OSPF [] On Behalf Of Balaji Ganesh (balagane)
Sent: Wednesday, July 05, 2017 14:01
To: OSPF WG List <<>>; Acee Lindem (acee) <<>>
Subject: Re: [OSPF] RFC 3101 NSSA/External Route preference clarification

Hi all,

Any views/comments on the below?


From: Acee Lindem (acee)
Sent: 19 June 2017 00:08
To: Balaji Ganesh (balagane) <<>>; OSPF WG List <<>>
Subject: Re: [OSPF] RFC 3101 NSSA/External Route preference clarification

Hi –  I encouraged Balaji to post to the list is that I think many implementations have ignored #3. I know that I changed the IBM implementation to compute ECMP routes to multiple ASBRs and had the Redback implementation do this from the start. Consequently, we’d like to solicit input as to what other implementations do. If we can reach consensus on this, we could issue an Errata to make this optional.


From: OSPF <<>> on behalf of "Balaji Ganesh (balagane)" <<>>
Date: Tuesday, June 13, 2017 at 4:13 AM
To: OSPF WG List <<>>
Subject: [OSPF] RFC 3101 NSSA/External Route preference clarification

Hi all,

When the metrics are same, RFC 3101 specifies the preference for NSSA/External routes as follows.
In the 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.