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

"Acee Lindem (acee)" <> Thu, 20 July 2017 14:49 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 60C9D131CDF for <>; Thu, 20 Jul 2017 07:49:24 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -14.522
X-Spam-Status: No, score=-14.522 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_HI=-5, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, RP_MATCHES_RCVD=-0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001, USER_IN_DEF_DKIM_WL=-7.5] autolearn=ham autolearn_force=no
Authentication-Results: (amavisd-new); dkim=pass (1024-bit key)
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id 2wi-AnuOnNp4 for <>; Thu, 20 Jul 2017 07:49:17 -0700 (PDT)
Received: from ( []) (using TLSv1.2 with cipher DHE-RSA-SEED-SHA (128/128 bits)) (No client certificate requested) by (Postfix) with ESMTPS id 1248C131CDC for <>; Thu, 20 Jul 2017 07:49:17 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple;;; l=37889; q=dns/txt; s=iport; t=1500562156; x=1501771756; h=from:to:cc:subject:date:message-id:references: in-reply-to:mime-version; bh=nnECcxnYtxRr7zsg2PynFY6CvqBosIZnC7X2POydy9U=; b=fW+b5Hz57u2sHrs/FqNN9p81FTwnEC5gVVS8x3SHRk03Af9L5L7AQAlL UzefshL1uMtrgAwxr0L2jfNrvdsZK+IMLmeF9uc4k+CtPcD/cHTIw72+t dgwzuAQlq6M0+4q9etpTKhbMU6cPgIfEg+i4ci5wJDIpbYxvxo7tm2Ih0 Q=;
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-AV: E=Sophos;i="5.40,384,1496102400"; d="scan'208,217";a="270583545"
Received: from ([]) by with ESMTP/TLS/DHE-RSA-AES256-SHA; 20 Jul 2017 14:49:15 +0000
Received: from ( []) by (8.14.5/8.14.5) with ESMTP id v6KEnFZG013892 (version=TLSv1/SSLv3 cipher=AES256-SHA bits=256 verify=FAIL); Thu, 20 Jul 2017 14:49:15 GMT
Received: from ( by ( with Microsoft SMTP Server (TLS) id 15.0.1210.3; Thu, 20 Jul 2017 10:49:14 -0400
Received: from ([]) by ([]) with mapi id 15.00.1210.000; Thu, 20 Jul 2017 10:49:14 -0400
From: "Acee Lindem (acee)" <>
To: Jeff Tantsura <>, David Lamparter <>
CC: Chao Fu <>, "Balaji Ganesh (balagane)" <>, OSPF WG List <>
Thread-Topic: [OSPF] RFC 3101 NSSA/External Route preference clarification
Thread-Index: AQHS6GH6q/Cb2aEok0aI9cREH6X2Mg==
Date: Thu, 20 Jul 2017 14:49:14 +0000
Message-ID: <>
References: <> <> <> <> <>
In-Reply-To: <>
Accept-Language: en-US
Content-Language: en-US
x-ms-exchange-messagesentrepresentingtype: 1
x-ms-exchange-transport-fromentityheader: Hosted
x-originating-ip: []
Content-Type: multipart/alternative; boundary="_000_D5963A95B9D72aceeciscocom_"
MIME-Version: 1.0
Archived-At: <>
Subject: Re: [OSPF] RFC 3101 NSSA/External Route preference clarification
X-Mailman-Version: 2.1.22
Precedence: list
List-Id: The Official IETF OSPG WG Mailing List <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Thu, 20 Jul 2017 14:49:24 -0000


I just looked at the Free Range Routing GitHub ( and they do not support NSSA (RFC 3107) and do NOT use the ASBR Router-ID to break ties in the AS External route calculation so this would support making the tie-breaker optional.


From: Jeff Tantsura <<>>
Date: Wednesday, July 5, 2017 at 3:31 PM
To: Acee Lindem <<>>
Cc: Chao Fu <<>>, "Balaji Ganesh (balagane)" <<>>, OSPF WG List <<>>
Subject: Re: [OSPF] RFC 3101 NSSA/External Route preference clarification


I find Acee's proposal reasonable and support it "as is".


On Jul 5, 2017, at 10:36, Acee Lindem (acee) <<>> wrote:

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.


OSPF mailing list<>