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

"Balaji Ganesh (balagane)" <> Wed, 05 July 2017 06:01 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 43BF3127698 for <>; Tue, 4 Jul 2017 23:01:36 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -14.521
X-Spam-Status: No, score=-14.521 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, URIBL_BLOCKED=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 EF1gzWyAXDY5 for <>; Tue, 4 Jul 2017 23:01:34 -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 47E16129B28 for <>; Tue, 4 Jul 2017 23:01:26 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple;;; l=24306; q=dns/txt; s=iport; t=1499234486; x=1500444086; h=from:to:subject:date:message-id:references:in-reply-to: mime-version; bh=qQBDUhW0SGGpWtW3gPp39WFc481qVU0wGTJCMoasW1A=; b=iMVaIyRi2Tx3cvu2zFt5Q3DD/PCkz4yVj5VcLON1xugr2vq2e1B0lnVF B5Yzwj9yhAGjXmv06O3Ugj+YuWI5u+CqKPguRDT3EgSix3Z8KV2Z8Aw4W a0ZMf0Ec6UQOy0Bj+Du4PyI3ZaNDmkv9S92EKkt51a0m2R4mLe6T50tyn s=;
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-AV: E=Sophos;i="5.40,310,1496102400"; d="scan'208,217";a="264139964"
Received: from ([]) by with ESMTP/TLS/DHE-RSA-AES256-SHA; 05 Jul 2017 06:01:25 +0000
Received: from ( []) by (8.14.5/8.14.5) with ESMTP id v6561Pa2014961 (version=TLSv1/SSLv3 cipher=AES256-SHA bits=256 verify=FAIL) for <>; Wed, 5 Jul 2017 06:01:25 GMT
Received: from ( by ( with Microsoft SMTP Server (TLS) id 15.0.1210.3; Wed, 5 Jul 2017 01:01:24 -0500
Received: from ([]) by ([]) with mapi id 15.00.1210.000; Wed, 5 Jul 2017 01:01:24 -0500
From: "Balaji Ganesh (balagane)" <>
To: OSPF WG List <>, "Acee Lindem (acee)" <>
Thread-Topic: [OSPF] RFC 3101 NSSA/External Route preference clarification
Thread-Index: AQHS6GH6q/Cb2aEok0aI9cREH6X2MqJE1v4Q
Date: Wed, 05 Jul 2017 06:01:24 +0000
Message-ID: <>
References: <>
In-Reply-To: <>
Accept-Language: en-US
Content-Language: en-US
x-ms-exchange-transport-fromentityheader: Hosted
x-originating-ip: []
Content-Type: multipart/alternative; boundary="_000_55cc5c59a333478a8af746881e5ac49aXCHALN017ciscocom_"
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: Wed, 05 Jul 2017 06:01:36 -0000

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.