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Re: [GROW] AS_Path prepend BCP

"Jakob Heitz (jheitz)" <jheitz@cisco.com> Mon, 27 July 2020 20:34 UTC

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From: "Jakob Heitz (jheitz)" <jheitz@cisco.com>
To: Michael McBride <michael.mcbride@futurewei.com>, "grow@ietf.org" <grow@ietf.org>
Thread-Topic: AS_Path prepend BCP
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Date: Mon, 27 Jul 2020 20:33:54 +0000
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Subject: Re: [GROW] AS_Path prepend BCP
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I have worked on more than one BGP implementation, but not all of them, of course.
On memory requirements for as-paths:
Attribute sets are shared among stored routes.
That means if two stored routes have the same attribute sets, the attribute set is stored only once.
As-paths are shared among attribute sets.
That means if two stored attribute sets have the same as-path, then the as-path is stored only once.
Storing them in the control plane is not a big problem.

However, as-paths can be sent in netflow.
Netflow is generated in the forwarding plane.
AS-paths are not stored in expensive fast memory on the forwarding plane, but still,
using memory on the forwarding plane has greater impact than on the control plane.

An as-path consists of AS_SEQUENCEs (and other elements). An AS_SEQUENCE can contain
a maximum of 255 ASNs. If the as-path is longer, then multiple AS_SEQUENCEs are
required. The code to parse them and create them is not often exercised and
is a potential for bugs in fresh code. The older implementations have these bugs
well and truly shaken out of them.

Regards,
Jakob.

From: GROW <grow-bounces@ietf.org> On Behalf Of Michael McBride
Sent: Sunday, July 26, 2020 11:42 AM
To: grow@ietf.org
Subject: [GROW] AS_Path prepend BCP

Hello wg,

We have submitted https://datatracker.ietf.org/doc/draft-mcbride-grow-as-path-prepend/ which is intended to be a bcp in the use of AS_Path prepend based on work of Doug Madory. As we state in the intro: AS_Path prepending is discussed in Use of BGP Large Communities [RFC8195] and this document provides additional, and specific, guidance to operators on how to be a good internet citizen with the proper use of AS_Path prepend.

We would encourage feedback on this document.

thanks,
mike

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