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Re: [Idr] Review Updates to draft-ietf-idr-bgp-prefix-sid

"Susan Hares" <shares@ndzh.com> Sun, 18 February 2018 15:54 UTC

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From: Susan Hares <shares@ndzh.com>
To: 'Robert Raszuk' <robert@raszuk.net>, "'Acee Lindem (acee)'" <acee@cisco.com>
Cc: idr@ietf.org
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Date: Sun, 18 Feb 2018 10:54:02 -0500
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Subject: Re: [Idr] Review Updates to draft-ietf-idr-bgp-prefix-sid
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Robert and Acee:

 

If we do “open surgery”, this document will need to go back through the process (WG LC, IETF LC, and IESG Review). 

 

Sue Hares

IDR co-chiar

 

From: Idr [mailto:idr-bounces@ietf.org] On Behalf Of Robert Raszuk
Sent: Sunday, February 18, 2018 5:57 AM
To: Acee Lindem (acee)
Cc: idr@ietf.org
Subject: Re: [Idr] Review Updates to draft-ietf-idr-bgp-prefix-sid

 

Hi Acee,

 

Since we are at the open surgery of this draft there is one more point which did not got resolved during our offline discussion. Feedback from WG is welcome here.

 

The draft RECOMMENDS the following procedure in case of a conflict: 

 

... it is RECOMMENDED that the first prefix using the label index is selected.

 

In BGP there is no guarantee about ordered delivery of BGP prefixes.  It may work such in some specific topologies but not in any arbitrary one. So if we recommend to pick one prefix on R1 and different prefix on R2 and install in forwarding the same SID for those I am afraid the data plane will get quite severely messed up and even finding it during troubleshooting will be quite hard. 

 

Also as pointed out already MPLS anycast SID may on purpose use the same label across nodes according to draft-ietf-spring-mpls-anycast-segments-02. Now if we want to accomplish the same in BGP we would need to advertise different prefixes with the same SID (in MPLS-SR index). the above paragraph does prevents that. Note that adding and advertising the _same_ prefix from different nodes will not work as BGP speakers may pick only one (best path) and use it for forwarding. IBGP or EBGP multipath may not always be enabled/use same set.

 

The crux of the issue when advertising SIDs in BGP is not so much about prefixes, but their next hops which unfortunately the draft does not even mention once.

 

Kind regards,

Robert.

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