Re: [Idr] Comments: Route Origin Community in SR Policy(draft-ietf-spring-segment-routing-policy)

Robert Raszuk <> Thu, 30 April 2020 09:15 UTC

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From: Robert Raszuk <>
Date: Thu, 30 Apr 2020 11:15:36 +0200
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To: "Ketan Talaulikar (ketant)" <>
Cc: "Chengli (Cheng Li)" <>, "" <>, idr wg <>, SPRING WG <>, Fangsheng <>, stefano previdi <>
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Subject: Re: [Idr] Comments: Route Origin Community in SR Policy(draft-ietf-spring-segment-routing-policy)
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Hi Chengli and Ketan,

Well I think (perhaps to your surprise) the current text is actually

See the overall idea of section 2.4 is not to define the real source of the
candidate path. That is done in section 2.5 The idea here is to keep
multiple *paths or versions* of the candidate paths in the local system

See if you continue reading section 2.6 demystifies the real objective:

   The tuple <Protocol-Origin, originator, discriminator> uniquely
   identifies a candidate path.

So the real originator is encoded in discriminator and here it just
means the peer candidate path was

received from. And if you read on this entire exercise only servers
best path selection as described in section 2.9.

.... the following order until only one valid best path is selected:

   1.  Higher value of Protocol-Origin is selected.

   2.  If specified by configuration, prefer the existing installed

   3.  Lower value of originator is selected.

   4.  Finally, the higher value of discriminator is selected.


      The originator allows an operator to have multiple redundant
      controllers and still maintain a deterministic behaviour over
      which of them are preferred even if they are providing the same
      candidate paths for the same SR policies to the headend.


On Thu, Apr 30, 2020 at 10:46 AM Ketan Talaulikar (ketant) <ketant=> wrote:

> Hi Cheng,
> I assume you are recommending the use of Route Origin Extended Community (
> for conveying the
> “Originator” when the SR Policy update is propagated over eBGP sessions via
> other eBGP/iBGP sessions instead of direct peering with the headend.
> I believe it does address the scenario you describe given that it is
> expected that SR Policy propagation via BGP is happening within a single
> administrative domain even if it comprises of multiple ASes.
> Also copying the IDR WG for inputs since this would likely need to be
> updated in draft-ietf-idr-segment-routing-te-policy.
> Thanks,
> Ketan
> *From:* spring <> *On Behalf Of *Chengli (Cheng Li)
> *Sent:* 30 April 2020 07:34
> *To:*
> *Cc:* SPRING WG <>; huruizhao <>;
> Fangsheng <>
> *Subject:* [spring] Comments: Route Origin Community in SR
> Policy(draft-ietf-spring-segment-routing-policy)
> Hi authors,
> In section 2.4 of [draft-ietf-spring-segment-routing-policy-06],
> introduced how the node-address of "Originator of CP(Candidate Path)" is
> generated when the Protocol-Origin is BGP. It says:
>     “Protocol-Origin is BGP SR Policy, it is provided by the BGP component
> on the headend and is:
>      o  the BGP Router ID and ASN of the node/controller signalling the
> candidate path when it has a BGP session to the headend, OR
>      o  the BGP Router ID of the eBGP peer signalling the candidate path
> along with ASN of origin when the signalling is done via one or  more
> intermediate eBGP routers, OR
>      o  the BGP Originator ID [RFC4456] and the ASN of the
> node/controller  when the signalling is done via one or more
> route-reflectors over  iBGP session.”
> In the operator's network, in order to reduce the number of  BGP sessions
> in controller and achieve scalability, the controller only establishes eBGP
> peer with the RR. And the RR establishes iBGP peers with the headends. As
> mentioned in the draft, the headend will use the RR's Router ID as the CP's
> node-address (the signaling is done via route transmission from RR to the
> headend instead of route reflection).  The headend needs to carry the CP's
> key when reporting the SR Policy status to the controller through BGP-LS.
> And there is a problem that the controller may not recognize the key
> because the node-address is generated by the RR node.
> For network robustness, two or more RRs are usually deployed. This will
> introduce another problem.. When the same CP advertised by the controller
> is delivered to the headend through different RRs, the headend cannot
> distinguish whether it is the same CP because the node-address in the CPs'
> key  comes from different RRs.
> To solve these problems,  We recommend carrying the Route Origin Community
> (defined in RFC 4360) directly when the controller advertises BGP routes.
> In this way, the key  of the CP is determined by the controller and will
> not change during the advertisement of BGP routes.
> Thanks,
> Cheng
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