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

["Ketan Talaulikar (ketant)" <[e-mail]>]

Hi Cheng,

I assume you are recommending the use of Route Origin Extended Community (https://tools.ietf.org/html/rfc4360#section-5) 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 <[e-mail]> On Behalf Of Chengli (Cheng Li)
Sent: 30 April 2020 07:34
To: [e-mail]
Cc: SPRING WG <[e-mail]>; huruizhao <[e-mail]>; Fangsheng <[e-mail]>
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