Re: [Idr] Questions from the IDR session

[Kaliraj Vairavakkalai <[e-mail]>]

Sorry, missed attaching the reference ppt. PFA..

From: Idr <[e-mail]> on behalf of Kaliraj Vairavakkalai <kaliraj=[e-mail]>
Date: Tuesday, July 27, 2021 at 6:05 PM
To: Susan Hares <[e-mail]>, [e-mail] <[e-mail]>, [e-mail] <[e-mail]>
Cc: 'idr-chairs' <[e-mail]>, [e-mail] <[e-mail]>
Subject: Re: [Idr] Questions from the IDR session
[External Email. Be cautious of content]

Sure Sue. I will copy Swadesh and Haibo to this thread here and answer the questions.

Hi Swadesh,

>1. Swadesh about the efficiency of sending two routes versus Color?

Ref: topology in slide#6 in attached presentation.

Similar to BGP-LU, BGP-CT gives the flexibility of originating CT-route at PE11, or at ASBR13/ASBR14.
So following options exist:

  1.  When originating at PE11, there will be one route received at PE25. NLRI: “RD_PE11: PE11”.
  2.  And yes, when originating at ASBRs with unique-RDs, there will be two routes received at PE21.  NLRIs: RD_ASBR13:PE11, RD_ASBR14:PE11.
            This helps in identifying who originated a particular route. ASBR13 or ASBR14.

  1.  If this is not desired, and loss of troubleshooting ability is OK, then both ASBRs can originate NLRI: “RD_X:PE11” prefix routes. Then it will become same as a).

In CAR, using NLRI: “C1:PE11” model, because of dual role of ‘Color’, option b) cannot be supported. i.e. we cannot identify which ASBR originated a particular route because the NLRI doesn’t have a distinguisher.
One of the ASBR originated-routes is hidden by path-selection at the first BGP-path-selection ‘pinch-point’. And upstream nodes cannot exercise that specific ASBR path if desired.
With CT that option exists. Because no path-hiding happens (thanks to RD), all available paths are available to upstream PE21.

Hi Haibo,

>2. Haibo’s question on using next-hop and color to match transport routes.
>The key in BGP CT route does not match with the key used for VPN route resolution.

                  For the VPN-route resolution, “BGP PNH and color-community” is used.
Similarly, For the CT-route resolution, “BGP PNH and transport-target community” is used.

So it is the same. The key for BGP route resolution is “BGP nexthop and mapping-community”.

IOW, in regular BGP nexthop-resolution, a BGP-route just resolves a <nexthop-address> received on the route.
And with CT, the Mapping-community (Color-community/Transport-target) additionally qualifies the type of transport-tunnel to be taken,
And so the resolution happens on basis of <nexthop-address , mapping-community> tuple. Hope I understood your question right.

Thanks
Kaliraj
From: Susan Hares <[e-mail]>
Date: Tuesday, July 27, 2021 at 4:57 PM
To: Kaliraj Vairavakkalai <[e-mail]>
Cc: 'idr-chairs' <[e-mail]>
Subject: Questions from the IDR session
[External Email. Be cautious of content]

Kaliraj:

Would you help the IDR WG by answer the questions
1. Swadesh about the efficiency of sending two routes versus Color?
2. Haibo’s question on using next-hop and color to match transport routes.
The key in BGP CT route does not match with the key used for VPN route resolution.

Thank you, Sue





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