Re: [Idr] Part 3 of CAR/CT Adoption call (7/14/2022 to 7/27/2022) - Operational Differences

[Shraddha Hegde <[e-mail]>]

IDR WG,

A number of adoption posts for CAR/CT have expressed concern over BGP-CT's "mapping community"
as not being compatible to SR-PCE architecture. I would like to clarify that BGP-CT is very much compatible to
SR-PCE architecture. "mapping community" is an abstraction that carries the desired intent.
In terms of protocol extensions it could be regular BGP community or Extended color community.


This has been specified in Section 2 Terminology section in https://datatracker.ietf.org/doc/draft-kaliraj-idr-bgp-classful-transport-planes/





Here is example topology and related description from CT authors



                                        Gold = 100 Bronze = 200
                                          |-------RSVP-TE-----|
                                +--------[ASBR2     (AS2)     PE2]--+
        Gold = 100 Bronze = 200 |                                   |
CE1-----[PE1      (AS1)     ASBR1]                                 CE2 (1.1.1.1)
          |--COL-SR-TUNNEL---|  |                                   |   <--SvcPfx1 (color:0:100)
                                +--------[ASBR3     (AS3)     PE3]--+   <--SvcPfx2 (color:0:200)
                                         |-----FLEX-ALGO-----|          <--SvcPfx3 (color:0:100200)
                                        Gold = 100 Bronze = 200

For BGP-CT routes, transport-target:0:<color> will be the mapping community as well


The resolution scheme for BGP-CT routes will be auto-created as the following in all relevant BNs and SNs

BGP-CT resolution-schemes:
--------------------------

Resolution Scheme Gold (Auto Created)
Mapping-community: transport-target:0:100
Transport Route DB: TC[100]

Resolution Scheme Bronze (Auto Created)
Mapping-community: transport-target:0:200
Transport Route DB: TC[200]

Service Route resolution-schemes in PE1
---------------------------------------

Resolution Scheme Gold_fallback_BE
Mapping-community: color:0:100
Transport Route DB: TC[100, BE]

Resolution Scheme Bronze_fallback_BE
Mapping-community: color:0:200
Transport Route DB: TC[200, BE]

Resolution Scheme Gold_fallback_Bronze
Mapping-community: color:0:100200
Transport Route DB: TC[100, 200]




------------------------------------------------------------------------------------------------------------------



For deployments that want to make use of extended color community, "mapping community" is nothing but extended color community.

While "mapping community" adopts itself well to the SR-PCE architecture it also works well for deployments that used

intent-aware paths and used technologies that existed prior to SR-PCE. "mapping community" is also

 flexible enough to satisfy evolving usecases. Example scenarios described below.



1.  A number of RSVP deployments use regular BGP communities carried in service prefixes to represent

 the intent-route that the service prefix should resolve on. These deployments would like to

 extend the intent-awareness multi-domain without having to go through the pain of re-mapping all

their service prefixes to start using extended color community.

"mapping community" caters well to this usecase

2.  The intent-awarenesss usecases  and requirements have evolved from SR-PCE days

Section 6 of https://datatracker.ietf.org/doc/draft-hr-spring-intentaware-routing-using-color/

describes a number of fallback requirements where

                           "Fallback schemes should be decoupled from primary.  For

            example, different service routes using same primary but

            different fallback schemes"



     In this usecase intent is more than just color.

BGP-CT solves this by carrying "mapping community" in service prefix that represents the

 intent which specified primary color and fallback color.

BGP-CT's approach for carrying intent is consistent. It's always in the "mapping community" no matter what

Kind of intent is being carried.



Question to CAR authors.



How are the uses cases 1 and 2 solved in BGP-CAR?





Rgds

Shraddha



Juniper Business Use Only