Re: [GROW] [Idr] draft-ietf-grow-diverse-bgp-path-dist: risk of inconsistent path selection

[Robert Raszuk <[e-mail]>]

Jakob,

Diverse-path works between RR and the client and client does not 
advertise back the best path to other IBGP peers so there is no issue 
with churn or oscillation.

Advertising different paths to possible EBGP peers is a feature not a bug.

By definition of diverse path such path will have different next hop 
hence the metric to next hop would be in most cases a tie breaker.

If metric to next hop is the same across both paths I see no reason to 
mandate any rules which would force different PEs to choose the same 
path. If operator wishes to prefer one path then the other he will do so 
by applying local preference.

The fact that you like to see all paths in BGP to be selected as the 
same is not sufficient reason to mandate it on the clients. I see more 
value and no issue keeping them diverse especially since you are 
pointing out last step in best path selection.

Cheers,
R.


> Again, if the tie breaker reaches step (f),
> the client will use the BGP Identifier of
> the RR it receives the path from to choose
> its best.
>
> In BGP, we like all speakers to choose the same
> path unless IGP metrics are different.
>
> With diverse-path, there is a possibility that
> different speakers choose different paths even
> if IGP metrics are the same.
>
> If the clients were to receive their paths
> from the source speaker, they would choose the
> same path at tie breaker (f).
>
> draft-pmohapat-idr-fast-conn-restore proposes
> the Edge_Discriminator attribute to fix it.
>
> It may not be a problem. However, the path selection is
> different than without diverse-path, therefore
> the behaviour needs to be stated in the draft.
>
> The rule I stated will remove the inconsistency.
>
> --
> Jakob Heitz.