Re: BGP:tie-breaking algorithm enhancement proposal

[Alexey Zinin <[e-mail]>]

Alvaro:

I know of at least one IX which has suffered and still
is suffering from this change in the path selection
process (I was handling this case).

I think what you propose may be a useful enhancement, but any
changes to the route selection algo must be implemented
with a configuration option, allowing to revert to standard
behavior, since people may (and actually do) rely on the Router-ID
as the determinating factor.

Alex.

> 
> 
> The current BGP4 draft (draft-ietf-idr-bgp4-09) reads:
> 
> -------------------------------------------------------------------
> 9.1.2.1 Breaking Ties (Phase 2)
> .....
>    The tie-breaking algorithm begins by considering all equally
>    preferable routes and then selects routes to be removed from
>    consideration.  The algorithm terminates as soon as only one route
>    remains in consideration.  The criteria must be applied in the order
>    specified.
> 
> .....
>       c) If at least one of the candidate routes was received from an
>       external peer in a neighboring autonomous system, remove from
>       consideration all routes which were received from internal peers.
> 
>       d) Remove from consideration all routes other than the route that
>       was advertised by the BGP speaker whose BGP Identifier has the
>       lowest value.
> -------------------------------------------------------------------
> 
> I would like to propose the following enhancement (to be put in place of
> the last step listed above):
> 
>       d) If the remaining routes are all external, remove from consideration 
>       all routes except the one that was received first and any others with 
>       the same BGP Identifier as it (the one received first).
> 
>       e) Remove from consideration all routes other than the ones advertised 
>       by the BGP speaker whose BGP Identifier has the lowest value.
> 
> 
> 
> In summary, the change will allow a BGP speaker to keep the oldest
> external route as the best path, if all the other conditions are the same.  
> Except for the case where more than one route is received from the same
> eBGP peer.
> 
> In keeping the oldest external, the number of unnecesary routing updates
> can be lowered.  Note that by advertising a new bestpath (if all the
> attributes, except the BGP Identifier, are the same), the exit point will
> not change.
> 
> In the case where the BGP Identifier is the same, then the "normal"
> procedure is used.
> 
> Any/all comment are welcome. :-)
> 
> Alvaro.
> 
> 
>