Re: Consistent routing policy?

[Martijn Schmidt via NANOG <nanog () nanog org>]

Hi Ben,

Prefix deaggregation and inconsistent announcements might work fine in 
the US where all paths are mostly of equal quality, but is the bane of 
my existence when it happens in regions like the Middle East or Africa 
where one transit used by a target ISP might be connected locally and 
the other connected in Europe. If you want to steer traffic you may be 
better off using BGP action communities, I'll leave you with this 
excellent presentation from RAS to provide some background on what might 
be achieved with that:

https://archive.nanog.org/meetings/nanog40/presentations/BGPcommunities.pdf

Best regards,
Martijn

On 9/16/19 3:05 PM, Mark Tinka wrote:
> On 16/Sep/19 14:47, Ben Logan wrote:
> > Thanks, Mark.  So the discrepancy between what's being advertised (/21
> > vs /22) shouldn't cause any issues?  That's the part I got a bit
> > confused about.  I don't see how it would, but I wanted to make sure.
> Longest match always wins... so provided your /22's are in the global
> table, traffic will follow the path toward them before the /21 is preferred.
>
> So, for example, if the upstream to whom you are sending the /21 doesn't
> do anything about how they learn the /22 from another source, (for their
> network) they will also send traffic back to you via the /22 path. This
> may or may not be preferred by you, or them. I suppose that's the main
> thing to think about.
>
> Mark.