[AusNOG] steering inbound BGP (Telstra)

[Ben]

On Sun, Feb 16, 2014 at 08:35:45PM -0800, Matthew Moyle-Croft wrote:
> 
> On 16 Feb 2014, at 8:26 pm, Ben <ben at meh.net.nz> wrote:
> 
> > On Sun, Feb 16, 2014 at 08:12:15PM -0800, Matthew Moyle-Croft wrote:
> >> 
> >> On 16 Feb 2014, at 8:09 pm, Joshua D'Alton <joshua at railgun.com.au> wrote:
> >> 
> >>> Whoops completely forgot about localpref!
> >>> 
> >>> @Andrew  2x24/s wouldn't work for Telstra inbound though as they'd still be preferring their learned /23 from their customer vs the 2x/24s from their peer(s), right? Or do you mean it would work for making the majority of the traffic come in via the other provider?
> >> 
> >> Yes it would - /24s are more specific so preferred.  /23 would only kick in if /24s stopped being advertised.
> > 
> > Although that may work, I wouldn't advocate it without fallback routes.
> 
> "Fallback routes"?

Err, advertising with less prepending for the same subnet. 
 
> More specifics are a common tool used to TE traffic.  The issue is if your /24s aren't globally advertised.  

Ok, so say Telstra advertise routes at Equninx Los Angeles, and the other provider doesn't advertise at Equninix Los Angeles, then
people close to that are going to often have local preference set to send via peering exchange.  But say they have a low end router
for peering, and theey decide to strip /24s, or default route+peering routes.  (the same can apply for domestic traffic, but it's
more significant and more obvious for international)

If you advertise a /23 to Telstra, and /23 and two /24s to your other provider, then when Telstra receive this traffic what are they
meant to do with it?  Is that clearer?

 
> The main issue is - if your other provider(s) don't have good domestic Australian connectivity, but again, I kind of assume anyone doing this has some clue and/or deserves some failure based learning.

Maybe I don't understand the problem completely.  From my understanding the idea was to get rid of all traffic off the fallback link, unless there's an actual outage, whilst still
advertising BGP routes.  That in itself is not acheivable from what I can tell.  And by advertising more specficially I can't see any advantage to just prepending normally.  If telstra
do accept the /24 over the /23, and send to the customer regardless, and still advertise out the /23 then there is a chance for misdirected traffic either going to the customer via
their alternative provider, or still going through their normal link.

Ben.