[AusNOG] steering inbound BGP (Telstra)

[Matthew Moyle-Croft]

On 16 Feb 2014, at 8:48 pm, Ben <ben at meh.net.nz> wrote:

...

>>> 
>> 
>> "Fallback routes"?
> 
> Err, advertising with less prepending for the same subnet. 

This is about more specifics.

> 
>> 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 they're stripping /24s and/or have default then they're going to be getting to the interwebz via a transit provider who WILL have full routes because they're clearly not big enough to have routers that cope with being in the DFZ.

> 
> 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?


They'll follow the /24s toward where you're advertising them.  Not ideal, but Telstra have decided to not offer fairly standard tools (BGP communities) so my empathy for them is as close to zero as you can get.

> 
> 
>> 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.

Again, prepending is irrelevant.  /24s will always win as they are more specific.  If you're not clear on that I'd do some reading up on routing.

MMC

> 
> Ben.