Keep in mind also that when you allow a customer to deprioritize a particular peer you can really blow up your COGS model (assuming you buy transit).
Cheers, Steve On Thu, Oct 26, 2017 at 1:05 PM, William Herrin <b...@herrin.us> wrote: > On Thu, Oct 26, 2017 at 2:47 PM, Jason Lixfeld <jason+na...@lixfeld.ca> > wrote: > > > Hi Bill, > > > > > On Oct 26, 2017, at 2:37 PM, William Herrin <b...@herrin.us> wrote: > > > > > > BGP routing is based on "distance". Distance in BGP is primarily > > calculated as the number of ASNs in the AS Path. Prepends make a path > more > > distance, encouraging routers to choose a different path if one is > > available. > > > > I understand how prepends fit in the context of best path selection, but > > my question was more the difference between a customer signalling the ISP > > to prepend their AS using a BGP community stamped to a prefix vs. the > > customer prepending their own AS instead. > > > > > Hi Jason, > > You'd only use communities like that if you want to signal the ISP to > deprioritize your advertisement on a particular peer or set of peers but > not others. That's when you're getting fancy. It's not the norm. The norm > is you want to deprioritize one of your paths as a whole. Maybe that link > has less capacity or is enough better connected that it would always > override your other links unless you detune it a little. > > I mean, you could tell the ISP to prepend everything based on a community, > assuming they support such a community, but why would you? That needlessly > makes things more complicated. > > Regards, > Bill Herrin > > > -- > William Herrin ................ her...@dirtside.com b...@herrin.us > Dirtside Systems ......... Web: <http://www.dirtside.com/> >
