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/>
