On Thu, Oct 26, 2017 at 1:58 PM, Jason Lixfeld <jason+na...@lixfeld.ca> wrote:
> Of all the ISPs that I am familiar with that have a BGP community > structure usable by their peering partners and/or downstream customers, > among other things, they allow the customer to signal the ISP to prepend > their own AS to the as-path of a particular prefix announcement. > > What functionality does a provider prepend support that is otherwise lost > in the absence of such a feature, but all the while, the customer would be > able to prepend their own AS to the same prefix announcement anyway? > Hi Jason, 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. So, prepends are the primary knob used for controlling which path gets taken. Is this a relic from before ISPs allowed for local preference adjustment, > or is there actually a use case for this? It's the exact opposite of a relic. Regards, Bill Herrin -- William Herrin ................ her...@dirtside.com b...@herrin.us Dirtside Systems ......... Web: <http://www.dirtside.com/>
