On Thu, Oct 26, 2017 at 1:58 PM, Jason Lixfeld <jason+nanog@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 ................ herrin@dirtside.com bill@herrin.us Dirtside Systems ......... Web: <http://www.dirtside.com/>