Re: What's the point of prepend communities?

[William Herrin <bill () herrin us>]

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/>