What's the point of prepend communities?

William Herrin bill at herrin.us
Thu Oct 26 18:37:56 CST 2017


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


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