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>

> 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

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.

Bill Herrin

William Herrin ................ herrin at dirtside.com  bill at herrin.us
Dirtside Systems ......... Web: <http://www.dirtside.com/>

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