What's the point of prepend communities?

joel jaeggli joelja at bogus.com
Fri Oct 27 03:44:55 CST 2017

On 10/26/17 10:58, Jason Lixfeld wrote:
> Hi,
> 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?
It has no effect on the provider itself since they prefer customer
routes anyway.

prepending towards a peer of your provider is to encourage them to
select an alternative (shorter) path.

prepending when it works is preferable to no export since the later
doesn't leave that provider available for fallback.
> Is this a relic from before ISPs allowed for local preference adjustment, or is there actually a use case for this?
local pref and med is um local to your upstream.
> Thanks!

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