What I'm trying to say here is that just saying "do not prepend" does not help. The purpose of as-path prepending is to de-prefer a route advertised to one AS with respect to the same route advertised to another AS.
We need to provide people with alternative methods to de-prefer a route. For example: To de-prefer a route at your ISP, use the communities as published by that ISP. They will not be susceptible to preference attacks once they leave this ISP. To de-prefer a route further afield in the internet, as-path prepending works in some cases, but not all. Usually 1, 2 or 3 prepends work in most cases. Looking glasses can be used to verify if the prepends are working. If as-prepending does not work, an alternative is to split the prefix to the preferred path. That means to advertise multiple more-specific prefixes that cover the range of the original prefix. Do we want to make these recommendations? My example illustrates one case where as-path prepending does not work to de-prefer a route. It shows a way that large ISPs can help to make as-path prepending work for this case. Regards, Jakob. From: GROW <[email protected]> On Behalf Of Jakob Heitz (jheitz) Sent: Wednesday, August 5, 2020 8:50 PM To: [email protected] Subject: Re: [GROW] AS_Path prepend BCP Consider a common problem [An Ink Drawing] Tier1-B sets local-preference for its customer to 120 and for its peer to 100. How does Customer cause Tier1-B to prefer the path: Content -> Tier1-B -> Tier1-A -> Regular-Provider -> Customer instead of its default path: Content -> Tier1-B -> Backup-Provider -> Customer ? Solution 1 -------------- Customer advertises split prefixes to Regular-Provider. Eg., 10.0.2.0/24 and 10.0.3/24 rather than 10.0.2/23. This works, but causes bigger FIBs for everybody. Solution 2 -------------- Customer advertises its routes with communities published by Tier1-B to lower its local-preference to Backup-Provider. This requires Backup-Provider to pass communities through and for Customer to know what Backup-Provider's upstreams are. It is operationally cumbersome. Solution 3 -------------- Tier1-B implements a route-policy like: if as-path length ge 15 then set local-preference 80 endif Then Customer can add lots of AS prepends that will actually work!! Regards, Jakob.
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