1) Are there any networks with routing policy that looks at prepends and says "if we see a peering path with >X number of prepends (or maybe just path length >X), demote the localpref to transit or lower"? "i.e. They obviously don't want us using this path, turn it into a backup path."
Yes. At a previous job, this is exactly what I did. If the path length was X or longer, set localpref to our last resort value. If path length was Y or longer, then I dropped completely, and at that point following defaults was just as good. Maybe once I hit something that caused a performance problem , but an email to that AS was all it took to fix ; they didn't realize they were prepending that much and corrected it. I have firsthand knowledge of some other networks that do similar things. On Thu, Oct 20, 2022 at 9:21 AM Jon Lewis <jlewis@lewis.org> wrote:
On Thu, 20 Oct 2022, Tom Beecher wrote:
1. Prepending by itself isn’t bad. Prepending past the point that it is effective in accomplishing anything is what you generally want to avoid. Even then, it’s not nearly as big a deal as some make it out to be in most cases.
To me, it's somewhat comical to see routes prepended 10-20 or more times. If one or two prepends doesn't do it, 10-20 isn't likely to either.
AFAIK, it's pretty common to use localpref to prefer peering (free) routes over transit (paid paths), and in cases where remote networks see your prepended path via peering, "no amount" of prepends is going change the fact that they prefer the free path.
While writing this though, two things occurred to me.
1) Are there any networks with routing policy that looks at prepends and says "if we see a peering path with >X number of prepends (or maybe just path length >X), demote the localpref to transit or lower"? "i.e. They obviously don't want us using this path, turn it into a backup path."
2) Particularly back when it was found some BGP implementations broke when encountering unusually long as-paths, I think it became somewhat common to reject routes with "crazy long" as-paths. If such policy is still in place in many networks, excessive prepending would actually have the desired effect for those networks. i.e. The excessive prepends would get that path rejected, keeping it from being used.
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