> > 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 <jle...@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. > > ---------------------------------------------------------------------- > Jon Lewis, MCP :) | I route > StackPath, Sr. Neteng | therefore you are > _________ http://www.lewis.org/~jlewis/pgp for PGP public key_________ >