Newbies Question: Do I really need to sacrifice Prefix-aggregation to do BGP Load-sharing?
Tom Beecher
beecher at beecher.cc
Thu Oct 20 14:49:02 UTC 2022
>
> 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 at 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_________
>
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