Re: Newbies Question: Do I really need to sacrifice Prefix-aggregation to do BGP Load-sharing?

[Tom Beecher <beecher () beecher cc>]

> 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.
>
> ----------------------------------------------------------------------
>   Jon Lewis, MCP :)           |  I route
>   StackPath, Sr. Neteng       |  therefore you are
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