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

Jon Lewis jlewis at
Thu Oct 20 13:20:22 UTC 2022

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

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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