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

William Herrin bill at
Thu Oct 20 14:05:40 UTC 2022

On Thu, Oct 20, 2022 at 5:13 AM Pirawat WATANAPONGSE via NANOG
<nanog at> wrote:
> I have considered the prepending myself, but dare not implement it yet
> for the fear that BGP (Human) Community will burn me alive, witch-hunt style,
> because of the following reasons:
> 1. I can see from looking glass(es) that my upstreams already practice prepending (some paths) at their level (at least 3 more hops [x4]), supposedly to “balance” their bandwidth.
> 2. Should I start prepending mine, I might upset their balance, causing them to prepend more, thus starting a “prepend war”. [I imagine that x20+ prepending starts out this way]
> The way I see it, prepending (or maybe even the whole BGP-Path thing) is a local-optimization problem: it’s only best for someone, not globally.
> And the Higher-Tiers (Lower Tier-Numbers) will always “engineer” me in the end.
> Worse yet, I might be out-voted by de-aggregation insider “cultists” anyway.

Hi Pirawat,

You asked the experts how it's done. It's done with prepends. Do you
really want to argue with the answer?

De-aggregation is a last resort, the bluntest tool in the toolchest.
And it costs other people money so they don't appreciate you doing it
unless you absolutely have to.

As others have said, no one is going to yell at you because you
prepended your AS two or three or even five times. If you don't get
the desired effect after 5, you're running up against a problem
prepends won't solve. The typical problem is that your upstream has
used "localprefs" to prefer a particular path to you, overriding AS
path length as the deciding factor. Competent upstreams that employ
this technique also allow you to set a "BGP community" on your
advertisement that overrides this behavior. A "BGP Community" is a
32-bit number often expressed as two 16-bit numbers the first of which
is the ISP's AS number. When detected by the router, the number causes
it to apply some locally-chosen rule to the route. If you ask them,
the ISP will provide you with a list of "BGP Communities" (numbers)
they allow you to set on your route advertisement along with what
action they will take if they see that number.

Bill Herrin

For hire.

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