Newbies Question: Do I really need to sacrifice Prefix-aggregation to do BGP Load-sharing? (the case of Multi-homed + Multi-routers + Multi-upstreams)

William Herrin bill at
Wed Oct 19 06:40:26 UTC 2022

On Tue, Oct 18, 2022 at 11:27 PM Pirawat WATANAPONGSE via NANOG
<nanog at> wrote:
> - Two upstreams (See the following lines), fully cross-connected to our gateways
> - One (pure) commercial ISP
> - One academic consortium ISP (who actually uses the above-mentioned commercial ISP as one of its upstreams as well)
> My current “situation”:
> - All inbounds “flock” in through the commercial ISP, overflowing the bandwidth;
> since (my guess) the academic ISP also uses that commercial ISP as its upstream, there is no way for its path to be shorter.

This is what "prepends" are for. You make the AS path sent to the
commercial ISP longer by adding extra copies of your own AS to the
front. This causes the AS path via the consortium to be the same
length or even shorter if you prefer. By default, BGP prefers the most
specific prefix first and then the shortest AS path second.

Depending on the ISP you may also need to "community" to tell it not
to prefer the direct path with you (which they may have done with a
localpref). But start with the prepends and see what happens.

Bill Herrin

For hire.

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