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London, UK
reply to kamikatze

Re: BGP Multihoming default-originate only setup

The easiest way to achieve it would be as pretty much mentioned before.

Advertise a longer AS path to your less preferred provider (perhaps also use one of their communities to append more of their AS to that path aswell), and give that providers default route a lower preference.

Advertise a shorter AS path to your preferred provider, and give its default route a higher preference.

This should take care of incomming and outcomming traffic going via the preferred ISP until it fails, afterwhich traffic will route in and out via the secondary ISP, and then return to the primary ISP once it is restored.

Keep in mind, though, that anyone connected to your less preferred ISP will still route traffic into your network via the link to that network, since it will likely be the preferred path from within that provider. However, anyone outside of that provider is likely to route in to you via your primary ISP where the AS path is much shorter.

Sometimes it is not always easy to achieve your desired outcomes as providers can and do do all sorts of things inside their own networks which mean that things dont behave as you would like.