How to avoid becoming your organisation as Transit AS?

When looking at BGP, the simplest/most-scalable way to prevent transit AS is with a filter-list.  This calls for regular expressions, which sometimes scare people…
ip as-path access-list 1 permit ^$
router bgp 100
neighbor 1.2.3.4 filter-list 1 out
And what you will do at that point is ONLY send out routes with an EMPTY as-path.  If you receive a route from ANY other eBGP peer, the as-path list will no longer be empty.  Therefore empty = your own internal routes!
And that way whether it’s 1 route today or 100 of them you are originating tomorrow, the rule set will still work.

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