BGP: how the networks of the internet find each other

The idea

With BGP, homeIsp, transitIsp and contentProvider, networks run by different organisations, tell each other which addresses they reach and through whom. A laptop at homeIsp reaches contentProvider's web server by the shortest list of networks.

Steps

1. Sessions over TCP

   homeIsp, a home ISP, swaps routes with two other organisations: transitIsp and contentProvider. In vtysh, FRR's shell, list its BGP sessions, then the connections under them.

   homeIsp:~# vtysh
   homeIsp:~# ss -tn state established '( sport = :179 or dport = :179 )'

2. Who can reach what: the AS path

   Read homeIsp's BGP table: every network it heard of, and through whom. Then read transitIsp's, in the middle. This table is how BGP chooses a way and avoids loops, so reading it is the core skill.

   homeIsp:~# vtysh
   transitIsp:~# vtysh