Hi Rene,
I really appreciate your nice explanation. I love the way you explained the things. I have one confusion in below statement.
"When two EBGP routers that are directly connected do not form a working BGP neighbor adjacency there could be a number of things that are wrong:
_Layer 2 down preventing us from reaching the other side._
_Layer 3 issue: wrong IP address on one of the routers._
_Access-list blocking TCP port 179 (BGP)._
_Wrong IP address configured for BGP neighbor router."
What do you mean by layer 2 down preventing us from reaching the other side? What could be the layer 2 issues except interface down?