|English: BGP MED (Photo credit: Wikipedia)|
|English: Border Gateway Protocol 4 Finite-state machine diagram (Photo credit: Wikipedia)|
As you can guess BGP v4 a complex protocol: good tutorials on BGP are really invaluable. So, where can you find the best tutorials on BGP?
In my opinion, Olivier Bonaventure has written some of the best tutorials on BGP. You can find them all on his webpage.
I recommend in particular the last part of his tutorial, where he describes the role of CE, P, and PE routers to permit the setup of VPNs among enterprise sites without too much manual configuration.