http://www.9tut.com/border-gateway-protocol-bgp-tutorial
BGP AS numbers can be between 1 to 65,535.
Internet Assigned Numbers Authority (IANA) manages the AS numbers from 1 to 64,512 for public use (similar to public IP addresses) while 64,512 to 65,535 numbers are reserved for private use (similar to private IP addresses).
BGP peer or BGP neighbor: Any two routers that have formed a TCP connection to exchange BGP routing information (as BGP runs over TCP on port 179, not UDP)
External BGP (eBGP): refers to the BGP neighbor relationship between two peers belongs to different AS. It is recommended that eBGP should be directly connected. Never run an IGP between eBGP peers.
Maybe you learned and understood about EIGRP, OSPF routing protocols. They are different but both of them have the same purpose: find the most optimal path to the destination. But when we are working in ISP level we don’t care much about this. In ISP we really want to control the path, even it is not the most optimal path, to the destination.
Another reason to use BGP is BGP can handle very big routing tables. The ISP level of routing do had large number of routes, which IGP cannot handle. BGP handle such large routes between AS. Currently the global Internet routing table contains over 500,000 routes.
BGP is a Layer 4 Protocol where peers have to be manually configured1 to form a TCP connection and begin speaking BGP to exchange routing information.
BGP peers exchange routing information between them via BGP sessions that run over TCP, which is a reliable, connection oriented & error free protocol.
https://blog.cdemi.io/beginners-guide-to-understanding-bgp/
For example, in 2008, when the Pakistan Government tried to ban YouTube, Pakistan Telecom (AS17557) used BGP to route YouTube's address block (AS36561) into a black hole.
BGP Black Hole
https://networklessons.com/bgp/internal-bgp-border-gateway-protocol-explained/
As it is today the Internet BGP routing tables have over 300,000 active forwarding entries and this is with summarization of over 2 billion addresses.
BGP just uses a path (Autonomous Systems - AS) hop count instead of a device hop count.
The first thing that must be understood is that each BGP device can have both internal and external BGP connections to other devices. Internal BGP connections are within the same AS while external BGP connections are between different AS's.