Border Gateway Protocol (BGP)

Basically helps with Dynamic routing - Wikipedia

Path-vector protocol, it exchanges best path to a destination between peers. The path is called ASPATH.
BGP serves as a network topology map, like how can we reach from A to C?

IMPORTANT

BGP helps with route optimization, since routers in one AS can provide best path if needed to other AS. Short path is by default. But ASPATH Prepending can make a route less optimal
In the screenshot below, direct path to Alice got artificially made look longer in order to prefer Fiber-optic link

AS and ASN
AS, autonomous system – a collection of routing prefixes (or more objectively – routers) in one or more networks
ASN, autonomous system number is how BGP identifies different networks over internet, allocated by IANA 0-65535, though 64512-65534 are private

Operates over TCP port 179 - TCP guaranties reliability.
Is not automatic, peering must be manually configured

iBGP – Internal BGP (within AS)
eBGP – External BGP (between ASes)