An Autonomous System (AS) is a set of Internet routable IP prefixes belonging to a network or a collection of networks that are all managed, controlled and supervised by a single entity or organization.
The Internet is a network of networks*, and autonomous systems are the big networks that make up the Internet. More specifically, an autonomous system (AS) is a large network or group of networks that has a unified routing policy. Every computer or device that connects to the Internet is connected to an AS.
Autonomous System Numbers (ASNs) range from 0 – 65535 and 64512-65534 are private and reserved.
iBGP – Interior Border Gateway Protocol – Routing within AS
eBGP – Exterior BOrder Gateway Protocol – Routing between AS
ASPATH:
BGP: BGP is the protocol for routing data packets between ASes.
Each AS uses BGP to announce which IP addresses they are responsible for and which other ASes they connect to. BGP routers take all this information from ASes around the world and put it into databases called routing tables to determine the fastest paths from AS to AS. When packets arrive, BGP routers refer to their routing tables to determine which AS the packet should go to next.
With so many ASes in the world, BGP routers are constantly updating their routing tables. As networks go offline, new networks come online, and ASes expand or contract their IP address space, all of this information has to be announced via BGP so that BGP routers can adjust their routing tables.
AS Path Prepending is used to artificially lengthen the least preferred path so routes can be forced to take a different / alternative short path.
Local Preference: used within an AS and highest local preference is preferred (default is 100)
AS Path Length: shortest AS path lenght preferred (e.g. 100 200 300 over 100 200 201 300)
Multi-Exit Discriminator (MED): path with lowest MED preferred.
eBGP over iBGP – prefer eBGP over iBGP