BGP - Border Gateway Protocol

• BGP is a routing protocol
• Used to control how data flows from point A to point B
• BGP is made up from a lot of self managing networks know as Autonomous Systems (AS)
• AS could be a large network, collection of routes etc. and is viewed as a black box from BGP perspective
• Each AS is allocated a number by IANA, named ASN
• ASNs are 16 bit in length and range from 0 to 65535, the range from 64512 to 65534 is private
• We can get 32 bit ASN numbers as well (this is out of scope for this exam)
• ASNs are used by the BGP to identify different entities on the network
• BGP is designed to be reliable and distributed, and it operates of TCP/179
• It is not automatic, the communication between to AS should be done manually
• Autonomous Systems do exchange network topology information between them
• BGP is a path-vector protocol: it exchanges the best path to a destination between peers, the path is called ASPATH (Autonomous System Path)
• BGP does not take into account link speed or condition, it focuses on path only
• iBGP - internal BGP, routing within an AS
• eBGP - external BGP, routing between AS’s
• BGP example:
• BGP always choses the shortest path. There are ways to influence the path by artificially expending the path (prepending itself to the path)