BGP (Border Gateway Protocol) is a protocol for sharing IP route information between administrative zones known as autonomous systems.
BGP was created mainly to help large network operators, like ISPs, apply routing policies to selectively allow and disallow traffic from other
BGP (Border Gateway Protocol) is a protocol for sharing IP route information between administrative zones known as autonomous systems.
BGP was created mainly to help large network operators, like ISPs, apply routing policies to selectively allow and disallow traffic from other networks.
- Autonomous systems
- BGP4 protocol
- Policy routing
- Network designs
Autonomous systems (ASes) are collections of connected routers managed by a single organization, for example an ISP, or a large company like Amazon. The Internet is made up of ASes that route traffic internally and externally between each other.
IP routers are responsible for the next hop an IP packet takes. In order to know where to forward a request to, IP routers need to share route information with each other. ASes use IGPs (Interior Gateway Protocols), like RIP and OSPF, to share route information within their AS, and EGPs (Exterior Gateway Protocols) to share routing information with other ASes.
BGP is an EGP. BGP1 was specified in 1989, and BGP4 was specified in 1993. BGP4 supports CIDR and subnetting [1, P. 101].
Note: Confusingly, the obsolete precursor to BGP is the Exterior Gateway Protocol (EGP), but both BGP and EGP are examples of exterior gateway protocols. In this document, EGP refers to exterior gateway protocols.
Exterior gateway protocols were developed primarily to allow ASes to set policies on their routing traffic. For example, two ASes might have a commercial agreement which means they can send traffic through each other, whereas another AS might want to disallow traffic from an AS that doesn’t pay it for its services. These policies can be configured using BGP [2, P. 301].
Each AS has a number (an ASN) assigned to it by either an internet registry or a provider. Like IP, there are reserved private AS numbers.
ASes can be categorized based on how they interact with other ASes.
Single-homed ASes connect to another AS via a single exit.
Single-homed ASes can be given a number from the private range of AS numbers [1, P. 97].
A multi-homed AS has at least two exit points to outside wo