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Border Gateway Protocol
From A to B
The Internet comprises a mix of autonomous systems (ASs) – networks and systems each under the administrative control of a specific provider – that have officially registered numbers known as AS numbers (ASNs). The Border Gateway Protocol (BGP), the latest version of which is BGP4, ensures accessibility between the autonomous systems and is designed and optimized for handling high volumes of routing information with a high level of stability.
Besides providers, large corporate and government customers also have to deal with BGP if they use or want to use multihoming (i.e., connecting your own autonomous system to several providers). BGP is also used on some internal networks and forms the basis for multiprotocol label switching (MPLS) in wide-area network (WAN) structures, but can also be used for Ethernet virtual private networks (EVPNs) or in combination with a virtual extensible local area network (VXLAN) in data center networks.
Today, BGP is capable of many more functions than simply distributing IP prefixes. The protocol therefore has a wide range of options for policy-based route selection.
Basics
In contrast to the various Interior Gateway Protocols (IGPs) such as the Routing Information Protocol (RIP), Open Shortest Path First (OSPF) protocol, or Enhanced Interior Gateway Routing Protocol (EIGRP) for internal networks, BGP is the only Exterior Gateway Protocol (EGP). As such, it is based on the path vector principle, which has similarities to the distance vector IGPs used to exchange routes within an autonomous system and optimized for fast convergence times to meet the stringent requirements for low downtimes – right down to the millisecond range. However, even smaller numbers of routes still need to be processed.
Extensions to BGP make it multiprotocol capable (MP-BGP4); that is, it supports IPv4 and IPv6. BGP can process and separate different types of
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