Border Gateway Protocol
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Introduction
The Border Gateway Protocol (BGP) is an inter-autonomous system routing protocol. As discussed in section 7.1.5, an autonomous system (AS) is a network or group of networks under a common administration and with common routing policies. BGP is used to exchange routing information for the Internet and is the protocol used between Internet service providers (ISP), which are different ASes.


BGP Characteristics
BGP is different from other routing protocols in several ways. Most important being that BGP is neither a pure distance vector protocol nor a pure page link state protocol. Let’s have a look at some of the characteristics that stands BGP apart from other protocols.
•Inter-Autonomous System Configuration: BGP’s primary role is to provide communication between two autonomous systems.
•Next-Hop paradigm: Like RIP, BGP supplies next hop information for each destination.
•Coordination among multiple BGP speakers within the autonomous system: If an Autonomous system has multiple routers each communicating with a peer in other autonomous system, BGP can be used to coordinate among these routers, in order to ensure that they all propagate consistent information.


BGP Attributes
BGP Attributes are the properties associated with the routes that are learned from BGP and used to determine the best route to a destination, when multiple routes are available. An understanding of how BGP attributes influence route selection is required for the design of robust networks. This section describes the attributes that BGP uses in the route selection process: