BGP General Operation and Messaging
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In the previous topics we have seen how BGP stores information about routes and uses it to determine paths to various networks. Let's now take a high-level look at how BGP operates in general terms. Like many other protocols covered in this Guide, BGP's operation can be described primarily in the form of messaging. The use of messages is the means by which route information is communicated between BGP peers. This eventually allows knowledge of how to reach networks to spread throughout the entire internetwork.
Before messaging can begin, BGP speakers must be designated, and then linked together. The BGP standard does not specify how neighboring speakers are determined; this must be done outside the protocol. Once accomplished, ASes are connected into a BGP-enabled internetwork. Topological linking provides the physical connection and the means for datagrams to flow between ASes. At this point the dance floor is prepared, but nobody is dancing; BGP can function but isn't yet in operation.
BGP operation begins with BGP peers forming a transport protocol connection. BGP uses TCP for its reliable transport layer, so the two BGP speakers establish a TCP session that remains in place during the course of the subsequent message exchange. When this is done, each BGP speaker sends a BGP Open message. This message is like an invitation to dance, and begins the process of setting up the BGP link between the devices. In this message, each router identifies itself and its autonomous system, and also tells its peer what parameters it would like to use for the link. This includes an exchange of authentication parameters. Assuming that each device finds the contents of its peer's Open message acceptable, it acknowledges it with a Keepalive message and the BGP session begins.
Under normal circumstances, most BGP speakers will maintain simultaneous sessions with more than one other BGP speaker, both within its own AS and outside the AS. Links between ASes are of course what enable BGP routers to learn how to route through the internetwork. Links within the AS are important to ensure that each BGP speaker in the AS maintains consistent information.
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