BGP uses TCP (port 179) to communicate with other routers.
The primary purpose of BGP4 is to exchange network reachability information with other BGP (autonomous) systems. This network layer reachability information (NLRI) includes information about each AS in the list of ASs that reachability information traverses. NLRI updates carry a network number, a list of the ASs through which the NLRI has passed (called the AS path), and a list of other path attributes that is used to construct a graph of network connectivity to remove routing loops.
Figure 25-2 illustrates how the Internet is constructed with multiple BGP autonomous systems.
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