Organizations requiring connectivity to the Internet must obtain an Autonomous System Number (ASN). An IGP is not required within an AS, and could use BGP as the only routing protocol in it, too.
If multiple IGPs or metrics are used within an AS, the AS must appear consistent to external ASs in routing policy. BGP prefers stability within the network, because a link flap could result in route computation for thousands of routes.įrom the perspective of BGP, an autonomous system (AS) is a collection of routers under a single organization’s control, using one or more IGPs, and common metrics to route packets within the AS. BGP does not advertise incremental updates or refresh network advertisements like OSPF or ISIS. BGP is the only protocol used to exchange networks on the Internet, which has more than 600,000 IPv4 routes and continues to grow. When BGP was created, the primary design consideration was for IPv4 inter-organization connectivity on public networks, such as the Internet, or private dedicated networks. RFC 1654 defines Border Gateway Protocol (BGP) as an EGP standardized path-vector routing protocol that provides scalability, flexibility, and network stability. EGP protocols may require a unique routing policy for every external organization that it exchanges routes. In IGP protocols, all routers use a common logic within the routing domain to find the shortest path to reach a destination. Routing protocols are classified as either an Interior Gateway Protocol (IGP) or an Exterior Gateway Protocol (EGP), which indicates whether the protocol is designed for exchanging routes within an organization or between organizations. Link flaps, router crashes, and other unexpected events could impact the best path, so the routers must exchange information with each other so that the network topology updates during these types of events. Routers try to select the best loop-free path in a network based on the destination network. A router learns about unattached networks through static configuration or through dynamic routing protocols that distribute network topology information between routers. The following topics are covered in this chapter:īGP Messages and Inter-Router Communicationīasic BGP Configuration for IOS, IOS XR, and NX-OSĪ router’s primary function is to move packets from one network to a different network.
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