An autonomous system (AS), also known as a domain, is a collection of routers that are under a common administration, such as a company's internal network or an Internet service provider's (ISP's) network.
Because the Internet is based on the AS concept, two types of routing protocols are required:
■ Interior gateway protocols (IGP) are intra-AS (inside an AS) routing protocols. Examples of IGPs include Routing Information Protocol (RIP) version 1 (RIPv1), RIP version 2 (RIPv2), Open Shortest Path First (OSPF), Integrated Intermediate System-to-Intermediate System (IS-IS), and Enhanced Interior Gateway Routing Protocol (EIGRP).
■ Exterior gateway protocols (EGP) are inter-AS (between autonomous systems) routing protocols. Border Gateway Protocol (BGP) is the only widely used EGP protocol on the Internet. BGP version 4 (BGP-4) is considered the acceptable version of BGP on the Internet. It is discussed in the "Border Gateway Protocol" section.
Different types of protocols are required for the following reasons:
■ Inter-AS connections require more options for manual selection of routing characteristics. EGPs should be able to implement various policies.
■ The speed of convergence (distribution of routing information) and finding the best path to the destination are crucial for intra-AS routing protocols.
Therefore, EGP routing protocol metrics (used to measure paths to a destination) include more parameters to allow the administrator to influence the selection of certain routing paths. EGPs are slower to converge and more complex to configure. IGPs use less-complicated metrics to ease configuration and speed up the decisions about best routing paths for faster convergence. The "Routing Protocol Metrics" section later in this chapter defines and explains routing protocol metrics.
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