Packet routing in the Internet is divided into two general groups: interior and exteriorrouting. Exterior routing occurs between autonomous systems, and is of concern to service providers and other large or complex networks. In TCP/IP terminology, these independent network systems are called autonomous systems.
Within an autonomous system (AS), routing information is exchanged using an interior routing protocol chosen by the autonomous system's administration. The exterior routing protocols, on the other hand are used between the autonomous systems. There are two main exterior protocols used in the network:
Exterior Gateway Protocol (EGP): EGP is for exchanging routing information between two neighbor gateway hosts in a network of autonomous systems. EGP is commonly used between hosts on the Internet to exchange routing table information. The protocol is based on periodic polling using Hello/I-Heard-You (I-H-U) message exchanges to monitor neighbor reachability and Poll commands to solicit Update responses. The routing table contains a list of known routers, the addresses they can reach and a cost metric associated with the path to each router so that the best available route is chosen. Each router polls its neighbor at intervals between 120 to 480 seconds and the neighbor responds by sending its complete routing table. EGP-2 is the latest version of EGP.
Border Gateway Protocol (BGP): BGP runs over TCP and is an inter-Autonomous System routing protocol. BGP is the only protocol that is designed to deal with a network of the Internet's size and the only protocol that can deal well with having multiple connections to unrelated routing domains. It is built on experience gained with EGP. The primary function of a BGP system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASs) that reachability information traverses. This information is sufficient to construct a graph of AS connectivity from which routing loops may be pruned and some policy decisions at the AS level may be enforced.
BGP-4 provides a new set of mechanisms for supporting classless interdomain routing (CIDR). These mechanisms include support for advertising an IP prefix and eliminate the concept of network "class" within BGP. BGP-4 also introduces mechanisms which allow aggregation of routes, including aggregation of AS paths. These changes provide support for the proposed supernetting scheme.
In the Internet, BGP is replacing EGP becoming the dominant exterior routing protocol.
Exterior Routing and Protocols
Related Terms: Border Gateway Protocol (BGP), Exterior Gateway Protocol (EGP), Routing Protocol, Classless Interdomain Routing (CIDR)