A route is a a set of addresses and the next hop used to send traffic to the addresses. A router is nominally a host that has more than one network interface and makes decisions about to which interface a given packet should be sent. As network topologies become more complex, the number of different routes increases, as does the frequency of routes changing. For this reason it is useful for routers to have a method for dynamically updating routes as the network topology changes, that is, as other routers come and go from the network either because of administrative changes or failures of routers or links between routers.
Routing protocols are a mechanism for routers to communicate routes with each other. Routers that communicate routes with each other are referred to as peers. When routes are sent between routers they contain information in addition to the addresses that the route covers and the next hop for this traffic. The additional information may be used to expire the route and to determine the cost of the route relative to other routes. When a router sends such a route it is said to be advertising. A route advertisement can be seen as a promise to deliver traffic for a given set of addresses. Advertising routes that cannot be satisfied leads to either routing loops or black-holing. A routing loop refers to traffic bouncing between routers until the maximum hop count is reached. Black-Holing refers to receiving traffic and then discarding it. In either case the addresses covered by the route is effectively removed from the network.
A prefix is a set of network addresses that a given route covers. In routing protocols this is given as either a classful network or in the case of more recently developed routing protocols a CIDR network. Classless Inter-Domain Routing (CIDR) is defined in RFC 1519 . CIDR networks allow networks to be defined as a network address and a netmask, enabling more flexible division of networks than classful routing.
When peers are configured to communicate routes with each other they are said to have a session running. When the session is established the routers advertise routes to each other and each router uses this information to determine the best route for each prefix that has been advertised to it or is advertised by it. When a session goes down, either administratively or because of a timeout, the prefixes advertised by the peer in question are removed, enabling the network to adapt to failures.
Routing protocols are divided into two types: Interior Gateway Protocol (IGP) and Exterior Gateway Protocol (EGP). An IGP is concerned with managing routes within a single network, ensuring that each point of the network is able to get to all other points in the network. An EGP communicates information about which addresses are within a network or may be accessed through a network. When BGP is used to communicate routes between different networks on the Internet, it is being used as an EGP.