Some protocols are routable and others are nonroutable. Nonroutable protocols are typically broadcast-based, which adds overhead and traffic to a network. Nonroutable protocols do not contain network layer addressing information that routers need to determine the destination of network or host traffic. However, any protocol that has layer three logical network addressing can be routed.
Routers serve as traffic forwarders for remote end systems. Routing protocols and mechanisms are needed to build and maintain route information. To forward traffic, routers need to know the destination address, from which source it can learn the path to a given destination, the best path, and a way of verifying the most current path upon which to take. When a router receives a frame, it identifies the destination network address and checks its route table to determine the best path. Routers use a combination of the following routing methods to build a router's route table: directly connected interface, static, default, and dynamic.
The term IGP (Interior Gateway Protocol) describes any routing protocol operating as a separate routing domain within an AS. EGPs (Exterior Gateway Protocols), such as BGP (Border Gateway Protocol), serve as a conduit for communication between autonomous systems.
Routing protocols fall into two main categories, Distance Vector or Link State. Distance Vector protocols determine best path on how far away the destination is (distance) while Link State protocols can use more sophisticated methods to determine the best path. Link State can take into consideration link variables, such as bandwidth, delay, reliability, and load. When determining the best path, Distance Vector protocols use hops or a combination of calculated metrics that represent a distance value.