Routers participating in the routing protocol interaction exchange information about the destinations they can directly or indirectly reach. In most cases, the information exchanged also carries information on the relative cost of moving toward the destination from the specific router, and possibly some policy information about what traffic is allowed to take particular paths.
Routing protocols generally fall into the categories of interior and exterior, a categorization based on their scope and the number of separate organizations involved in administering them. They also have somewhat different route selection concepts and provide slightly different information.
Interior routing protocol
Interior routing protocols (IGP) concentrate on finding the best path to a destination, "best" being specific to the protocol and possible configuration-specific tuning. The most common interpretation of "best" is giving a route to a destination that has the highest usable bandwidth; the route is no better than the lowest bandwidth of any of its hops.
In general, interior routing assumes that if it can find a path to a destination, it may use it. Some IGPs, or more often their specific implementations, may allow prioritizing forwarding or perhaps assigning only certain types of packets to a given route.
Typically, a given routing domain, or instance of a routing protocol with common policies and definition of "best", is under a single network administration authority.
Exterior routing protocols
Exterior routing protocols, such as the Border Gateway Protocol as the only unicast exterior routing protocol in the Internet, are more "reachabiity" protocols than "best path" protocol. They can provide a significant amount of information about the various points and autonomous systems a route traverses, perhaps information on the acceptable use policy for a route (e.g., only participants in a high-energy physics research program), etc.
Where IGPs typically need to split into separate routing domains when dealing with low thousands of routers and routes, BGP, in a high-performance router, may need to handle millions of routes.
Unicast routing protocols
The most basic type of routing protocol is unicast, which gives destinations to specific addresses, or to address ranges (e.g., IP subnets) that contain specific addresses. Unicast routing protocols can be interior or exterior.
Interior routing protocols are under the control of a single administrative authority that defines the cost factors and policies that will affect the advertising and acceptance of routing information. That scope of control by the single authority is called a routing domain. More than one routing domain may coexist in an autonomous system, which is a set of routers and addresses, under one or more administrative authorities, that present a single set of route advertisement to the Internet or a private network that interconnects non-public autonomous systems.
Paradigms and protocols
Interior unicast routing protocols use one of two paradigms, with different algorithms and implementations. Three standard, and one proprietary, protocols of this type are in common use:
- Open Shortest Path First (OSPF), an Internet Engineering Task Force (IETF) protocol, which supports hierarchy and primarily uses link state algorithms.
- Intermediate System to Intermediate System (IS-IS), also a IETF link-state protocol but with different assumptions, characteristics, and optimal applications than OSPF.
- Routing Information Protocol (RIP), an IETF protocol using a simple version of the distance vector paradigm. It is far less scalable than the other unicast routing protocols, but is simple and can be useful in small networks.
- Interior Gateway Routing Protocol (IGRP), an obsolete distance vector protocol from Cisco Systems
- Enhanced Interior Gateway Protocol (EIGRP), an "advanced distance vector", or "diffusing update", protocol proprietary to Cisco Systems.
One unicast exterior routing protocol is in general use, the IETF-standardized Border Gateway Protocol.