Routing is a core concept of the Internet and many other networks. Routing provides the means of discovering paths along which information (usually, but not always, packets) can be sent.
Automatic routing makes networks autonomous. Such networks can use their routing to find the best route to deliver data to a destination; choices are made depending upon goals such as finding the shortest distances and the fastest links available through a choice of network connections. This allows the network to route around network failures and blockages, and can make many aspects of the day to day running of such networks automatic, and free from the need for human intervention.
The actual process of passing logically addressed packets from their local subnetwork toward their ultimate destination is called forwarding. It is closely related to routing, in that routing tells the forwarding where to send packets, but they are logically completely separate. In large networks, packets may pass through many intermediary destinations before reaching their destination. Routing and forwarding both occur at layer 3 of the OSI seven-layer model.
Hubs and switches move data on what appears (to the connected computers) to be the local network, and are invisible to connected computers, while the router is explicitly visible to them.
Knowing where to send packets requires a knowledge of the structure of the network. In small networks, routing can be very simple, and is often configured by hand. In large networks the topology of the network can become complex, and may change constantly, making the problem of constructing the routing tables very complex.
As routers can only recalculate the best routes very slowly relative to the rate of arrival of packets, routers keep a routing table that maintains a record of only the best possible routes to certain network destinations and the routing metrics associated with those routes.
Consult Routing for more detail on the topic.
The Interior Gateway Protocols (IGPs) exchange routing information within a single autonomous system. he IGP is a protocol used by gateways to communicate routing information between each other. It can then be used by the IP protocol to direct packets from a gateway to the rest of a network. Common examples include:
The Routing Information Protocol (RIP) is one of the most commonly used Interior Gateway Routing Protocols on internal networks (and to a lesser extent, the Internet), which helps routers dynamically adapt to changes of network connections by communicating information about which networks each router can reach and how far away those networks are. Although RIP is still actively used, it is generally considered to have been obsoleted by Link-state routing protocols such as OSPF and EIGRP.
See RIP for more detail.
Open Shortest Path First (OSPF) is a link-state, hierarchical Interior Gateway Protocol (IGP) routing algorithm. The well known Dijkstra's algorithm is used to calculate the shortest path tree. It uses cost as its routing metric. A link state database is constructed of the network topology which is identical on all routers in the area.
See OSPF for more detail.
Interior Gateway Routing Protocol (IGRP) is a proprietary distance-vector routing protocol invented by Cisco, used by routers to exchange routing data within an autonomous system.
IGRP was created in part to overcome the limitations of RIP (maximum hop count, and a single routing metric) when used within large networks. IGRP supports multiple metrics for each route, including bandwidth, load, delay, and relability; to compare two routes these metrics are combined together into a single metric, using a formula which can be adjusted through the use of pre-set constants. The maximum hop count of IGRP-routed packets is 255.
See IGRP for more detail.
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol based on their original IGRP. EIGRP is a distance vector routing protocol, with optimizations to minimize both the routing instability incurred after topology changes, as well as the use of bandwidth and processing power in the router.
See EIGRP for more detail.
Exterior Gateway Protocols (EGPs) route between separate autonomous systems. EGPs include:
The original Exterior Gateway Protocol used to connect to the former Internet backbone network and is now obsolete.
See EGP for more detail.
The Border Gateway Protocol (BGP) is one of the core routing protocols in the Internet. It works by maintaining a table of IP networks or 'prefixes' which designate network reachability between autonomous systems (AS). It is described as a path vector protocol. BGP does not use technical metrics, but makes routing decisions based on network policies or rules. The current version of BGP, BGP version 4 (BGPv4), was adopted in 1995 and is specified in request for comment RFC 1771.
See BGP for more detail.