Route
wood router to bore holes or grooves.
Routing is a core concept of the Internet and many other networks. Routing provides the means of forwarding logically addressed packets from their local subnetwork toward their ultimate destination. In large networks, packets may pass through many intermediary destinations before reaching their destination. Routing occurs at layer 3 of the OSI seven-layer model.
Hubs and switches move data on the local network, while the router has an awareness of the neighboring routers. See the router article for a description of the hardware technology and manufacturers of routers. The rest of this article describes routing in a hardware-independent way.
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.
Routed versus Routing protocol
There is often confusion between routed protocol and routing protocol:
- Routed protocol: Any network protocol that provides enough information in its network layer address to allow a packet to be forwarded from host to host based on the addressing scheme. Routed protocols define the format and use of the fields within a packet. Packets generally are conveyed from end system to end system. IP is an example of a routed protocol.
There is also a third method called hybrid:
Hybrid protocols like EIGRP is a combination of link-state and distance-vector routing protocols.
Hybrid protocols have rapid convergence (like link-state protocols) but use much less memory and processor power than link-state protocols. Hybrid protocols use distance-vectors for more accurate metrics and to determine the best path to distination networks.
A routing metric consists of any value used by routing algorithms to determine whether one route is superior to another. Metrics can cover such information as bandwidth, delay, hop count, path cost, load, MTU, reliability, and communication cost. The routing table stores only the best possible routes, while link-state or topological databases may store all other information.
Depending on the relationship of the router relative to other autonomous systems, various classes of routing protocols exist:
- Interior Gateway Protocols (IGPs) exchange routing information within a single autonomous system. Common examples include:
- IGRP (Interior Gateway Routing Protocol)
- EIGRP (Enhanced Interior Gateway Routing Protocol)
- OSPF (Open Shortest Path First)
- RIP (Routing Information Protocol)
- IS-IS (Intermediate System to Intermediate System)
