Home > Routing

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. Circuit-based networks, such as the voice telephone network, also perform routing, to find paths for calls through the network fabric.

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.

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 one host to another 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.

• Routing protocols: facilitate the exchange of routing information between networks, allowing routers to build routing tables dynamically. Traditional IP routing stays simple because it uses next-hop routing where the router only needs to consider where it sends the packet, and does not need to consider the subsequent path of the packet on the remaining hops.

Although this dynamic routing can become very complex, it makes the Internet very flexible, and has allowed it to grow in size by more than eight orders of magnitude over the years since adopting IP in 1983.

Routing algorithms use two basic techologies:

• telling the world who your neighbors are: link-state routing protocols such as OSPF
• telling your neighbors what the world looks like to you: distance-vector routing protocolA distance-vector routing protocol is a routing protocol used in routing of packet-switched networks in computer communications, as in for example the Routing Information Protocol for Internet traffic. Examples of distance-vector routing protocols includes such as RIPThis article is chiefly about the Routing Information Protocol for IPv4. The Routing Information Protocol (RIP) is one of the most commonly used Interior Gateway Protocols on internal networks (and to a lesser extent, the internet), which helps routers dy

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 destination.

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 bandwidthAnalog Bandwidth is the width, usually measured in hertz, of a frequency band f f. It can also be used to describe a signal, in which case the meaning is the width of the smallest frequency band within which the signal can fit. It is usually notated B, W,, delayDelay is: In sound effects, any of a class of effect that adds one or more delayed versions of the original signal, to create effects such as echo or flanger. In telecommunications, either: # In radar, the electronic delay of the start of the time base us, hop countIn telecommunication, the term hop count has the following meanings: #In a data communications network, the number of legs traversed by a packet between its source and destination. Note Hop count may be used to determine the time to live for some packets., 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:

• Ad hoc network routing protocols appear in networks with no or little infrastructure. For a list of a couple of the proposed protocols, see the Ad hoc protocol list

• 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)

• Exterior Gateway Protocols (EGPs) route between separate autonomous systems. EGPs include:
  • EGP (the original Exterior Gateway Protocol used to connect to the former Internet backbone network -- now obsolete)
  • BGP (Border Gateway Protocol: at version BGPv4 as of 2002)