Routing table

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In computer networking a routing table, or Routing Information Base (RIB), is a data structure in the form of a table-like object stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics associated with those routes. The routing table contains information about the topology of the network immediately around it. The construction of routing tables is the primary goal of routing protocols. Static routes are entries made in a routing table by non-automatic means and which are fixed rather than being the result of some network topology 'discovery' procedure.

Routing tables are generally not used directly for packet forwarding in modern router architectures; instead, they are used to generate the information for a smaller forwarding table which contains only the routes which are chosen by the routing algorithm as preferred routes for packet forwarding, often in a compressed or pre-compiled format that is optimized for hardware storage and lookup. The remainder of this article will ignore this implementation detail, and refer to the entire routing/forwarding information subsystem as the "routing table".

Contents

Basics

A routing table utilizes the same idea that one does when using a map in package delivery. Whenever a node needs to send data to another node on a network, it must know where to send it, first. If the node cannot directly connect to the destination node, it has to send it via other nodes along a proper route to the destination node. Most nodes do not try to figure out which route(s) might work; instead, a node will send an IP packet to a gateway in the LAN, which then decides how to route the "package" of data to the correct destination. Each gateway will need to keep track of which way to deliver various packages of data, and for this it uses a Routing Table. A routing table is a database which keeps track of paths, like a map, and allows the gateway to provide this information to the node requesting the information.

With hop-by-hop routing, each routing table lists, for all reachable destinations, the address of the next device along the path to that destination; the next hop. Assuming that the routing tables are consistent, the simple algorithm of relaying packets to their destination's next hop thus suffices to deliver data anywhere in a network. Hop-by-hop is the fundamental characteristic of the IP Internetwork Layer [1] and the OSI Network Layer, in contrast to the functions of the IP End-to-End and OSI Transport Layers. Current router architecture separates the Control Plane function of the routing table from the Forwarding Plane function of the forwarding table [2]

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