Intro
Routing is the process of selecting a path for traffic in a network, or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone network (PSTN), and computer networks, such as the Internet.
In packet switching networks, routing is the higher-level decision making that directs network packets from their source toward their destination through intermediate network nodes by specific packet forwarding mechanisms. Packet forwarding is the transit of network packets from one network interface to another. Intermediate nodes are typically network hardware devices such as routers, gateways, firewalls, or switches. General-purpose computers also forward packets and perform route, although they have no specially optimized hardware for the task.
The process usually directs forwarding on the basis of route tables, which maintain a record of the routes to various network destinations. Routing tables may be specified by an administrator, learned by observing network traffic or built with the assistance of routing protocols.
A route is the path that data takes when travelling through a network from one host to another. Routing is the process by which the path, or some subset of it, is determined. One of the characteristic features of the Internet, as compared to other network architectures, is that each node that receives a packet will typically determine for itself what the next step in the path should be.
Routing
IP route decisions are generally made based on the destination of network traffic. When an IP packet is sent from a node on the network, it will consult its route table to determine the next hop device that the traffic should be sent to, in order for it to reach its final destination.
Dynamic route protocols allow network attached devices to learn about the structure of the network dynamically from peer devices. This reduces the administrative effort required to implement and change routin throughout a network. Some examples of dynamic router protocols are:
- RIP (Routing Information Protocol)
- OSPF (Open Shortest Path First)
- ISIS (Intermediate system to intermediate system)
- BGP (Border Gateway Protocol)
- IGRP (Interior Gateway Routing Protocol)
ISIS and OSPF are link-state protocols, meaning each node part of the same zone, will know the state of all the link in the mesh. Due to the exponential number of link in a mesh, thoses protocols are for small mesh such as an ISP national backbone.
RIP is usually used to easily announce customer’s routes in a backbone.
BGP is used as an external routing protocol to exchange routes with other entities. ISP use BGP extensivly to “trade” their routes.
