Routing protocol
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[edit] Definition
Routing protocols allow different computer networks to communicate. Routing protocols specify the set of rules that help layer 3(OSI model) devices pass information among themselves on the topology of the network. It helps the Layer 3 device decide on the best available path to send network traffic.
The list of routing protocols and their classification is provided in the List of routing protocols article. Examples of routing protocols are interior gateway protocols (IGP) like RIP, OSPF, EIGRP, IS-IS, IGRP and exterior gateway protocols like (the confusingly named) Exterior Gateway Protocol (EGP) (EGP is a type of EGP and now 'obsolete') and Border Gateway Protocol (BGP).
Routing protocols are different from Routed protocols in that routed protocols are responsible for the transfer of traffic between L3 devices while routing protocols help the L3 device in the decision-making on which path to take. This is explained in greater detail in the Routing article
[edit] How Routing Protocols Work
Routing protocols keep the information about all the paths or routes available to the L3 decive in a Routing Table. The Routing Table would be populated with route information from information it learnt from other L3 devices or from information configured by a programmer.
The essential functions of the routing protocol is to send routes, receive routes and making routing decisions based on the routes in the routing table.
The three important parameters on which the routing protocol makes the decision of the best available path and which is specified in the Routing Table are-
1. Metric
2. Administrative distance
3. Prefix length
[edit] Metric
Metric is the parameter that determines the suitability of a route for a particular network subnet. The calculation of the metric differs for each routing protocol and can be based on a large number of criteria like hop count, bandwidth, delay, load and reliability. The metric can be manually configured in many routing protocols as well.
[edit] Administrative distance
Administrative distance (AD) is the parameter that specifies the routing protocol through which the Routing Table on the L3 device learnt the route. The importance of the AD is that it helps the L3 device to make its routing decision based on the routing protocol. For example on Cisco routers, the AD for the routing protocol for OSPF is 110, EIGRP is 90 and static route is 1. If the Routing Table has OSPF, EIGRP and a static routes for the same route, it will prefer to use the route learnt via EIGRP as the best path.
[edit] Prefix length
Prefix length is the parameter that helps the L3 device decide on the route in the Routing Table that is the closest match to the destination address. This is determined on the subnet that is has the most number of similar bits to the bits in the destination address. In other words, the route with the longest prefix is selected from the available routes in the routing table.
