Dynamic Routing Concepts

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Dynamic routing is when protocols are used to find networks and update routing tables on routers.

this is easier than using static or default routing, but it’ll cost you in terms of router CPU pro- cesses and bandwidth on the network links.

A routing protocol defines the set of rules used by a router when it communicates routing information between neighbor routers.

The routing protocol I’m going to talk about in this chapter is Routing Information Protocol (RIP) versions 1 and 2, with a bit of Interior Gateway Routing Protocol (IGRP) thrown in.

Two types of routing protocols are used in internetworks: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs).

IGPs are used to exchange routing information with routers in the same autonomous system (AS). An AS is a collection of networks under a common administrative domain, which basically means that all routers sharing the same routing table information are in the same AS. EGPs are used to communicate between ASes.

An example of an EGP is Border Gateway Protocol (BGP).

Since routing protocols are so essential to dynamic routing, I’m going to give you the basic information you need to know about them next .


Administrative Distances

The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.

If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table.

If both advertised routes to the same network have the same AD, then routing protocol metrics (such as hop count or bandwidth of the lines) will be used to find the best path to the remote network.


Connected interface 0 Static route 1 OSPF 110 RIP 120 Unknown 255 (this route will never be used)


Routing Protocols

There are three classes of routing protocols: Distance vector The distance-vector protocols find the best path to a remote network by judging distance. Each time a packet goes through a router, that’s called a hop. The route with the least number of hops to the network is determined to be the best route. The vector indicates the direction to the remote network. Both RIP and BGP are distance-vector routing protocols. They send the entire routing table to directly connected neighbors.


Link state In link-state protocols, also called shortest-path-first protocols, the routers each create three separate tables. One of these tables keeps track of directly attached neighbors, one determines the topology of the entire internetwork, and one is used as the routing table. Link-state routers know more about the internetwork than any distance-vector routing protocol. OSPF is an IP routing protocol that is completely link state.

Link-state protocols send updates containing the state of their own links to all other routers on the network.


Source : CCNA Book , Todd Lammle

Reza Moghadam


--MikroTik Certified Trainer 16:18, 12 April 2013 (UTC)