Routing Protocols
Introduction
Routing
protocols were created for routers. These protocols have
been designed to allow the exchange of routing tables,
or known networks, between routers. There are a lot of
different routing protocols, each one designed for
specific network sizes, so I am not going to be able to
mention and analyse them all, but I will focus on the
most popular.
The two main types of routing:
Static routing and
Dynamic routing
The router
learns about remote networks from neighbor routers or
from an administrator. The router then builds a routing
table, the creation of which I will explain in detail,
that describes how to find the remote networks. If the
network is directly connected then the router already
knows how to get to the network. If the networks are not
attached, the router must learn how to get to the remote
network with either static routing
(administrator manualy enters the routes in the router's
table) or dynamic routing
(happens automaticlly using routing protocols).
The routers
then update each other about all the networks they know.
If a change occurs e.g a router goes down, the dynamic
routing protocols automatically inform all routers about
the change. If static routing is used, then the
administrator has to update all changes into all routers
and therefore no routing protocol is used.
Only
Dynamic routing uses
routing protocols, which enable routers to:
-
Dynamically discover and maintain routes
-
Calculate routes
-
Distribute routing updates to other routers
- Reach
agreement with other routers about the network
topology
Statically
programmed routers are unable to discover routes, or
send routing information to other routers. They send
data over routes defined by the network Administrator.
A Stub
network is so called because it is a dead end in the
network. There is only one route in and one route out
and, because of this, they can be reached using
static routing, thus saving
valuable bandwidth.
Dynamic Routing Protocols
There are 3
types of Dynamic routing
protocols, these differ mainly in the way that they
discover and make calculations about routes (click to
select):
1)
Distance
Vector
2)
Link State
3)
Hybrid
-
Distance Vector routers
compute the best path from information passed to
them from neighbors
-
Link State routers each
have a copy of the entire network map
-
Link State routers
compute best routes from this local map
The Table
below (clickable) shows the
main characteristics of a few different types of
dynamic routing protocols:
You can also
clasify the routing protocols in terms of their location
on a network. For example, routing protocols can exist
in, or between, autonomous systems.
Exterior Gateway Protocols
(EGP's) are found between autonomous systems, whereas
Interior Gateway Protocols
(IGP'S) are found within autonomous systems:
Example of an
EGP is the Border Gateway
Protocol (BGP) which is also used amongst the Internet
routers, whereas examples of IGP
protocols are RIP, IGRP, EIGRP.
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