routing algorithms & protocols

7/31/2019 Routing Algorithms & Protocols

1/23

Routing Algorithms & Protocols

By

Utkarsh Hathi


2/23

Routing

A process of selecting paths in a network todecide where to send the network traffic.

In most subnets, packets require multiplehops to make the journey.

Even in broadcast networks, routing is an issueif source and destination are not on the same

network.


3/23

Routing Algorithm

Routing Algorithm is that part of the network

layer software responsible for deciding which

output line an incoming packet should be

transmitted on.

Overall performance of your network depends

on the routing algorithms


4/23

Continued

Routing Algorithm cant be changed, only

protocols can be changed and as different

protocols use different algorithms; changing it

will change the algorithm as well.

Now, we will have a look at the different

routing protocols by the algorithms they

support.


5/23

Routing Metric

A property of a route, consisting of a value

used by algorithms to decide which route will

perform better

Different algorithms and protocols use

different metrics and that can help us to

decide which protocol to use by the type of

services we want.


6/23

IGP and EGP

IGP Interior Gateway Protocol, also known

as intra domain routing algorithm

Works within an autonomous system

Can be divided into three categories

Distance vector routing protocols

Link state routing protocols

Balanced Hybrid routing protocol


7/23

Autonomous system (IGP)


8/23

Distance Vector Routing Protocol

Requires that a router informs its neighbors

of changes in topology periodically; can be

also a triggered update if there is an instant

change

Each node maintains a table of minimum

distances to every node.

Examples: RIP V1 ,V2 and IGRP


9/23

Routing Information Protocol (V1)

As a distance vector routing protocol, it uses

hop count as a metric.

Limitation of 15 hops for a packet to travel to

avoid the routing loops. Thus it limits the size

of network it can support.

Initially used to send update every 30 seconds

but as networks became large it became

difficult.


10/23

Continued

RIP is not preferred choice for routing as its

convergence time and scalability are poor

compared to EIGRP, OSPF or IS-IS.

RIP V1 doesnt support subnet while RIP V2

does thus supporting Classless Inter-domain

routing.


11/23

IGRP

Interior gateway routing protocol (IGRP),

another distance vector routing protocol

invented by CISCO and increases the hop limit

to 255 from 15 of RIP.

Supports multiple metrics such as bandwidth,

delay, load, MTU and reliability.


12/23

IGRP metric count


13/23

Link state routing protocol

Every node constructs the map of the

connectivity to the network, in the form of a

graph showing which nodes are connected to

other nodes

Then each node calculates the best logical

path from it to every possible destination.


14/23

Each node independently runs

an algorithm over the map to determine

the shortest path from itself to every other

node in the network

Uses link as the metric and will choose the

path from where the packet will reach the

fastest


15/23

Example:


16/23

Open Shortest Path First (OSPF)

The most widely-used interior gatewayprotocol (IGP) in large enterprise networks

Support variable length Subnet maskingsupporting Classless Inter domain routing.

Detects changes in the topology, such as linkfailures, very quickly and converges on a newloop-free routing structure within seconds


17/23

Continued


18/23

OSPF Areas

Allows an AS to be split into different areas for

ease of management and the routing

information transmitted between areas is

summarized to reduce network bandwidth

consumption.

Special area called the backbone


19/23

OSPF Computation

OSPF route computation

OSPF route computation is described as follows:

1. Based on the network topology around itself, each routergenerates Link State Advertisements (LSA) and sends them to otherrouters in update packets.

2. Each OSPF router collects LSAs from other routers to compose aLSDB (Link State Database). An LSA describes the network topologyaround a router, so the LSDB describes the entire network topologyof the AS.

3. Each router transforms the LSDB to a weighted directed graph,which actually reflects the topology architecture of the entirenetwork. All the routers have the same graph.

4. Each router uses the SPF algorithm to compute a Shortest PathTree that shows the routes to the nodes in the autonomous system.The router itself is the root of the tree.


20/23

Border Gateway Protocol

Was created to replace Exterior gateway

protocol, that is used for interconnecting

different autonomous systems.

Most Internet service providers must use BGP

to establish routing between one another


21/23

BGP between different AS


22/23

The exchange of the routing information

between two routers using BGP takes place in

a session.

For that BGP uses the services of TCP; when a

TCP connection is created for BGP it lasts for

long.


23/23

Types of BGP messages

routing algorithms & protocols

Documents