Routing A broad Perspective

reekydizzyNetworking and Communications

Oct 28, 2013 (4 years and 7 months ago)



A broad Perspective


Vasudha Chandrasekaran (Vas)

Prajakta Vaidya (Priya)

Routing is…

What fans do at football games

What pigs do for truffles

Send corps of infantrymen into full scale,
disorganized retreat.

Uh! So what is Routeing?

Something that Brits use, to distinguish
between what happens in Networks from what
happened to them in New Orleans in 1814.

What ISO and CCITT use in their documents

And Yes, it is…

Routeing is…

The process of determining, selecting the
best outgoing path that a packet has to
take in a Internetwork.

But, with due apologies to British Standards

Institute, lets stick to Routing!

OSI Routing Scheme

A set of Routing Protocols

Allows end systems and intermediate systems
to collect and distribute information necessary
to determine routes

Routing Information base

Table containing all routing relevant

Routing Algorithms

Uses information in routing information base to
derive routes between end systems

OSI Routing Architecture

For the Next 10 min….

Design Goals of Routing Algorithms

Algorithm Types

Routing Metrics

Dynamic Routing Algorithms

Design Goals


Simplicity and low Overhead

Robustness and stability

Rapid Convergence


Algorithm types

Static Vs Dynamic

Single Path Vs Multiple Path

Host Intelligent Vs Router Intelligent

Intradomain Vs Interdomain

Routing Metrics

Path Length





Communication cost

Static Algorithms

Shortest Path Algorithm


Flow based Routing

Dynamic Algorithms

Distance Vector Routing

Link State Routing

Hierarchical Routing

Distance Vector Routing

Bellman Ford/ Ford Fulkerson Algorithm

ARPANET routing algorithm

AppleTalk and CISCO routers use improves
distance vector protocols

Each router knows the id of every other router in
the network.

Each router maintains a vector with an entry for
every destination that contains:

The cost to reach the destination from this router.

The direct link that is on that least cost path.

Each router, periodically sends its vector to his
direct neighbors.

Upon receiving a vector a router updates the local

vector based

on the direct link’s cost and the
received vector

Link State Routing

Discover the neighbors and learn the
network addresses

Measure the delay or cost to each of the

Construct a packet telling all it has just

Send this packet to all other routers

Compute the shortest path to every other

Link State Routing contd..

Each router knows the id of every other
router in the network.

Each router maintains a topology map of
the whole network.

Each router, periodically floods its link
state updates (with its direct connectivity

Upon receiving a vector, a router updates
the local topology map and re
shortest paths

Increasing Network Size

Router memory requirements increase.

More CPU time for look up and more
bandwidth needed to send status reports.

Routing table size increases with increase in
network size.

Hierarchical Routing

Routers divided into Regions.

Regions > Clusters > Zones > Groups >

Internal structure of a region known only to
routers within that region.

Different networks do not need to know the
topological structure of other ones.

Example of Hierarchical Routing

Penalty for Hierarchical Routing

Path length may increase. But this increase
is sufficiently small and usually acceptable.

The optimum number of levels for an N
router subnet is ln N, with a total of e ln N
entries per router.

Internet Control Protocols

Routing Information Protocol

Open Shortest Path First

Interior Gateway Routing Protocol

Exterior Gateway protocol

Border Gateway Protocol

Routing Information Protocol

The protocol is limited to networks whose longest
path is 15 hops.

Inappropriate for larger networks.

Uses fixed metrics to compare alternative routes.

Not appropriate for routing based on dynamic

Cannot operate within a hierarchical structure.

Open Shortest Path First

OSPF is a link
state routing protocol that calls for
the sending of link
state advertisements to all
other routers within the same hierarchical area.

As OSPF routers accumulate link
information, they use the SPF algorithm to
calculate the shortest path to each node using
Dijkstra’s Shortest Path Algorithm .

Can operate in a hierarchy.

Supports multi
path routing and type of service
based routing.

Interior Gateway Routing Protocol

A distance
vector routing protocol.

Replaced RIP

Network Size

Single Metric

IGRP uses five criteria to determine the best path:
the link's speed, delay, packet size, loading and

The weighting factors for each of these metrics
can be fixed up by Network Administrators.

Permits multi
path routing.

Exterior Gateway Protocol

First exterior routing protocol to gain
widespread accepted on the internet.

Dynamic routing protocol with a very
simple design used for communication of
reachability information.

It does not use metrics and therefore cannot
make intelligent routing decisions

Border Gateway Protocol

Internet standard for inter
domain exterior routing.

BGP is used to exchange routing information
between Internet service providers (ISP).

External / Interior BGP

Extremely robust and scalable

Supports classless inter
domain routing.

4, the latest version, lets administrators
configure cost metrics based on policy statements.

Routers using BGP, exchange routing information
using TCP connection.

BGP contd.

BGP communicates with autonomous (local)
networks using Internal BGP (IBGP) since it
doesn't work well with IGP. The routers inside the
autonomous network thus maintain two routing
tables: one for the interior gateway protocol and
one for IBGP.

Policies are manually configured into each BGP

Never put Iraq on a route starting at Pentagon.

Traffic starting and ending at IBM should not transit

Questions ?