Chapter 5 – Routing Protocols: IGRP - Chabot College

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Oct 28, 2013 (3 years and 9 months ago)

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Chapter 5


Routing Protocols:
IGRP

Building a Network

To Be Reliable


provide error detection and
ability to correct errors

To Provide Connectivity


incorporate a variety of
hardware & software products so they can
function together

To Be Easy To Use


users need to have no concern
for the network’s structure and implementation

To Be Easy To Modify


allow itself to evolve and
adapt as needs change

Be Easy To Implement


must follow industry
standards and allow a variety of configurations

Path Determination

Usually the responsibility of the
router


Path Determination

Enables a router to evaluate the available
paths to a destination and establish best
path for the packet

Most routing protocols use simply use the
shortest and best path

Analogy


Packet routing is like driving a
car: Routers through the use of protocols
make path decisions based on routing
tables, and people driving cars determine
their paths by reading road signs


Metrics

Routing tables must be updated and
accurate

Each routing protocol interprets the “best
path” in its own way

Protocol generates a value, METRIC, for
each path through the network

Smaller the metric the better the path

Metrics differ depending on the protocol
being used

Metrics

Metrics

Metrics

Router Forwarding Decisions

Router examines the packet’s destination
protocol address


it either knows or does
not know how to forward to the next hop

Does not know and no default route
assigned
-

the packet is typically dropped

Network portion of IP address used by
router within the network cloud

The source and destination IP addresses
never changes as a packet traverses the
network

Router Forwarding Decisions

Network portion of the address is
used to make path selection

Router responsible for passing the
packet to the next network along the
path

Switching function allows the router
to accept a packet on one interface
and forward it another interface

Routing Protocols

ROUTED protocols


moved over a
network

TCP/IP

IPX

AppleTalk

ROUTING protocols


route routed protocols
through a network

IGRP

EIGRP

OSPF

BGP

OSI routing

Advanced Peer
-
to
-
Peer
Networking(APPN)

Intermediate System
-
to
-
Intermediate
System(IS
-
IS)

RIP


Routing Protocols Continued

End systems (computers) use routed
protocols to talk to each other

IP

Routers (intermediate systems) use
routing protocols to talk to each
other


about networks and paths

Multi
-
Protocol Routing

Routers are capable of supporting multiple
independent routing protocols

IGRP

RIP

Allows routers to deliver packets from
several routed protocols over the same
data links

TCP/IP

IPX

Apple Talk

Differentiating Routing
Protocols from One Another

Can be differentiated from one
another by:

Particular goals of the designer

Various types
-

each has a different
effect on the network

Routing protocols use a variety of
metrics to identify best paths

Routing Protocols

Interior Protocols

Used for routing
information within
networks that are
under a common
administration

All IP interior protocols
must be specified with
a list of associated
networks before
routing can occur

CISCO supports RIP
and IGRP


Exterior Protocols

Used to exchange
information between
networks

Require the following
before routing can
begin

List of neighbor
routers

List of networks to
advertise as directly
reachable

EGP and BGP

Goals of Routing Protocols

Optimal Route

Select best route

Depends on metric and metric weighting

Simplicity and Efficiency

Efficiency is important when software
implementing the routing protocol must run on a
computer with limited resources

Robustness

Should perform correctly at all times

Hardware failures, high load conditions and incorrect
implementation

Goals of Routing Protocols
Continued

Rapid Convergence

Must converge rapidly

Speed and ability of a group of devices to
agree on the topology after a change has
occurred

Flexibility

Quickly and accurately adapt to a
variety of network circumstances

Routing Loops

Packet continues to bounce back and forth
between to devices until:

Device is updated

Packet is switched the maximum number of
times allowed

Different routing protocols have different
maximums

IGRP has a maximum hop count of 255 it
defaults to 100

Usually set to 50 or less

Static and Dynamic Routing

Classifications of Routing
Protocols

Most can be classified into three basic
approaches

Distance vector routing

Determines the direction and distance to any link in
the network

IGRP and RIP

Link
-
state routing (shortest path first)

Re
-
creates exact topology of the entire network

OSPF, IS
-
IS, NLSP

Hybrid approach

Combines both aspect of link
-
state and distance
vectoring

EIGRP

Classes of Routing Protocols

IP Routing Configuration

Any routing protocol must follow two steps:

Create routing process with one of the router
commands

Configure the protocol specifics

Interior routing protocols also must have a
list of networks specified before routing
begins

IGRP requires an AS (Autonomous System)
number


IP Routing Configuration

Choosing a routing protocol


consider the
following:

Network size and complexity

Network traffic levels

Security needs

Reliability needs

Network delay characteristics

Organizational policies

Organizational acceptance of change




IGRP Overview

IGRP Metrics

CISCO proprietary


developed to
supercede RIP

Distance vector interior routing
protocol

Uses a combination of metrics

Network delay, bandwidth, reliability
and load all factored into the routing
decision


IGRP Continued

Provides a wide range for its metrics

For example

Reliability


value between 1 and 255

Bandwidth


values reflecting speeds from 1200
bps to 10 Gbps

Delay


value from 1 to 224

Default values related to weightings for
IGRP give the most importance to
bandwidth


makes IGRP superior to RIP

Contrast with RIP(does not weigh metrics)


it only uses one

Distance
-
Vector Routing

IGRP Operation

Goal of IGRP was create a robust protocol
for routing within an AS

IGRP uses a combination of user
-
configurable metrics

IGRP advertises three types of routes:

Interior

System

Exterior

IGRP Operation

Interior Routes

Interior routes


between subnets in
the network attached to a router
interface

If the network attached to router is
not subnetted


IGRP does NOT
advertise interior routes

Subnet information is not included in
IGRP updates


IGRP Operation
-
System Routes

Routes to other major networks
within AS

Router derives system routes from
directly connected interfaces and
system route information from other
routers that use IGRP

System routes do NOT include
subnetting information


IGRP Operation
-
Exterior Routes

Routes to networks outside the AS that
are considered when identifying the
gateway of last resort

Router uses the gateway of last resort if it
does not have a better route

If AS has more than one connection to an
external network
-

different routers can
choose different exterior routers as the
gateway of last resort

Autonomous Systems

Command Sequence for Enabling
IGRP on a Router

Features that Enhance Stability
of IGRP
-

Holddowns

Used to avoid routing loops in the network

Effect of increasing topology convergence
time

Used to prevent regular update messages
from reinstating a route that may have
gone bad

Tell routers to hold down any changes that
might effect routes for some period of
time

Period of time is calculated to be just
greater than time to update entire network


Features that Enhance Stability
of IGRP


Split Horizons

Occur when router tries to send
information about a route back in the
direction that it came

Helps prevent routing loops

Implemented in IGRP


Features that Enhance Stability
of IGRP


Poison Reverse Updates

Intended to defeat larger routing
loops

Poison reverse updates are sent to
remove the route and place it in
holddown

Poisoning the route can speed
convergence

IGRP Metrics and routing
Updates

IGRP uses several types of metric
information

For each path through an AS IGRP
records:

Segment with lowest bandwidth

Accumulated delay

Smallest maximum transmission unit (MTU)

Reliability and load



IGRP Metrics and routing
Updates

Bandwidth by default is given the most
importance

Router running IGRP sends updates every 90
seconds

Declares route inaccessible if an update from
first router is not sent within 3 update
periods (270 seconds)


after 5 update periods (450 seconds) router
removes the route from the routing table

Uses flash update and poison reverse to speed
up convergence

IGRP Metrics and routing
Updates

Flash Update

Sending of update sooner than standard
periodic update interval

Poison Reverse Updates

Intended to defeat larger routing loops

Sent to remove a route and place it in
holddown

IGRP


Maximum Hop Count

Maximum hop count 255

Normally set lower than the default
of 100

Should be number at least greater
than the maximum number of routers
a route might have to go through


The End