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 Systemto-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 userconfigurable 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