Chapter 5 – Routing Protocols: IGRP

Building a Network

To Be Reliable – provide error detection and ability to correct errorsTo Provide Connectivity – incorporate a variety of hardware & software products so they can function togetherTo Be Easy To Use – users need to have no concern for the network’s structure and implementationTo Be Easy To Modify – allow itself to evolve and adapt as needs changeBe 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 packetMost routing protocols use simply use the shortest and best pathAnalogy – 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 accurateEach routing protocol interprets the “best path” in its own wayProtocol generates a value, METRIC, for each path through the networkSmaller the metric the better the pathMetrics 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 hopDoes not know and no default route assigned - the packet is typically droppedNetwork portion of IP address used by router within the network cloudThe 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 selectionRouter 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/IPIPXAppleTalk

ROUTING protocols – route routed protocols through a network

IGRPEIGRPOSPFBGPOSI routingAdvanced 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

IPRouters (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

IGRPRIP

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

TCP/IPIPXApple Talk

Differentiating Routing Protocols from One Another

Can be differentiated from one another by:

Particular goals of the designerVarious types - each has a different effect on the networkRouting protocols use a variety of metrics to identify best paths

Routing Protocols

Interior ProtocolsUsed for routing information within networks that are under a common administrationAll IP interior protocols must be specified with a list of associated networks before routing can occurCISCO supports RIP and IGRP

Exterior ProtocolsUsed to exchange information between networksRequire 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 RouteSelect best routeDepends on metric and metric weighting

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

RobustnessShould perform correctly at all times

Hardware failures, high load conditions and incorrect implementation

Goals of Routing Protocols Continued

Rapid ConvergenceMust converge rapidly

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

FlexibilityQuickly 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 100Usually set to 50 or less

Static and Dynamic Routing

Classifications of Routing Protocols

Most can be classified into three basic approaches

Distance vector routingDetermines the direction and distance to any link in the network

IGRP and RIPLink-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 commandsConfigure the protocol specifics

Interior routing protocols also must have a list of networks specified before routing beginsIGRP requires an AS (Autonomous System) number

IP Routing Configuration

Choosing a routing protocol – consider the following:

Network size and complexityNetwork traffic levelsSecurity needsReliability needsNetwork delay characteristicsOrganizational policiesOrganizational acceptance of change

IGRP Overview

IGRP Metrics

CISCO proprietary – developed to supercede RIPDistance vector interior routing protocolUses a combination of metricsNetwork delay, bandwidth, reliability and load all factored into the routing decision

IGRP Continued

Provides a wide range for its metricsFor example

Reliability – value between 1 and 255Bandwidth – values reflecting speeds from 1200 bps to 10 GbpsDelay – value from 1 to 224

Default values related to weightings for IGRP give the most importance to bandwidth – makes IGRP superior to RIPContrast 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 ASIGRP uses a combination of user-configurable metricsIGRP advertises three types of routes:

InteriorSystemExterior

IGRP Operation–Interior Routes

Interior routes – between subnets in the network attached to a router interfaceIf the network attached to router is not subnetted – IGRP does NOT advertise interior routesSubnet information is not included in IGRP updates

IGRP Operation-System Routes

Routes to other major networks within ASRouter derives system routes from directly connected interfaces and system route information from other routers that use IGRPSystem 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 resortRouter uses the gateway of last resort if it does not have a better routeIf 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 networkEffect of increasing topology convergence timeUsed to prevent regular update messages from reinstating a route that may have gone badTell routers to hold down any changes that might effect routes for some period of timePeriod 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 cameHelps prevent routing loopsImplemented in IGRP

Features that Enhance Stability of IGRP – Poison Reverse Updates

Intended to defeat larger routing loopsPoison reverse updates are sent to remove the route and place it in holddownPoisoning the route can speed convergence

IGRP Metrics and routing Updates

IGRP uses several types of metric informationFor each path through an AS IGRP records:

Segment with lowest bandwidthAccumulated delaySmallest maximum transmission unit (MTU)Reliability and load

IGRP Metrics and routing Updates

Bandwidth by default is given the most importanceRouter running IGRP sends updates every 90 secondsDeclares 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 tableUses flash update and poison reverse to speed up convergence

IGRP Metrics and routing Updates

Flash UpdateSending of update sooner than standard periodic update interval

Poison Reverse UpdatesIntended to defeat larger routing loopsSent to remove a route and place it in holddown

IGRP – Maximum Hop Count

Maximum hop count 255Normally set lower than the default of 100Should be number at least greater than the maximum number of routers a route might have to go through

The End