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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.

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Presentation on theme: "Chapter 5 – Routing Protocols: IGRP. Building a Network To Be Reliable – provide error detection and ability to correct errors To Provide Connectivity."— Presentation transcript:

1 Chapter 5 – Routing Protocols: IGRP

2 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

3 Path Determination Usually the responsibility of the router

4 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

5 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

6 Metrics



9 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

10 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

11 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

12 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


14 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


16 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

17 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


19 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

20 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

21 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

22 Static and Dynamic Routing

23 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

24 Classes of Routing Protocols

25 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

26 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

27 IGRP Overview

28 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

29 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

30 Distance-Vector Routing

31 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

32 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

33 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

34 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

35 Autonomous Systems

36 Command Sequence for Enabling IGRP on a Router

37 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

38 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

39 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

40 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

41 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

42 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

43 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

44 The End

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