represents bandwidth and the metric K3 represents delay. By default the values of the metrics K1 and K3 are set to 1, while K2, K4 and K5 are set to 0.This composite metric is more accurate than the hop count metric that RIP uses when choosing a path to a destination. The path that has the smallest metric value is the best route. The metrics that IGRP uses are: IGRP uses a composite metric. This metric is calculated as a function of bandwidth, delay, load, and reliability. By default, only bandwidth and delay are considered. The other parameters are considered only if enabled via configuration. Delay and bandwidth are not measured values, but are set via the delay and bandwidth interface commands. The show ip route command in the example shows the IGRP metric values in brackets. A link with a higher bandwidth will have a lower metric, and a route with a lower cumulative delay will have a lower metric. Web Links Configuring IGRP http://www.cisco.com/en/US/products/sw/ iosswrel/ps1831/ products_configuration_guide_ chapter09186a00800d97f6.html
Content 7.3 IGRP 7.3.3 IGRP routes IGRP advertises three types of routes: Interior
Interior routes are routes between subnets of a network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes. System
System routes are routes to networks within an autonomous system. The Cisco IOS software derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking routers or access servers. System routes do not include subnet information. Exterior
Exterior routes are routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. The Cisco IOS software chooses a gateway of last resort from the list of exterior routes that IGRP provides. The software uses the gateway (router) of last resort if a better route is not found and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort. Interactive Media Activity Checkbox: IGRP Routes After completing this activity, the student will be able to understand IGRP routes. Web Links Interior Gateway Protocol - IGRP http://www.firewall.cx/ index.php?c=igrp
Content 7.3 IGRP 7.3.4 IGRP stability features IGRP has a number of features that are designed to enhance its stability, such as: Holddowns
Holddowns are used to prevent regular update messages from inappropriately reinstating a route that may not be up. When a router goes down, neighboring routers detect this via the lack of regularly scheduled update messages. Split horizons
Split horizons are derived from the premise that it is usually not useful to send information about a route back in the direction from which it came. The split horizon rule helps prevent routing loops. Poison reverse updates
Split horizons prevent routing loops between adjacent routers, but poison reverse updates are necessary to defeat larger routing loops. Generally speaking, increases in routing metrics indicate routing loops. Poison reverse updates then are sent to remove the route and place it in holddown. With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater. IGRP also maintains a number of timers and variables containing time intervals. These include an update timer, an invalid timer, a holddown timer, and a flush timer. The update timer specifies how frequently routing update messages should be sent. The IGRP default for this variable is 90 seconds. The invalid timer specifies how long a router should wait in the absence of routing-update messages about a specific route before declaring that route invalid. The IGRP default for this variable is three times the update period. The holddown timer specifies the amount of time for which information about poorer routes is ignored. The IGRP default for this variable is three times the update timer period plus 10 seconds. Finally, the flush timer indicates how much time should pass before a route is flushed from the routing table. The IGRP default is seven times the routing update timer. Today, IGRP is showing its age, it lacks support for variable length subnet masks (VLSM). Rather than develop an IGRP version 2 to correct this problem, Cisco has built upon IGRP's legacy of success with Enhanced IGRP. Web Links An Introduction to IGRP http://www.cisco.com/en/US/tech/tk365/
tk352/technologies_white_paper09186a0
0800c8ae1.shtml
Content 7.3 IGRP 7.3.5 Configuring IGRP To configure the IGRP routing process, use the router igrp configuration command. To shut down an IGRP routing process, use the no form of this command. RouterA(config)#router igrp as-number
RouterA(config)#no router igrp as-number The Autonomous System number is one that identifies the IGRP process. It is also used to tag the routing information. To specify a list of networks for IGRP routing processes, use the network router configuration command. To remove an entry, use the no form of the command. Figure is an example of how to configure IGRP using AS 101. Lab Activity Lab Exercise: Configuring IGRP This lab is to setup an IP addressing scheme using class C networks. Lab Activity e-Lab Activity: Configuring IGRP In this lab, the students will learn to configure IGRP. Web Links Configuring IGRP http://www.cisco.com/en/US/products/sw/ iosswrel/ps1831/ products_configuration_guide_ chapter09186a00800d97f6.html
Content 7.3 IGRP 7.3.6 Migrating RIP to IGRP With the creation of IGRP in the early 1980s, Cisco Systems was the first company to solve the problems associated with using RIP to route datagrams between interior routers. IGRP determines the best path through the internetwork by examining the bandwidth and delay of the networks between routers. IGRP converges faster than RIP, thereby avoiding the routing loops caused by disagreement over the next routing hop to be taken. Further, IGRP does not share the hop count limitation of RIP. As a result of this and other improvements over RIP, IGRP enabled many large, complex, topologically diverse internetworks to be deployed.These are the steps to follow to convert from RIP to IGRP.
  1. Verify existing routing protocol (RIP) on the routers to be converted.
  2. Configure IGRP on RouterA and RouterB
  3. Enter show ip protocols on RouterA and RouterB
  4. Enter show ip route on RouterA and RouterB
Lab Activity Lab Exercise: Default Routing with RIP and IGRP This lab is to configure a default