Migrating RIP to IGRP

Migrating RIP to IGRP

7.3.6

Migrating RIP to IGRP

Migrating RIP to IGRP

7.3.6

Configuring IGRP

Configuring IGRP

7.3.5 This page will introduce the commands that are used to configure 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.

The command syntax is as follows:

RouterA(config)#router igrpas-number

RouterA(config)#no router igrpas-number

The AS number identifies the IGRP process.

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 shows an example of how to configure IGRP for AS 101.

The Lab Activities will help students configure IGRP.

The next page will explain how to convert a router from RIP to IGRP.

IGRP stability features

IGRP stability features

7.3.4 This page will describe three features that are designed to enhance the stability of IGRP:

• Holddowns

• Split horizons

• Poison reverse updates

Holddowns

Holddowns are used to prevent regular update messages from reinstating a route that may not be up. When a router goes down, neighbor routers detect this from the lack of regularly scheduled update messages.

Split horizons

Split horizons are derived from the premise that it is not useful to send information about a route back in the direction from which it came. The split horizon rule helps prevent routing loops between adjacent routers.

Poison reverse updates

Poison reverse updates are used to prevent larger routing loops. Increases in routing metrics usually 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 many timers and variables that contain 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 route before it declares 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.

IGRP lacks support for VLSM. Cisco has created Enhanced IGRP to correct this problem.

The next page will show students how to configure IGRP.

IGRP routes

IGRP routes

7.3.3 This page will introduce the three types of routes that IGRP advertises:

• Interior

• System

• Exterior

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 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 a gateway of last resort is identified. The Cisco IOS software chooses a gateway of last resort from the list of exterior routes that IGRP provides. The software uses the gateway 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.

The Interactive Media Activity will help students understand the different types of IGRP routes.

The next page will introduce three features that increase the stability of IGRP.

IGRP metrics

IGRP metrics

7.3.2 This page will describe the metrics that IGRP uses.

The show ip protocols command displays parameters, filters, and network information about the routing protocols in use on the router. The algorithm used to calculate the routing metric for IGRP is shown in the graphic. It defines the value of the K1 to K5 metrics and provides information about the maximum hop count. The metric K1 represents bandwidth and the metric K3 represents delay. By default the values of the metrics K1 and K3 are set to 1, and K2, K4, and K5 are set to 0.

This composite metric is more accurate than the hop count metric that RIP uses to choose a path to a destination. The path that has the smallest metric value is the best route.

IGRP uses the following metrics:

• Bandwidth – The lowest bandwidth value in the path

• Delay – The cumulative interface delay along the path

• Reliability – The reliability on the link toward the destination as determined by the exchange of keepalives
• Load – The load on a link toward the destination based on bits per second

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 through configuration. Delay and bandwidth are not measured values, but are set with 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.

The next page will discuss the IGRP routes.

IGRP / IGRP features

IGRP

IGRP features

7.3.1 This page will explain the main features and functions of IGRP.

IGRP is a distance vector IGP. Distance vector routing protocols measure distances to mathematically compare routes. This measurement is known as the distance vector. Routers that use distance vector protocols must send all or a portion of their routing table in a routing update message at regular intervals to each neighbor router. As routing information spreads throughout the network, routers perform the following functions:

• Identify new destinations

• Learn of failures

IGRP is a distance vector routing protocol developed by Cisco. IGRP sends routing updates at 90 second intervals. These updates advertise all the networks for a particular AS. Key design characteristics of IGRP are a follows:

• The versatility to automatically handle indefinite, complex topologies

• The flexibility needed to segment with different bandwidth and delay characteristics

• Scalability for functioning in very large networks

By default, the IGRP routing protocol uses bandwidth and delay as metrics. Additionally, IGRP can be configured to use a combination of variables to determine a composite metric. These variables are as follows:

• Bandwidth
• Delay
• Load
• Reliability

The next page will introduce the IGRP metrics.