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Configuring IGRP

Configuring IGRP

This chapter describes how to configure the Interior Gateway Routing Protocol (IGRP). For a complete description of the IGRP commands in this chapter, refer to the "IGRP Commands" chapter of the Network Protocols Command Reference, Part 1. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.

IGRP is a dynamic distance-vector routing protocol designed by Cisco in the mid-1980s for routing in an autonomous system that contains large, arbitrarily complex networks with diverse bandwidth and delay characteristics.

For protocol-independent features, see the chapter "Configuring IP Routing Protocol-Independent Features" in this document.

Cisco's IGRP Implementation

IGRP uses a combination of user-configurable metrics, including internetwork delay, bandwidth, reliability, and load.

IGRP also advertises three types of routes: interior, system, and exterior, as shown in Figure 18. Interior routes are routes between subnets in the network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes.

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 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 it does not have a better route for a packet 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.

Figure 18:
Interior, System, and Exterior Routes

IGRP Updates

By default, a router running IGRP sends an update broadcast every 90 seconds. It declares a route inaccessible if it does not receive an update from the first router in the route within 3 update periods (270 seconds). After 7 update periods (630 seconds), the Cisco IOS software removes the route from the routing table.

IGRP uses flash update and poison reverse updates to speed up the convergence of the routing algorithm. Flash update is the sending of an update sooner than the standard periodic update interval of notifying other routers of a metric change. Poison reverse updates are intended to defeat larger routing loops caused by increases in routing metrics. The poison reverse updates are sent to remove a route and place it in holddown, which keeps new routing information from being used for a certain period of time.

IGRP Configuration Task List

To configure IGRP, perform the tasks in the following sections. Creating the IGRP routing process is mandatory; the other tasks are optional.

Also see the examples in the "IGRP Configuration Examples" section at the end of this chapter.

Create the IGRP Routing Process

To create the IGRP routing process, use the following required commands starting in global configuration mode:
Step Command Purpose

1 . 

router igrp process-id

Enable an IGRP routing process, which places you in router configuration mode.

2 . 

network network-number

Associate networks with an IGRP routing process.

IGRP sends updates to the interfaces in the specified networks. If an interface's network is not specified, it will not be advertised in any IGRP update.

It is not necessary to have a registered autonomous system number to use IGRP. If you do not have a registered number, you are free to create your own. We recommend that if you do have a registered number, you use it to identify the IGRP process.

Apply Offsets to Routing Metrics

An offset list is the mechanism for increasing incoming and outgoing metrics to routes learned via IGRP. This is done to provide a local mechanism for increasing the value of routing metrics. Optionally, you can limit the offset list with either an access list or an interface. To increase the value of routing metrics, use the following command in router configuration mode:
Command Purpose

offset-list [access-list-number | name] {in | out} offset [type number]

Apply an offset to routing metrics.

Allow Unicast Updates for IGRP

Because IGRP is normally a broadcast protocol, in order for IGRP routing updates to reach nonbroadcast networks, you must configure the Cisco IOS software to permit this exchange of routing information.

To permit information exchange, use the following command in router configuration mode:
Command Purpose

neighbor ip-address

Define a neighboring router with which to exchange routing information.

To control the set of interfaces with which you want to exchange routing updates, you can disable the sending of routing updates on specified interfaces by configuring the passive-interface command. See the discussion on filtering in the "Filter Routing Information" section in the "Configuring IP Routing Protocol-Independent Features" chapter.

Define Unequal-Cost Load Balancing

IGRP can simultaneously use an asymmetric set of paths for a given destination. This feature is known as unequal-cost load balancing. Unequal-cost load balancing allows traffic to be distributed among multiple (up to four) unequal-cost paths to provide greater overall throughput and reliability. Alternate path variance (that is, the difference in desirability between the primary and alternate paths) is used to determine the feasibility of a potential route. An alternate route is feasible if the next router in the path is closer to the destination (has a lower metric value) than the current router and if the metric for the entire alternate path is within the variance. Only paths that are feasible can be used for load balancing and included in the routing table. These conditions limit the number of cases in which load balancing can occur, but ensure that the dynamics of the network will remain stable.

The following general rules apply to IGRP unequal-cost load balancing:

If these conditions are met, the route is deemed feasible and can be added to the routing table.

By default, the amount of variance is set to one (equal-cost load balancing). You can define how much worse an alternate path can be before that path is disallowed by using the following command in router configuration mode:
Command Purpose

variance multiplier

Define the variance associated with a particular path.

Note By using the variance feature, the Cisco IOS software can balance traffic across all feasible paths and can immediately converge to a new path if one of the paths should fail.

See the "IGRP Feasible Successor Relationship Example" at the end of this chapter.

Control Traffic Distribution

If variance is configured as described in the preceding section "Define Unequal-Cost Load Balancing," IGRP or Enhanced IGRP will distribute traffic among multiple routes of unequal cost to the same destination. If you want to have faster convergence to alternate routes, but you do not want to send traffic across inferior routes in the normal case, you might prefer to have no traffic flow along routes with higher metrics.

To control how traffic is distributed among multiple routes of unequal cost, use the following command in router configuration mode:
Command Purpose

traffic-share {balanced | min}

Distribute traffic proportionately to the ratios of metrics, or by the minimum-cost route.

Adjust the IGRP Metric Weights

You have the option of altering the default behavior of IGRP routing and metric computations. This allows, for example, tuning system behavior to allow for transmissions via satellite. Although IGRP metric defaults were carefully selected to provide excellent operation in most networks, you can adjust the IGRP metric. Adjusting IGRP metric weights can dramatically affect network performance, however, so ensure that you make all metric adjustments carefully.

To adjust the IGRP metric weights, use the following command in router configuration mode. Because of the complexity of this command, we recommend that you only use it with guidance from an experienced system designer.
Command Purpose

metric weights tos k1 k2 k3 k4 k5

Adjust the IGRP metric.

By default, the IGRP composite metric is a 24-bit quantity that is a sum of the segment delays and  the lowest segment bandwidth (scaled and inverted) for a given route. For a network of homogeneous media, this metric reduces to a hop count. For a network of mixed media (FDDI, Ethernet, and serial lines running from 9600 bps to T1 rates), the route with the lowest metric reflects the most desirable path to a destination.

Adjust Timers

Routing protocols use several timers that determine such variables as the frequency of routing updates, the length of time before a route becomes invalid, and other parameters. You can adjust these timers to tune routing protocol performance to better suit your internetwork needs. You can make the following timer adjustments:

It also is possible to tune the IP routing support in the software to enable faster convergence of the various IP routing algorithms, and, hence, quicker fallback to redundant routers. The total effect is to minimize disruptions to end users of the network in situations where quick recovery is essential.

To adjust the timers, use the following command in router configuration mode:
Command Purpose

timers basic update invalid holddown flush [sleeptime]

Adjust routing protocol timers.

Disable Holddown

When the Cisco IOS software learns that a network is at a greater distance than was previously known, or it learns the network is down, the route to that network is placed in holddown. During the holddown period, the route is advertised, but incoming advertisements about that network from any router other than the one that originally advertised the network's new metric will be ignored. This mechanism is often used to help avoid routing loops in the network, but has the effect of increasing the topology convergence time.

To disable holddowns with IGRP, use the following command in router configuration mode. All devices in an IGRP autonomous system must be consistent in their use of holddowns.
Command Purpose

no metric holddown

Disable the IGRP holddown period.

Enforce a Maximum Network Diameter

The Cisco IOS software enforces a maximum diameter to the IGRP network. Routes whose hop counts exceed this diameter are not advertised. The default maximum diameter is 100 hops. The maximum diameter is 255 hops.

To configure the maximum diameter, use the following command in router configuration mode:
Command Purpose

metric maximum-hops hops

Configure the maximum network diameter.

Validate of Source IP Addresses

By default, the system validates the source IP addresses of incoming IGRP routing updates. To disable this function, use the following command in router configuration mode:
Command Purpose

no validate-update-source

Disable the checking and validation of the source IP address of incoming routing updates.

Enable or Disable Split Horizon

Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router out of any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks (such as Frame Relay and SMDS), situations can arise for which this behavior is less than ideal. For these situations, you might want to disable split horizon. This applies to IGRP and RIP.

If an interface is configured with secondary IP addresses and split horizon is enabled, updates might not be sourced by every secondary address. One routing update is sourced per network number unless split horizon is disabled.

To enable or disable split horizon, use one of the following commands in interface configuration mode:
Command Purpose

ip split-horizon

Enable split horizon.

no ip split-horizon

Disable split horizon.

Split horizon for Frame Relay and SMDS encapsulation is disabled by default. Split horizon is not disabled by default for interfaces using any of the X.25 encapsulations. For all other encapsulations, split horizon is enabled by default.

See the "Split Horizon Examples" section at the end of this chapter for examples of using split horizon.

Note In general, changing the state of the default is not recommended unless you are certain that your application requires making a change in order to advertise routes properly. Remember: If split horizon is disabled on a serial interface (and that interface is attached to a packet-switched network), you must disable split horizon for all routers in any relevant multicast groups on that network.

IGRP Configuration Examples

This section contains the following IGRP configuration examples:

IGRP Feasible Successor Relationship Example

In Figure 19, the assigned metrics meet the conditions required for a feasible successor relationship, so the paths in this example can be included in routing tables and used for load balancing.

Figure 19: Assigning Metrics for IGRP Path Feasibility

The feasibility test would work as follows:

router igrp 109
  variance 10

A maximum of four paths can be in the routing table for a single destination. If there are more than four feasible paths, the four best feasible paths are used.

Split Horizon Examples

The following sample configuration illustrates a simple example of disabling split horizon on a serial link. In this example, the serial link is connected to an X.25 network.

interface  serial  0
  encapsulation  x25
  no  ip  split-horizon

In the next example, Figure 20 illustrates a typical situation in which the no  ip  split-horizon interface configuration command would be useful. This figure depicts two IP subnets that are both accessible via a serial interface on Router C (connected to Frame Relay network). In this example, the serial interface on Router C accommodates one of the subnets via the assignment of a secondary IP address.

The Ethernet interfaces for Router A, Router B, and Router C (connected to IP networks,, and all have split horizon enabled by default, while the serial interfaces connected to networks and all have split horizon disabled by default. The partial interface configuration specifications for each router that follow Figure 20 illustrate that the ip  split-horizon command is not explicitly configured under normal conditions for any of the interfaces.

Figure 20: Disabled Split Horizon Example for Frame Relay Network

In this example, split horizon must be disabled in order for network to be advertised into network, and vice versa. These subnets overlap at Router C, interface S0. If split horizon were enabled on serial interface S0, it would not advertise a route back into the Frame Relay network for either of these networks.

Configuration for Router A
interface ethernet 1
  ip address
interface serial 1
  ip address
  encapsulation frame-relay 
Configuration for Router B
interface ethernet 2
  ip address
interface serial 2
  ip address
  encapsulation frame-relay 
Configuration for Router C
interface ethernet 0
  ip address
interface serial 0
  ip address
  ip address secondary
  encapsulation frame-relay 

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