Cisco Systems 78-11741-02 manual Igrp Feasible Successor Relationship Example, IPC-220

Configuring IGRP

IGRP Configuration Examples

IGRP Feasible Successor Relationship Example

In Figure 37, 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 be used for load balancing.

Figure 37 Assigning Metrics for IGRP Path Feasibility

Route to Network A

Metric = m = 10876

Router C1	Route to Network A
	Metric = n = 12776

Route to Network A

Metric = p = 10776

Router C2 56620

The feasibility test would work as follows:

•Assume that Router C1 already has a route to Network A with metric m and has just received an update about Network A from Router C2. The best metric at Router C2 is p. The metric that Router C1 would use through Router C2 is n.

•If both of the following two conditions are met, the route to Network A through Router C2 will be included in the routing table of Router C1:

–If m is greater than p.

–If the multiplier (value specified by the variance router configuration command) times m is greater than or equal to n.

•The configuration for Router C1 would be 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 configuration shows 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 38 illustrates a typical situation in which the no ip split-horizoninterface 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 12.13.50.0, 10.20.40.0, and 20.155.120.0, respectively) all have split horizon enabled by default, while the serial interfaces connected to networks 128.125.1.0 and 131.108.1.0 all have split horizon disabled by default. The partial interface configuration specifications for each router that follow Figure 38 illustrate that the ip split-horizoninterface configuration command is not explicitly configured under normal conditions for any of the interfaces.

Cisco IOS IP Configuration Guide

IPC-220