Problem: IGRP Is Not Installing All Possible Equal-Cost Paths-Cause: maximum-paths Restricts IGRP to a Maximum of Four Paths by Default

Problem: IGRP Is Not Installing All Possible Equal-Cost Paths ‚ Cause: maximum-paths Restricts IGRP to a Maximum of Four Paths by Default

By default, Cisco routers support only four equal paths, for load-balancing purposes. The com-mand maximum- path can be used for up to six equal-cost paths. If the command is not configured properly, it can cause problems, as discussed in this section. The command maximum-path is incorrectly used, so it allows only one path to the destination even though more than one path exists. The maximum-path 1 command should be used only when load balancing is not desired.

Figure 5-14 shows the network setup that produces the problem of IGRP not installing all possible equal-cost paths.

Figure 5-14. Network Setup Vulnerable to Equal-Cost-Path Routes Not Being Installed by IGRP

Example 5-38 shows the routing table entry of Router R1. Only one route is being installed in the routing table.

Example 5-38 Routing Table for R1 in Figure 5-14

 R1#  show ip route igrp  131.108.0.0/24 is subnetted, 1 subnets  I   131.108.2.0 [100/8976] via 131.108.5.3, 00:00:09, Ethernet2  

Figure 5-15 shows the flowchart to follow to solve this problem.

Figure 5-15. Problem-Resolution Flowchart

Debugs and Verification

Example 5-39 shows the output of the debug ip igrp transaction command on Router R1, revealing that Router R1 is receiving two equal-cost route paths.

Example 5-39 debug ip igrp transaction Command Output Reveals the Number of Equal-Cost Paths Received

 R1#  debug ip igrp transaction  IGRP protocol debugging is on IGRP: received update from 131.108.5.3 on  Ethernet2   network 137.99.0.0, metric 8976 (neighbor 501)  IGRP: received update from 131.108.1.2 on  Ethernet1   network 137.99.0.0, metric 8976 (neighbor 501)  

Only one route is installed in the routing table. You see only one route in the routing table instead of two because the operator has configured maximum-paths 1 in the configuration.

Example 5-40 shows the current configuration for Router R1. The maximum-path setting is set to 1, which prevents IGRP from installing more than one path in the routing table. By default, maximum-path is set to 4.

Example 5-40 Current R1 Configuration

  router igrp 1   network 131.108.0.0    maximum-paths 1   

Solution

By default, Cisco IOS Software allows up to four equal-cost routes to be installed into the routing table. This can be increased up to six routes if configured properly. If there is a desire to do a load balancing over six links, use this command.

Example 5-41 shows the configuration that installs six equal-cost route paths in the routing table.

Example 5-41 Configuring R1 to Accept Up to Six Equal-Cost Route Paths in the Routing Table

 R1#  router igrp 1   maximum-paths 6  

This example makes more sense when you have more than four paths and only four are getting installed in the routing table. Because four equal-cost routes is a default, you must configure maximum-paths to accommodate the fifth and (possibly sixth ) route.