Problem: RIP Is Not Installing All Possible Equal-Cost Paths-Cause: maximum-path Command Restricts RIP from Installing More Than One Path

Problem: RIP Is Not Installing All Possible Equal-Cost Paths ‚ Cause: maximum- path Command Restricts RIP from Installing More Than One Path

By default, Cisco routers support only four equal paths for the purpose of load balancing. The maximum-path command can be used for up to six equal-cost paths. If the command is not configured properly, it can cause a problem, as discussed in this section. When con-figured improperly, the maximum-path command allows only one path to the destination, even though more than one path exists. Configuring the command as maximum-path 1 should be done only when load balancing is not desired.

Figure 3-19 and Example 3-60 provide a network scenario that will be used as the basis for troubleshooting when the maximum-path command restricts RIP from installing more than one path, resulting in the omission of all possible equal-cost paths. The sections that follow carefully dissect how to troubleshoot this problem.

Figure 3-19. RIP Network Vulnerable to an Equal-Cost Path Problem

Figure 3-19 shows the network setup that produces the problem of RIP not installing all possible equal-cost paths.

Example 3-60 shows the routing table of Router R1. Only one route is being installed in the routing table. By default, any routing protocol supports equal-cost multipaths (load balancing). If more than one equal path exists, it must be installed in the routing table.

Example 3-60 R1 Installs Only One Path for 131.108.2.0/24

 R1#  show ip route rip  131.108.0.0/24 is subnetted, 1 subnets  R   131.108.2.0 [120/1] via 131.108.5.3, 00:00:09, Ethernet2  

Figure 3-20 shows the flowchart to follow to solve this problem based on this cause.

Figure 3-20. Flowchart to Solve Why RIP Routes Don't Show Up in a Routing Table

Debugs and Verification

Example 3-61 shows the output of debug ip rip on Router R1. The output shows that Router R1 is receiving two equal-cost routes.

Example 3-61 debug ip rip Output on R1 Shows R1 Receiving Two Updates for the 131.108.2.0 Network

 R1#  debug ip rip  RIP protocol debugging is on R1# RIP: received v2 update from 131.108.5.3 on Ethernet2  131.108.2.0/24 -> 0.0.0.0 in 1 hops  RIP: received v2 update from 131.108.1.2 on Ethernet1  131.108.2.0/24 -> 0.0.0.0 in 1 hops  

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

Example 3-62 shows the current configuration for Router R1.

Example 3-62 R1 Is Configured with maximum-path 1

 R1#  router rip   version 2   network 131.108.0.0    maximum-paths 1   

Solution

By default, Cisco IOS Software allows up to four equal-cost routes to be installed in the routing table. This could be increased up to six routes if configured as in Example 3-63.

Example 3-63 shows the configuration that installs six equal-cost path routes in the routing table.

Example 3-63 Allowing the Maximum of Six Paths in the Routing Table

 R1#  router rip   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, maximum-paths needs to be increased to accommodate the fifth and possibly sixth route.