for router R1, where only one route is being installed in the routing table. Debugs and Verification
Figure shows the output of debug ip igrp transactions command on router R1, revealing that R1 is receiving two equal-cost path routes. However, the routing table showed that only one route is being installed. Figure shows the current configuration for router R1. The maximum-path command has been configured with the value of 1, which prevents IGRP from installing more than one path in the routing table. By default, the maximum-path is set to four. If the command maximum-path 1 is used, it will only allow one path to the destination even though more than one path exists. The maximum-path 1 command should only be used when load balancing is not desired. 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. Figure shows the configuration that installs six equal-cost route paths in the routing table. This example makes more sense when there are more than four paths and only four are being installed in the routing table. Because four equal-cost routes is the default, the maximum-paths command can be configured to accommodate a fifth or possibly sixth route.
Content 5.5 Troubleshooting IGRP 5.5.5 Candidate default Except for IGRP, the way to set the gateway of last resort for all other routing protocols is to define a static route to 0.0.0.0 with a mask of 0.0.0.0. That route can then be propagated within RIP and OSPF with the default-information originate command, and within EIGRP with the redistribute static command. IGRP does not understand the 0.0.0.0 static route, so there is a different way to set the gateway of last resort in IGRP. Figure shows a sample network where router R1 will send default traffic out the 155.155.155.0/24 network. Router R1 wants to propagate this gateway of last resort to router R2. Figure shows the configuration of router R1, without a gateway of last resort configured. Debugs and Verification
Figure shows the routing table in R2, which R2 is receiving 155.155.155.0/24, but it is not a candidate for default. Solution
IGRP is incapable of carrying the 0.0.0.0/0 (also known as a default route). Instead, IGRP uses the default-network command to mark a network as a candidate for default. In this example, R1 is sending 155.155.155.0/24 and it is desirable to make R1 a candidate for default. To do that, the configuration on R1 must be changed to establish 155.155.0.0 as the default network. Upon doing this, IGRP will automatically start treating 155.155.155.0/24 as the candidate for default and will set the gateway of last resort on router R2. Figure shows the configuration for default-network on R1. This ip default-network statement must always point toward a major network, not a subnet, otherwise, it will not set the gateway of last resort. Figure illustrates that after the configuration change on R1, the debug ip igrp transactions output shows IGRP treating 155.155.155.0/24 as an exterior route because it is marked as a candidate for default route. Figure now shows that the gateway of last resort is set and that 155.155.155.0/24 is marked as a candidate for default. Also, the * next to the I in the routing table means that this entry is a candidate for default route. Note: When troubleshooting issues regarding default routes with RIP and OSPF, be sure a static 0.0.0.0/0 route and the default-information originate command is configured on the entrance router (or in the case of OSPF, the ASBR). With EIGRP, the static 0.0.0.0/0 route and the redistribute static command must be configured on the entrance router.
Content 5.6 Troubleshooting EIGRP 5.6.1 Mismatched K values For EIGRP to establish its neighbors, the K constant value to manipulate the EIGRP metric must be the same. In the EIGRP metric calculation, the default for the K value is set so that only the bandwidth and the delay of the interface are used to calculate the EIGRP metric. At times, the network administrator might want other interface values, such as load and reliability, to determine the EIGRP metric. Therefore, the K values must be changed. Because only bandwidth and delay are used in the calculations, the remaining K values are set to a value of 0 by default. Note: Cisco usually recommends that network administrators do not modify the default metric of using bandwidth and delay. However, the K values must be the same for all the routers or EIGRP will not establish a neighbor relationship. An example of two routers with mismatched K values is illustrated. Modifying the K values will affect how much, if any, of bandwidth, delay, reliability, and load will have on the metric calculation. Modifying these K values will have the following affect: Debugs and Verification
Figure shows the K values on RTR B have been changed to include load and reliability, along with bandwidth and delay. K1, K2, K3, and K4 have all been set to 1. The metric weights command shows that these values have been modified. The first value is the type of service (TOS) which is not supported, but must be included. RTR A retains the default K values of K1 and K3 set to 1 for bandwidth and delay only, with all other K values set to 0. If router such as RTR A is using the default, normally the metric weights command would not be included in the configuration. The command show ip eigrp neighbors on both RTR A and RTB B would show an empty list. Troubleshooting this problem requires careful scrutiny of the routers configuration. Solution
The solution is to configure the same K values on all routers in the EIGRP domain. Figure shows the modification made to RTR A to match the K values on RTR B. At this point both routers will be able to establish a neighbor relationship and exchange routing information.
Content 5.6 Troubleshooting EIGRP 5.6.2 Mismatched AS number EIGRP will not form any neighbor relationships with routers that have different autonomous system numbers. If the AS numbers are mismatched, no adjacency is formed. This problem is usually caused by misconfiguration on the routers. Figure shows two routers running EIGRP. Debugs and Verification
Figure shows the configurations of both routers. Because the two routers have different AS numbers, using the command show ip eigrp neighbors on both RTR A and RTB B would show an empty list. Solution
The solution is to modify one of the two routers so the AS numbers are the same. Figure shows both routers configured with the same AS number, EIGRP AS 1. At this point, both routers will be able to establish a neighbor relationship and exchange routing information.
Content 5.6 Troubleshooting EIGRP 5.6.3 Stuck in active – determining the problem When troubleshooting an EIGRP stuck in active problem, two questions need to be answered:
  1. Why is the route active?
  2. Why is the route stuck?
Determining why the route is active is not necessarily a difficult task. Sometimes, the route that constantly is going active could be due to a flapping link. Or, if the route is a host route (/32 route), it is possible that it is from a dial-in connection that gets disconnected. A more important and difficult task is determining why an active route becomes stuck. Usually, an active route gets stuck for one of the following reasons: By default, the stuck in active timer is only 180 seconds. If the EIGRP neighbor does