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.