Troubleshooting IGRP Route Installation

This section discusses the most common scenarios that can prevent IGRP routes from getting installed in the routing table. This is the most useful section in this chapter because the most common problem in IGRP is that routes are not in the routing table. If a specific destination is not in the routing table, the packet destined for that address will be dropped.

The easiest way to find out whether a specific route is in the routing table is with the show ip route x.x.x.x command, where x.x.x.x is the specific destination (that is, an IP address of a host or a server).

Three possible sources exist for problems when routes do not to get installed in the routing table:

• Receiver problem

• Intermediate media problem (Layer 2)

• Sender problem

Receiver problems refer to when the routing update was sent by the sender. Because of some problems at the receiver's end, the receiving router cannot install the routes in the routing table.

Intermediate media problems actually refer to any medium that is in the middle of two routers exchanging routing updates. In this case, the sender already has sent the routing update, but the receiving router never received it because the medium in the middle is experiencing some kind of problem. There could be various forms of media, from a simple hub to a complex switch.

The sender's problem refers to an instance in which the routing updates are never sent by the sender, so the receiving router never receives the routing updates. The sender's problem is discussed in the section "Troubleshooting IGRP Routes Advertisement." Two problems exist in IGRP routes installation:

• IGRP routes are not in the routing table.

• IGRP is not installing all equal-cost- path routes.

In the first case, IGRP is not installing a particular route or is not installing any routes in the routing table. However, in the second case, there are some routes in the routing table for a particular destination, but some of the routes are not being installed. These two problems are discussed in detail in the sections that follow.