Content

| 5.3 |  | Common IGP Routing Protocol Issues, Causes, and Solutions |
| 5.3.1 |  | Introduction |

This section discusses several possible scenarios that can prevent routes from being installed in the routing table. These are some of the problems that are common to most routing protocols.

If routes are not installed in the routing table, the router will not forward the packets to the destinations that are missing. It is possible that the packets could be incorrectly forwarded using a supernet or default route. This scenario was previously discussed in the section, Using Discard Routes.

Missing routes in the routing table create reachability problems. Users start complaining that they cannot reach a server or a printer. When this problem is investigated, one of the first things to look for is if the appropriate routers have a route for this destination in their routing tables.

Three possibilities exist for routes not being installed in the routing table:

1. Receiver problem – The router is receiving the updates, but it is not installing the routes.
2. Intermediate media problem, Layer 2 – The sender has sent the updates, but they were lost along the way and the receiver did not receive them.
3. Sender problem – The sender is not advertising the routes, so the receiving side is not seeing the routes in the routing table.

Some of the common causes for routes not being installed in the routing table are:

- Missing or incorrect network or neighbor statement
- Layer 1/2 down
- Distribute-list in/out blocking (sender/receiver)
- Access list blocking
- Advertised Network Interface is down
- Passive interface

| Lab Activity Lab Exercise: Troubleshooting Problems at the Physical, Data Link, and Network Layers After completing this lab, the student will be able to follow a logical troubleshooting process to define, isolate, and correct problems outlined in a trouble ticket.