Introduction

This section discusses several possible scenarios that can prevent routes from being installed in the routing table.

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 II

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.