Internet Core Protocols: The Definitive Guide

Misconfigured Routing Tables

The most common cause of connectivity problems across a network is that the routing tables have not been properly defined. In this scenario, your datagrams are going out to the remote destination, and datagrams are being sent back to your system but are taking a bad route on the way to your network. This problem occurs when the advertised routes for your network point to the wrong router (or do not point to any router).

This is a very common problem with new or recently changed networks. It is not at all unusual for somebody to forget to define the route back to your new network. Just because the datagrams are going out does not mean that return datagrams are coming back in on the same route.

The only way to successfully diagnose this problem is to use the traceroute program from both ends of a connection, seeing where in the network path the problem occurs. If you stop getting responses after the second or third hop on outbound tests, then it is highly likely that the router at that juncture has an incorrect routing entry for your network, or doesn’t have any entry at all. For more information on traceroute, refer to Section 5.3.3 in Chapter 5.