Internet Core Protocols: The Definitive Guide

Misconfigured Routing Tables

It is also quite possible that your datagrams are going out to the remote destination, and that datagrams are being sent back to your system, but that they are taking a bad route on the way back to your network. This problem occurs when the advertised routes for your network point to an illegal route.

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