I commonly run into issues where a GRE tunnel (CentOS server to server, not router based) works and pings fine from both ends but the IP / Range being tunneled does not function.
This almost always turns out to be some routing issue at the tunnel source but it's a joke trying to get these issues solved as the impetus is basically on me, the client to prove to the provider the issue is not with the tunnel itself.
Other than ping and route command read outs what are some methods to troubleshoot such an issue? Please list the tools and example commands if possible.