Troubleshoot IPv6 Issues
From ARIN IPv6 Wiki
Use this page to collect tips on how to troubleshoot IPv6 issues. For instance, adding an AAAA record for a service to your DNS will result in some people losing connectivity and others experiencing unusually high latency. The root cause for connectivity loss is usually that the end user is on another network with IPv6 on their PC but no form of IPv6 connectivity. In fact, their company may be blocking Teredo and 6to4 traffic. The root cause for high latency is usually that the end user is using some form of tunnel (ISATAP, tunnel broker, 6to4, Teredo) that causes their traffic to trombone out to a distant tunnel endpoint and then come back again, or perhaps the tunnel endpoint is on a network that has poor IPv6 connectivity.
As you deploy IPv6 and as other networks deploy IPv6, your helpdesk will see new and different problems. Also, problems that seem familiar will turn out to have different rot causes from before. You will need to deal with this issue even if you are delaying implementation of IPv6 in your own network.
In order to debug connectivity issues, you can easily traceroute from various places to your network by setting up an IPv4-only host and setting up various kinds of transition technologies on it (6to4, Teredo, Hexago and SixXS tunnels, etc.)
