Is Teredo in my router a back door?

You shouldn't be worried about it. It looks as if Teredo is a IPv6 tunneling technology. According to this Wikipedia article it allows for IPv6 connectivity by tunneling IPv6 packets through your router encapsulated in IPv4/UDP datagrams (so you can still talk IPv6 even though your router doesn't).

All this is quite simple to work around, just turn Teredo off. From 'Accessories', run 'Command Prompt' with a right-click 'run as Administrator', then when you get the command prompt, type in:

netsh interface teredo set state disable

then exit and close the CMD prompt window. You will need a reboot to effect the change completely.

If you find you now can't go to your favorite website, chat club, MMORPG, etc., then you know what Teredo was actually doing, but why it is on your Win7 or higher machine is not really a mystery - it is 'down talking' to IP4 systems using it (in theory -however, in reality, since this functionality is already built-in to Windows without Teredo, chances are, it's some junky crawler you picked up....ewww....iccck!)

If you need to turn Teredo back on for some legitimate reason, just enter:

netsh interface teredo set state enable

at that same CMD prompt. SIMPLE! (oh, and reboot...of COURSE).

Teredo can be used for malicious purposed. unfortunately without seeing a traffic capture of what is actually being send across the port it is difficult to determine what its purpose is. I recommended using wireshark to capture traffic destined for that port on the local machine. It is also possible that it simply is Teredo. I will be happy to assist you with traffic analysis if necessary. At the end of the day if your not using Teredo an you have no use for the port than simply close it up.

http://www.symantec.com/avcenter/reference/Teredo_Security.pdf