Got 2 Xbox360's working fine!
Uses fixed ip's on them, no Portforwarding for Xbox live has been set!
If you do only that Xbox Will work with live..
(Also I did disable SPI under firewall settings)
The drawback is you get some NAT limitations, depending on WHO you want to chat with and that
persons NAT mode in use!
But it Will mostly work fine in 98% of the cases!
Don't do portforwarding!
Only UPNP otherwise you Will limit Xbox live to
thebox you do forward the packets to (that particular ip)
Maybe you just have done a portforwarding to an dhcp assigned ip address! Causing the first Xbox to boot and get an ip address fr.o.m. The router to get online! That Xbox Will then keep that ip until you reeboot the router or it's lease time is reached! (Sounds like it! Remove any portforwarding rules for Xbox live)
There was some kind of firmware update in the US 1.01N i guess it was called!
But i dont remember if that was for an Xbox issue or not!
Remember it is only for the US version of the router.
Also don't use the MS wireless adapters, i had alot of compatibility issues with those!
Ensure you setup uniq IP's correct subnetmask, gateway and dns and it should work.
Also offcource you can't use one Xbox Live account on more than one Xbox at the time, but
i guess you know that.
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Gaming and QoS for XBL