ok guys here it is all out: You need to have a GOOD "Gateway" router. I tried getting 2 xbox's working on a t-mobile linksys router and couldn't get it to work, I later figured out that it wasn't a Gateway Router, but it was just a plain router (w/voip of course). So I got a Linksys WRT54G2 v1 and did this: I left the security settings on default:
SPI Firewall: ON
Block Internet Requests:ON
Block NAT Redirection:OFF
uPNP: ENABLED (this cancels the need for port forwarding or port triggering)
You shouldn't need to Port Forward or Port Trigger ANYTHING for xbox, AS LONG AS you have a GOOD Gateway Router with latest firmware OR DD-WRT firmware!!! THAT'S KEY!!!
Make sure box xbox's are set to AUTOMATICALLY receive IP Address and DNS settings. If you assign a manual STATIC IP to xboxes, then uPNP doesn't open ports for them, therefore making uPNP useless.
If you have Cable Internet, your MTU can be 1500, but you can play with it if you want. 1364 is the MINIMUM requirement for Xbox Live. So I would say try 1500, if you notice lag, bring it down to 1450 and try again if no difference bring it down to 1425 or 1400.
If this does not work for you, then get a new router!!! Invest in a wrt54g2 or the N version of that style if u want.
If you have 2 routers in your equation, MAKE SURE YOU DO THE FOLLOWING:
the router connected to modem should be GATEWAY ROUTER. That Gateway router should have all the settings I mentioned above. Out of any port of that router, should connect to the 2nd router's WAN port. The 2nd Router should be in BRIDGE mode. So the trick is, on the second router, shut off ALL firewall settings. Set router to automatically receive IP (AKA DHCP). Make SURE that on this 2nd router, you DISABLE uPNP, dmz, port forwarding, port triggering and QoS. BTW, when you set router into bridge mode, it should automatically disable DHCP Server (meaning that it will disable the ability to ASSIGN ip's automatically, if it doesn't disable, then make sure you disable it)
Hope This helps everyone out!!!! And if you can't get it to work, GET A BETTER ROUTER!!!