I run a (much smaller) LAN in South Africa...
I was wondering how you set your LAN up... (If you don't want to give out this info on the forums, you are more than welcome to mail me)
What physical topology do you use?
You also mentioned you are blocking certain ports over your backbone.. Are you using Cisco's to do this?
With regards to IP addressing, how do you handle the large LAN? do you use different subnets? DHCP?
Any info you can give me would be great (the more detailed / technical, the better)