 want to allow for growth: 340 + 51 = 411, and 411 / 48 = 8.56. This means you'll need nine modules, which will provide a total of 432 ports. Nine is an odd number, which means one switch will have more modules than the other. This may or may not matter in your environment, but I like to have each side match exactly. It's always better to have too many ports, rather than too few, SO long as the budget allows it. Rounding out the modules to 5 on each chassis brings you to 10 modules, totaling 480 ports. Now you have 480 ports available, and a present requirement for 340. This allows for a growth factor of almost 30 percent. Chances are you've missed something (I always do), SO having room for more than 15 percent growth is not a bad thing. In cases like this, I make sure to reserve extra ports for expansion purposes. They always come in handy later. Another requirement that engineers often forget is the need for inter- switch trunks. Switches in failover pairs need to be connected to each other, usually with multigigabit links. If you're using Firewall Services Modules, they should have dedicated trunks. Content Service Mod- ules should also have their own trunks, as should RSPAN, if you'll be using it. If you assume a 2 Gbps EtherChannel for each of these trunks, you've just allocated 16 ports (8 on each side). I've worked on high-bandwidth networks where each of these trunks required 4 Gbps EtherChannels. A design like this would require 32 ports (16 on each side). That's almost an entire 48-port module just for inter-switch communication! Chapter 33: Designing Networks Once you've determined how many ports you'll require, map out the devices you'll need, and figure out what devices will serve what purposes. Here, I've decided that 6509 switches will be used in the core, and 2811 routers will be used for Internet and backend connectivity to the corporate web site, located in a collocation facility. Figure 33-1 shows my spreadsheet for this information. Interfaces Device Function Location Slots Core-1 6509-E Core