When the number of ports and interfaces is not available, the next best method is to use the number of routers and switches to make an estimate:

• Obtain the total number of routers and the total number of switches in the network. Include virtual routers in this count of physical systems if they are used in the network. VMware, Inc. has developed two multipliers to represent the typical number of ports and interfaces for these devices: 25 interfaces per router and 60 ports per switch. These values are typical in most deployments:

 Total ports = 60 * switches
 Total interfaces = 25 * routers
• Using the total ports and total interfaces, estimate the number of managed ports and interfaces. Use the managed ratios for ports (5 percent) and interfaces (90 percent) to calculate managed ports and interfaces:

 Managed ports = 5 percent total ports
 Managed interfaces = 90 percent total interfaces
• Add 30 percent to the estimate of managed ports and interfaces for uncertainty:

 Estimated managed ports and interfaces = (managed ports + managed interfaces) + 30 percent
• For example, if a network has 95 routers and 305 switches, the estimate for the number of managed ports and interfaces is calculated as follows:

 Total ports = 60 * switches = 60 * 305 = 18,300
 Managed ports = 5 percent total ports = (5/100) * 18,300 = 915
 Total interfaces = 25 * routers = 25 * 95 = 2,375
 Managed interfaces = 90 percent total interfaces = (90/100) * 2,375 = 2,138
 Estimated managed ports and interfaces = (managed ports + managed interfaces)+ 30 percent = (915 + 2,138) + 30 percent = 3,053 + ((30/100) * 3,053) = 3,053 + 916 = 3,969

All values that were less than one were rounded up in these calculations.