BGP

Chapter 3. Physical Design Considerations

“The OC-3 circuit is online again. The telco reports nettles had grown into the A/C exhaust.”

This chapter deals with the costly parts of the network: the physical properties, such as hardware, locations, and topology, and with ISPs and bandwidth.

Availability

There is a lot of theory about network reliability. If you are building a network that has to conform to a specific uptime figure, it’s a good idea to spend some time reading up on this. You’ll learn how to calculate availability figures for your network using known Mean Time Between Failures (MTBF) and Mean Time To Repair (MTTR) values for your equipment. For a single component, the availability is just the MTTR as a percentage of the MTTR plus the MTBF:

A component with a 100,000-hour MTBF (more than 11 years between failures) and an eight-hour MTTR has an availability of 99.992%, statistically. The easiest way to increase this figure is by buying equipment that runs longer before it fails (higher MTBF value) and takes less time to repair or be replaced by a spare (lower MTTR value).

Calculating availability becomes more complex as the number of components increases, but it basically boils down to this: as you add more components that must all be “up” for your network to function, your availability figure drops accordingly. To get the availability for the system as a whole, multiply the ...