Service providers sell an interconnection to the cloud and are responsible for that portion only. Service providers/carriers then interconnect their own networks known as autonomous systems between each other. Not all interconnections are the same. Often, providers make routing decisions based on their own criteria that may not necessarily benefit the end customer. Common factors are connection speed, link price, equipment capabilities, and inter-carrier relations. Service providers may make routing changes dynamically, possibly affecting the end customer. As an Internet customer, you will have to consume what you are given.
Don’t like putting all your eggs in the same basket? You may choose to connect to multiple service providers (multi-homing). But, how do you advertise your existing network to both? What if you have more than one network and would like to be in control of when and how it is advertised?
Border gateway protocol will allow you to:
- Be part of the cloud — be the Internet;
- Be in control of your route advertisements;
- Be able to control outbound traffic path;
- Be able to influence inbound traffic path;
- Be able to multi-home.
The figure above illustrates an example of a multi-homed customer. They are advertising a more exact path to their preferred ISP to influence inbound traffic while maintaining redundancy. Note: Letter of Authorization (LOA) allowing to advertise a network that belongs to another ISP and contiguous /23 are required.