IP Routing

Connecting to the Internet

Several key design issues should be considered when connecting to the Internet. These issues impact the reliability, performance and cost of Internet connectivity. I will examine several design alternatives in the following sections, then look at a case study.

Design Alternatives

There are three alternatives regarding physical connectivity between the client and the ISP(s):

Singly-homed

A single circuit may be adequate for a small organization generating a trickle of traffic. The client organization must choose an ISP and decide on the speed of the access circuit. However, if that single circuit breaks, the entire organization will be without Internet service. Hence, if the organization’s access to the Internet is critical, multi-homing is warranted (even when the traffic volume is small).

Multi-homed to the same ISP

An organization may decide to implement multiple circuits between itself and its ISP for reliability. An organization that decides to multi-home has a more complex task at hand. Should the links be of the same speed? What should the link speeds be? How should inbound traffic be distributed over the links? How should outbound traffic be distributed over the links?

Multi-homed to different ISPs

A multi-homed organization may consider using more than one ISP for additional reliability. This will guard against failures in a single ISP network.

There are several options regarding the routing of traffic between the client organization and the Internet. ...