Content

| 9.1 |  | Autonomous Systems |
| 9.1.3 |  | Multihomed nontransit autonomous systems |

An AS is a multihomed system if it has more than one exit point to outside networks. An AS connected to the Internet can be multihomed to a single provider or multiple providers. A nontransit AS does not allow transit traffic to pass through it. Transit traffic is any traffic that has a source and destination outside the AS. Figure illustrates a multihomed and nontransit AS, shown as AS 24, which is connected to two providers, ISP1 and ISP2.

A nontransit AS would advertise only its own routes to both providers to which it connects. It would not advertise routes that it learned from one provider to another. This makes certain that ISP1 will not use AS 24 to reach destinations that belong to ISP2, and ISP2 would not use AS 24 to reach destinations that belong to ISP1. Of course, ISP1 or ISP2 can force traffic to be directed to AS 24 by way of default or static routing. As a precaution against this, the router at the border of AS 24 could filter incoming traffic to prevent transit traffic from passing through its border router.

Multihomed nontransit autonomous systems do not really need to run BGP4 with their providers. It is usually recommended and often required by ISPs. As it will be seen later in this module, BGP4 offers numerous advantages, including increased control of route propagation and filtering.