8.1 Autonomous Systems
8.1.3 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. The figure illustrates a multihomed and nontransit 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 ensures that ISP1 will not use AS 24 to reach destinations that belong to ISP2, and vice versa. Of course, ISP1 or ISP2 can force traffic to be directed to AS 24 via 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.

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