8.1 Autonomous Systems
8.1.5 Multihomed nontransit AS
An AS is multihomed if it has more than one exit point onto the internetwork; an AS connected to the Internet can be multihomed to a single provider or multiple providers. A nontransit AS does not allow transit traffic -- any traffic that has a source and destination outside the AS -- to pass through it. The Figure illustrates an AS (AS1) that is nontransit and multihomed to two providers, ISP1 and ISP2.

A nontransit AS would only advertise its own routes to both the providers it connects to -- it would not advertise routes it learned from one provider to another. This ensures traffic not destined to a network within the AS would not be directed to the AS by either of the providers. For example, in the figure, AS1 is learning about 10.1.0.0/16 and 10.2.0.0/16 from ISP1, and routes 192.168.30.0/24 and 192.168.40.0/24 from ISP2. The routers at the border of AS1 aren't advertising these routes back out -- they are only advertising local routes (172.18.20.0/24 and 172.18.21.0/24). Since AS1 isn't advertising 10.1.0.0/16 to ISP2, the routers in ISP2's AS will not see the path through AS1 as a viable path to that destination. Of course, ISP1 or ISP2 can force their traffic to be directed to AS1 via default or static routing. As a precaution against this, the routers at the border of AS1 could filter any traffic coming toward them to prevent traffic not destined to a network within AS1 from passing through.

Multihomed nontransit ASs do not really need to run BGP4 with their providers, although it is recommended -- and most of the time it's required by the providers. As you will see later on, running BGP4 with the providers has many advantages in controlling route propagation and filtering.