| 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.
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