| 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.
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